Observations and impacts of bleach washing on indoor chlorine chemistry

HOCl Formation Driven by Photochemical Processes Enhanced Atmospheric Oxidation Capacity in a Coastal Atmosphere

Chlorine (Cl) radicals can profoundly affect the atmospheric oxidation capacity and the fates of pollutants. Hypochlorous acid (HOCl) is a potentially crucial Cl precursor, yet the understanding of its formation mechanisms and atmospheric impacts is still limited. Here, we observed high concentrations of HOCl in a coastal city of Southeast China during the autumn of 2022, with an average daytime peak of 181 ppt. Machine learning analysis identified Cl2, O3, nitrate, temperature, and iron as the primary factors affecting HOCl distribution. Beyond Cl2 photolysis, both nitrate photolysis and aerosol iron photochemistry also contributed to Cl radical production, which drove daytime HOCl production through reactions involving ClO and HO2 radicals in the presence of O3. Both OH and Cl radicals released via HOCl photolysis increased the levels of ROx radicals by ∼10%, thereby enhancing the daytime O3 generation and atmospheric oxidation capacity. Our findings emphasize the significant role of HOCl in atmospheric chemistry and suggest that controlling O3 levels could alleviate Cl radical production and its adverse impacts on air quality.

Modelling indoor radical chemistry during the HOMEChem campaign

In the indoor environment, occupants are exposed to air pollutants originating from continuous indoor sources and exchange with the outdoor air, with the highest concentration episodes dominated by activities performed indoors such as cooking and cleaning. Here we use the INdoor CHEMical model in Python (INCHEM-Py) constrained by measurements from the House Observations of Microbial and Environmental Chemistry (HOMEChem) campaign, to investigate the impact of a bleach cleaning event and cooking on indoor air chemistry. Measurements of the concentrations of longer-lived organic and inorganic compounds, as well as measured photolysis rates, have been used as input for the model, and the modelled hydroxyl (OH) radicals, hydroperoxyl radicals, and nitrous acid (HONO) concentrations compared to the measured values. The peak modelled OH, , and HONO concentrations during cooking and cleaning activities are about 30%, 10%, and 30% higher than the observations, respectively, within experimental uncertainties. We have determined rates for the rapid loss of HONO formed through cooking activities onto a wet surface during the cleaning events and also for the subsequent slow release of HONO from the cleaned surface back into the gas-phase. Using INCHEM-Py we have also predicted peak concentrations of chlorine (Cl) atoms, (0.75-2.3) × 105 atom per cm3 at the time of cleaning. Model predictions of the Cl atom and OH radical reactivities were also explored, showing high Cl atom reactivity throughout the day, peaking around 5000-9000 s-1. The OH reactivity was found to increase from a background value close to urban outdoor levels of 20-40 s-1, to levels exceeding observations in outdoor polluted areas following cooking and cleaning activities (up to 160 s-1). This underlines the high oxidation capacity of the indoor atmospheric environment through determining the abundance of volatile organic compounds.

A Near-Explicit Reaction Mechanism of Chlorine-Initiated Limonene: Implications for Health Risks Associated with the Concurrent Use of Cleaning Agents and Disinfectants

Limonene, a key volatile chemical product (VCP) commonly found in personal care and cleaning agents, is emerging as a major indoor air pollutant. Recently, elevated levels of reactive chlorine species during bleach cleaning and disinfection have been reported to increase indoor oxidative capacity. However, incomplete knowledge of the indoor transformation of limonene, especially the missing chlorine chemistry, poses a barrier to evaluating the environmental implications associated with the concurrent use of cleaning agents and disinfectants. Here, we investigated the reaction mechanisms of chlorinated limonene peroxy radicals (Cl-lim-RO2•), key intermediates in determining the chlorine chemistry of limonene, and toxicity of transformation products (TPs) using quantum chemical calculations and toxicology modeling. The results indicate that Cl-lim-RO2• undergoes a concerted autoxidation process modulated by RO2• and alkoxy radicals (RO•), particularly emphasizing the importance of RO• isomerization. Following this generalized autoxidation mechanism, Cl-lim-RO2• can produce low-volatility precursors of secondary organic aerosols. Toxicological findings further indicate that the majority of TPs exhibit increased respiratory toxicity, mutagenicity, and eye/skin irritation compared to limonene, presenting an occupational hazard for indoor occupants. The proposed near-explicit reaction mechanism of chlorine-initiated limonene significantly enhances our current understanding of both RO2• and RO• chemistry while also highlighting the health risks associated with the concurrent use of cleaning agents and disinfectants.

Commercial kitchen operations produce a diverse range of gas-phase reactive nitrogen species

Gas-phase reactive nitrogen species (Nr) are important drivers of indoor air quality. Cooking and cleaning are significant direct sources indoors, whose emissions will vary depending on activity and materials used. Commercial kitchens experience regular high volumes of both cooking and cleaning, making them ideal study locations for exploring emission factors from these sources. Here, we present a total Nr (tNr) budget and contributions of key species NO, NO2, acidic Nr (primarily HONO) and basic Nr (primarily NH3) using novel instrumentation in a commercial kitchen over a two-week period. In general, highest tNr was observed in the morning and driven compositionally by NO, indicative of cooking events in the kitchen. The observed HONO and basic Nr levels were unexpectedly stable throughout the day, despite the dynamic and high air change rate in the kitchen. After summing the measured NOx, HONO and Nr,base fractions, there was on average 5 ppbv of Nr unaccounted for, expected to be dominated by neutral Nr species. Using co-located measurements from a proton transfer reaction mass spectrometer (PTR-MS), we propose the identities for these major Nr species from cooking and cleaning that contributed to Nr,base and the neutral fraction of tNr. When focused specifically on cooking events in the kitchen, a vast array of N-containing species was observed by the PTR-MS. Reproducibly, oxygenated N-containing class ions (C1-12H3-24O1-4N1-3), consistent with the known formulae of amides, were observed during meat cooking and may be good cooking tracers. During cleaning, an unexpectedly high level of chloramines was observed, with monochloramine dominating the profile, as emitted directly from HOCl based cleaners or through surface reactions with reduced-N species. For many species within the tNr budget, including HONO, acetonitrile and basic Nr species, we observed stable levels day and night despite the high air change rate during the day (>27 h-1). The stable levels for these species point to large surface reservoirs which act as a significant indoor source, that will be transported outdoors with ventilation.

Interaction of Cooking-Generated Aerosols on the Human Nervous System and the Impact of Caloric Restriction Post-Exposure

Background: The inhalation of cooking-generated aerosols could lead to translocation to the brain and impact its function; therefore, the effects of cooking-generated aerosols on healthy adults were investigated using an electroencephalograph (EEG) during the 2 h period post-exposure. Methods: To explore any changes from the impact of exposure to cooking-generated aerosols on the human brain due to the absence of food intake during exposure, we divided the study participants into three groups: (A) no food intake for 2 h (2 h-zero calorie intake), (B) non-zero calorie intake, and (C) control group (simulated cooking). Results: The ultrafine particle concentrations increased from 9.0 × 103 particles/cm3 at the background level to approximately 8.74 × 104 particles/cm3 during cooking. EEGs were recorded before cooking (step 1), 60 min after cooking (step 2), 90 min after cooking (step 3), and 120 min after cooking (step 4). Comparing the non-zero calorie group with the control group, it was concluded that exposure to cooking-generated aerosols resulted in a 12.82% increase in the alpha band two hours post-exposure, compared to pre-exposure. The results revealed that zero calorie intake after exposure mitigated the impacts of cooking-generated aerosols for the alpha, beta3, theta, and delta bands, while it exacerbated effects on the whole brain for the beta1 and beta2 bands. Conclusions: While these are short-term studies, long-term exposure to cooking-generated ultrafine particles can be established through successive short-term exposures. These results underscore the need for further research into the health impacts of cooking-generated aerosols and the importance of implementing strategies to mitigate exposure.

Factors Affecting Chlorinated Product Formation from Sodium Hypochlorite Bleach and Limonene Reactions in the Gas Phase

During use of sodium hypochlorite bleach, gas-phase hypochlorous acid (HOCl) and chlorine (Cl2) are released, which can react with organic compounds present in indoor air. Reactivity between HOCl/Cl2 and limonene, a common constituent of indoor air, has been observed. The purpose of this study was to characterize the chemical species generated from gas-phase reactions between HOCl/Cl2 and limonene. Gas-phase reactions were prepared in Teflon chambers housing HOCl, Cl2, and limonene. The resulting chemical products were analyzed using gas-phase preconcentration, followed by gas chromatography and high-resolution mass spectrometry. Several chlorinated products were detected, including limonene species containing one, two, and three chlorines and limonene chlorohydrin. Product concentrations and yields were estimated for the most abundant products, and greater than 80% of transformed limonene was represented in the detected products. Temporal sampling of the reactions allowed time courses to be plotted for limonene decay and chlorinated limonene product generation under different conditions, including the treatments of HOCl/Cl2, Cl2 only, high vs low relative humidity, and ± ozone. These experiments add product speciation, yield estimates, and an understanding of environmental factors affecting product formation to previous studies, further highlighting the chemical transformations initiated by sodium hypochlorite bleach in indoor air.

Cleaning products: Their chemistry, effects on indoor air quality, and implications for human health

The use of cleaning and disinfecting products both at work and at home increased during the COVID-19 pandemic. Those products often include surfactants, acids/bases, carcinogens such as chloroform, and endocrine-disrupting chemicals, such as cyclosiloxanes, phthalates, and synthetic fragrances, which may cause harmful health effects among professional cleaners as well as among people exposed at home or in their workplaces. The aim of this study was to synthesize the effects of the commonly used chemical, surface cleaning and disinfecting products on indoor air quality, focusing on chemical and particulate matter pollutants, exposure, and human health in residential and public buildings. We also provide a summary of recommendations to avoid harmful exposure and suggest future research directions. PubMed, Google Scholar, Scopus, and Web of Science (WoS) were used to search the literature. Analysis of the literature revealed that the use of cleaning products and disinfectants increase occupants' exposure to a variety of harmful chemical air contaminants and to particulate matter. Occupational exposure to cleaning and disinfectant products has been linked to an increased risk of asthma and rhinitis. Residential exposure to cleaning products has been shown to have an adverse effect on respiratory health, particularly on asthma onset, and on the occurrence of asthma(-like) symptoms among children and adults. Efforts to reduce occupants' exposure to cleaning chemicals will require lowering the content of hazardous substances in cleaning products and improving ventilation during and after cleaning. Experimentally examined, best cleaning practices as well as careful selection of cleaning products can minimize the burden of harmful air pollutant exposure indoors. In addition, indirect ways to reduce exposure include increasing people's awareness of the harmfulness of cleaning chemicals and of safe cleaning practices, as well as clear labelling of cleaning and disinfecting products.

Oxidation Mechanism and Toxicity Evolution of Linalool, a Typical Indoor Volatile Chemical Product

Linalool, a high-reactivity volatile chemical product (VCP) commonly found in cleaning products and disinfectants, is increasingly recognized as an emerging contaminant, especially in indoor air. Understanding the gas-phase oxidation mechanism of linalool is crucial for assessing its impact on atmospheric chemistry and human health. Using quantum chemical calculations and computational toxicology simulations, we investigated the atmospheric transformation and toxicity evolution of linalool under low and high NO/HO2· levels, representing indoor and outdoor environments. Our findings reveal that linalool can undergo the novel mechanisms involving concerted peroxy (RO2·) and alkoxy radical (RO·) modulated autoxidation, particularly emphasizing the importance of cyclization reactions indoors. This expands the widely known RO2·-dominated H-shift-driven autoxidation and proposes a generalized autoxidation mechanism that leads to the formation of low-volatility secondary organic aerosol (SOA) precursors. Toxicological analysis shows that over half of transformation products (TPs) exhibited higher carcinogenicity and respiratory toxicity compared to linalool. We also propose time-dependent toxic effects of TPs to assess their long-term toxicity. Our results indicate that the strong indoor emission coupled with slow consumption rates lead to significant health risks under an indoor environment. The results highlight complex indoor air chemistry and health concerns regarding persistent toxic products during indoor cleaning, which involves the use of linalool or other VCPs.

The chemical assessment of surfaces and air (CASA) study: using chemical and physical perturbations in a test house to investigate indoor processes

The Chemical Assessment of Surfaces and Air (CASA) study aimed to understand how chemicals transform in the indoor environment using perturbations (e.g., cooking, cleaning) or additions of indoor and outdoor pollutants in a well-controlled test house. Chemical additions ranged from individual compounds (e.g., gaseous ammonia or ozone) to more complex mixtures (e.g., a wildfire smoke proxy and a commercial pesticide). Physical perturbations included varying temperature, ventilation rates, and relative humidity. The objectives for CASA included understanding (i) how outdoor air pollution impacts indoor air chemistry, (ii) how wildfire smoke transports and transforms indoors, (iii) how gases and particles interact with building surfaces, and (iv) how indoor environmental conditions impact indoor chemistry. Further, the combined measurements under unperturbed and experimental conditions enable investigation of mitigation strategies following outdoor and indoor air pollution events. A comprehensive suite of instruments measured different chemical components in the gas, particle, and surface phases throughout the study. We provide an overview of the test house, instrumentation, experimental design, and initial observations - including the role of humidity in controlling the air concentrations of many semi-volatile organic compounds, the potential for ozone to generate indoor nitrogen pentoxide (N2O5), the differences in microbial composition between the test house and other occupied buildings, and the complexity of deposited particles and gases on different indoor surfaces.

Direct generation of DBPs from city dust during chlorine-based disinfection

Chlorine-based disinfectants, such as sodium hypochlorite, are extensively used in our daily lives. In particular, during the recent Covid-19 pandemic and post-pandemic period, excessive amounts of chlorine-based disinfectants were used both indoors and outdoors to interrupt virus transmission. However, the interaction between disinfectants and city dust during the disinfection process has not been sufficiently evaluated. In this study, we conducted a comprehensive investigation into the intrinsic characteristics (e.g. morphology, size, elemental composition, and organic content, etc.) of dust collected from various indoor and outdoor areas. The results showed that the organic carbon content of indoor dust reached 6.14 %, with a corresponding measured dissolved organic carbon value of 4.17 ± 0.23 mg/g (normalized to the dust weight). Concentrations of regulated DBPs, resulting from the interaction between dust and NaClO, ranged from 57.78 ± 2.72 to 102.80 ± 22.63 µg/g for THMs and from 119.18 ± 6.50 to 285.14 ± 36.95 µg/g for HAAs (normalized to the dust weight). More significantly, using non-target analysis through gas chromatography quadrupole time-of-flight mass spectrometry (GC-qTOF-MS), we identified a total of 68, 89, and 87 types of halogenated DBPs from three typical indoor and outdoor sites (R-QH, C-JS, and W-BR, respectively). These unknown DBPs included compounds with higher toxicity compared to regulated DBPs. These findings highlight that city dust is a significant source of DBP generation during chlorine-based disinfection, posing potential harm to both the ecological environment and human health.

Gas-Phase and Surface-Initiated Reactions of Household Bleach and Terpene-Containing Cleaning Products Yield Chlorination and Oxidation Products Adsorbed onto Indoor Relevant Surfaces

The use of household bleach cleaning products results in emissions of highly oxidative gaseous species, such as hypochlorous acid (HOCl) and chlorine (Cl2). These species readily react with volatile organic compounds (VOCs), such as limonene, one of the most abundant compounds found in indoor enviroments. In this study, reactions of HOCl/Cl2 with limonene in the gas phase and on indoor relevant surfaces were investigated. Using an environmental Teflon chamber, we show that silica (SiO2), a proxy for window glass, and rutile (TiO2), a component of paint and self-cleaning surfaces, act as a reservoir for adsorption of gas-phase products formed between HOCl/Cl2 and limonene. Furthermore, high-resolution mass spectrometry (HRMS) shows that the gas-phase reaction products of HOCl/Cl2 and limonene readily adsorb on both SiO2 and TiO2. Surface-mediated reactions can also occur, leading to the formation of new chlorine- and oxygen-containing products. Transmission Fourier-transform infrared (FTIR) spectroscopy of adsorption and desorption of bleach and terpene oxidation products indicates that these chlorine- and oxygen-containing products strongly adsorb on both SiO2 and TiO2 surfaces for days, providing potential sources of human exposure and sinks for additional heterogeneous reactions.

Addressing new chemicals of emerging concern (CECs) in an indoor office

Indoor pollutants change over time and place. Exposure to hazardous organics is associated with adverse health effects. This work sampled gaseous organics by Tenax TA tubes in two indoor rooms, i.e., an office set as samples, and the room of chassis dynamometer (RCD) set as backgrounds. Compounds are analyzed by a thermal desorption comprehensive two-dimensional gas chromatography-quadrupole mass spectrometer (TD-GC × GC-qMS). Four new chemicals of emerging concern (CECs) are screened in 469 organics quantified. We proposed a three-step pipeline for CECs screening utilizing GC × GC including 1) non-target scanning of organics with convincing molecular structures and quantification results, 2) statistical analysis between samples and backgrounds to extract useful information, and 3) pixel-based property estimation to evaluate the contamination potential of addressed chemicals. New CECs spotted in this work are all intermediate volatility organic compounds (IVOCs), containing mintketone, isolongifolene, β-funebrene, and (5α)-androstane. Mintketone and sesquiterpenes may be derived from the use of volatile chemical products (VCPs), while (5α)-androstane is probably human-emitted. The occurrence and contamination potential of the addressed new CECs are reported for the first time. Non-target scanning and the measurement of IVOCs are of vital importance to get a full glimpse of indoor organics.

Kinetics of hypochlorous acid reactions with organic and chloride-containing tropospheric aerosol

Chlorine plays an important role in tropospheric oxidation processes, in both marine and continental environments. Although modeling studies have explored the importance of halogen chemistry, uncertainty remains in associated chemical mechanisms and fundamental kinetics parameters. Prior kinetics measurements of multiphase halogen recycling reactions have been largely performed with dilute, bulk solutions, leaving unexplored more realistic chemical systems which have high solute concentrations and are internally mixed with both halide and organic components. Here, we address the multiphase kinetics of gaseous HOCl using an aerosol flow tube and aerosol mass spectrometer to study its reactions with particulate chloride, using atmospherically relevant particle acidity, solute concentrations, and ionic strength. We also investigate the chemistry that results when biomass burning (BB) aerosol components and chloride are internally mixed. Using pH-buffered deliquesced particles, we show that the rate constant for reaction of dissolved HOCl with H+ and Cl- at high relative humidity (RH) (80-85%) is within a factor of two of the literature value for bulk phase conditions. However, at lower RH values (60-70%) where the particles are considerably more concentrated, the rate constant for chloride loss from the particles is an order of magnitude higher. For pure organic compounds commonly found in biomass burning (BB) aerosol, such as coniferaldehyde, salicylic acid and furfural, an increase in the aerosol chlorine content occurs with HOCl exposure, indicating the formation of organochlorine species. Together, these independent findings explain results for internally mixed aerosol particles with both chloride and BB components present where we observed behavior consistent with both chloride loss and organochlorine formation occurring simultaneously upon HOCl exposure. Our results indicate that chlorine recycling via HOCl uptake by chloride-containing particles will occur in the atmosphere efficiently over a wide range of RH conditions, even when reactive organic compounds are present in the same particles as chloride. Simultaneously, formation of organochlorine compounds, which are commonly toxic, is likely occurring when reactive organic components are present.

Impacts of non-microbial soils on polychloramide disinfectants

The presence of some nonmicrobial chemicals and surfaces, herein called "soils", are known to degrade the performance of biocides, and biocidal assays often include mixtures of materials to mimic the effects of soils. We hypothesized that water-soluble anionic polychloramide biocides were less sensitive to soil interference than cationic polymeric biocides. The relationships between soil composition and antimicrobial polymer biocidal activity were compared for an anionic polychloramide, a cationic polychloramide, and a cationic poly(quaternary ammonium) biocide. The nanoscale soil models individually investigated were polyacrylic acid (PAA), cellulose nanocrystals (CNCs), and bovine serum albumin. The low molecular weight model soils were ammonium chloride, glycine, and succinimide. Three types of soil impacts were identified: 1) sequestration, whereby the soil physically inhibited transport of the biocide to microbes; 2) extraction, whereby the soil reduced or extracted oxidative chlorine, decreasing or eliminating the oxidative chlorine strength; and 3) extraction whereby the biocidal activity increases in the presence of a low molecular weight chemical that carries oxidative Cl from the polymer to the microbes. PAA and CNCs inhibit cationic biocides by sequestration but have little impact on anionic polychloramide. Glycine and BSA extract oxidative chlorine, lowering the biocidal activity of the anionic and cationic polychloramides while not impacting the poly(quaternary ammonium) biocide. Finally, the presence of succinimide increased bacteria deactivation of both anionic and cationic polychloramides. We propose that succinimide extracts oxidative chlorine from the polychloramides and transports it to the bacteria.

Chloramines as an important photochemical source of chlorine atoms in the urban atmosphere

Monochloramine, dichloramine and trichloramine (NH2Cl, NHCl2, NCl3) are measured in the ambient atmosphere, in downtown Toronto in summer (median 39, 15 and 2.8 ppt) and winter (median 11, 7.3 and 0.7 ppt). NCl3 and NHCl2 were also measured in summer (median 1.3 and 14 ppt) from a suburban Toronto location. Measurements at two locations demonstrate prevalence of chloramines in an urban atmosphere. At both sites, NCl3 exhibits a strong diel pattern with maximum values during the night, and photolytic loss with sunrise. At the downtown site, a strong positive correlation between NH2Cl and NHCl2 in the summer night indicates a common source, with daily average peak mixing ratios approaching 500 and 250 ppt, respectively. As a previously unidentified source of chlorine (Cl) atoms, we demonstrate that NCl3 photolysis contributes 49 to 82% of the total local summertime Cl production rate at different times during the day with an average noontime peak of 3.8 × 105 atoms/cm3/s, with smaller contributions from ClNO2 and Cl2. Photolysis of NH2Cl and NHCl2 may augment this Cl production rate. Our measurements also demonstrate a daytime enhancement of chloroacetone in both the summer and winter, demonstrating the importance of Cl photochemistry. The results suggest that chloramines are an important source of Cl atoms in urban areas, with potential impacts on the abundance of organic compounds, ozone, nitrogen oxides, and particulate matter. Future studies should explore the vertical gradients of chloramines and their contribution to Cl production throughout the boundary layer.

Differences in airborne stability of SARS-CoV-2 variants of concern is impacted by alkalinity of surrogates of respiratory aerosol

The mechanistic factors hypothesized to be key drivers for the loss of infectivity of viruses in the aerosol phase often remain speculative. Using a next-generation bioaerosol technology, we report measurements of the aero-stability of several SARS-CoV-2 variants of concern in aerosol droplets of well-defined size and composition at high (90%) and low (40%) relative humidity (RH) upwards of 40 min. When compared with the ancestral virus, the infectivity of the Delta variant displayed different decay profiles. At low RH, a loss of viral infectivity of approximately 55% was observed over the initial 5 s for both variants. Regardless of RH and variant, greater than 95% of the viral infectivity was lost after 40 min of being aerosolized. Aero-stability of the variants correlate with their sensitivities to alkaline pH. Removal of all acidic vapours dramatically increased the rate of infectivity decay, with 90% loss after 2 min, while the addition of nitric acid vapour improved aero-stability. Similar aero-stability in droplets of artificial saliva and growth medium was observed. A model to predict loss of viral infectivity is proposed: at high RH, the high pH of exhaled aerosol drives viral infectivity loss; at low RH, high salt content limits the loss of viral infectivity.

Indoor and outdoor air quality impacts of cooking and cleaning emissions from a commercial kitchen

Gas and particulate emissions from commercial kitchens are important contributors to urban air quality. Not only are these emissions important for occupational exposure of kitchen staff, but they can also be vented to outdoors, causing uncertain health and environmental impacts. In this study, we chemically speciated volatile organic compounds and measured particulate matter mass concentrations in a well-ventilated commercial kitchen for two weeks, including during typical cooking and cleaning operations. From cooking, we observed a complex mixture of volatile organic gases dominated by oxygenated compounds commonly associated with the thermal degradation of cooking oils. Gas-phase chemicals existed at concentrations 2-7 orders of magnitude lower than their exposure limits, due to the high ventilation in the room (mean air change rate of 28 h^-1 during operating hours). During evening kitchen cleaning, we observed an increase in the signal of chlorinated gases from 1.1-9.0 times their values during daytime cooking. Particulate matter mass loadings tripled at these times. While exposure to cooking emissions in this indoor environment was reduced effectively by the high ventilation rate, exposure to particulate matter and chlorinated gases was elevated during evening cleaning periods. This emphasizes the need for careful consideration of ventilation rates and methods in commercial kitchen environments during all hours of kitchen operation.

Hydrogen peroxide emissions from surface cleaning in a single-family residence

High levels of reactive chemicals may be emitted to the indoor air during household surface cleaning, leading to poorer air quality and potential health hazards. Hydrogen peroxide (H₂O₂)-based cleaners have gained popularity in recent years, especially in times of COVID-19. Still, little is known regarding the effects of H₂O₂ cleaning on indoor air composition. In this work we monitored time-resolved H₂O₂ concentrations during a cleaning campaign in an occupied single-family residence using a cavity ring-down spectroscopy (CRDS) H₂O₂ analyzer. During the cleaning experiments, we investigated how unconstrained (i.e., "real-life") surface cleaning with a hydrogen peroxide solution influenced the indoor air quality of the house, and performed controlled experiments to investigate factors that could influence H₂O₂ levels including surface area and surface material, ventilation, and dwell time of the cleaning solution. Mean peak H₂O₂ concentrations observed following all surface cleaning events were 135 ppbv. The factors with the greatest effect on H₂O₂ levels were distance of the cleaned surface from the detector inlet, type of surface cleaned, and solution dwell time.

Bleach Emissions Interact Substantially with Surgical and KN95 Mask Surfaces

Mask wearing and bleach disinfectants became commonplace during the COVID-19 pandemic. Bleach generates toxic species including hypochlorous acid (HOCl), chlorine (Cl₂), and chloramines. Their reaction with organic species can generate additional toxic compounds. To understand interactions between masks and bleach disinfection, bleach was injected into a ventilated chamber containing a manikin with a breathing system and wearing a surgical or KN95 mask. Concentrations inside the chamber and behind the mask were measured by a chemical ionization mass spectrometer (CIMS) and a Vocus proton transfer reaction mass spectrometer (Vocus PTRMS). HOCl, Cl₂, and chloramines were observed during disinfection and concentrations inside the chamber are 2-20 times greater than those behind the mask, driven by losses to the mask surface. After bleach injection, many species decay more slowly behind the mask by a factor of 0.5-0.7 as they desorb or form on the mask. Mass transfer modeling confirms the transition of the mask from a sink during disinfection to a source persisting >4 h after disinfection. Humidifying the mask increases reactive formation of chloramines, likely related to uptake of ammonia and HOCl. These experiments indicate that masks are a source of chemical exposure after cleaning events occur.

Emerging investigator series: an instrument to measure and speciate the total reactive nitrogen budget indoors: description and field measurements

Reactive nitrogen species (N_r), defined here as all N-containing compounds except N₂ and N₂O, have been shown to be important drivers for indoor air quality. Key N_r species include NO_x (NO + NO₂), HONO and NH₃, which are known to have detrimental health effects. In addition, other N_r species that are not traditionally measured may be important chemical actors for indoor transformations (e.g. amines). Cooking and cleaning are significant sources of N_r, whose emission will vary depending on the type of activity and materials used. Here we present a novel instrument that measures the total gas-phase reactive nitrogen (tN_r) budget and key species NO_x, HONO, and NH₃ to demonstrate its suitability for indoor air quality applications. The tN_r levels were measured using a custom-built heated platinum (Pt) catalytic furnace to convert all N_r species to NO_x, called the tN_r oven. The measurement approach was validated through a series of control experiments, such that quantitative measurement and speciation of the total N_r budget are demonstrated. The optimum operating conditions of the tN_r oven were found to be 800 °C with a sampling flow rate of 630 cubic centimetres per minute (ccm). Oxidized nitrogen species are known to be quantitatively converted under these conditions. Here, the efficiency of the tN_r oven to convert reduced N_r species to NO_x was found to reach a maximum at 800 °C, with 103 ± 13% conversion for NH₃ and 79-106% for selected relevant amines. The observed variability in the conversion efficiency of reduced N_r species demonstrates the importance of catalyst temperature characterization for the tN_r oven. The instrument was deployed successfully in a commercial kitchen, a complex indoor environment with periods of rapidly changing levels, and shown to be able to reliably measure the tN_r budget during periods of longer-lived oscillations (>20 min), typical of indoor spaces. The measured NO_x, HONO and basic N_r (NH₃ and amines) were unable to account for all the measured tN_r, pointing to a substantial missing fraction (on average 18%) in the kitchen. Overall, the tN_r instrument will allow for detailed survey(s) of the key gaseous N_r species across multiple locations and may also identify missing N_r fractions, making this platform capable of stimulating more in-depth analysis in indoor atmospheres.

Determinants Analysis Regarding Household Chemical Indoor Pollution

Indoor household pollution is not yet sufficiently studied in the general population. Over 4 million people die prematurely every year due to air pollution in households. This study aimed to propose quantitative data research through the administration of a KAP (Knowledge, Attitudes, and Practices) Survey Questionnaire. This cross-sectional study administered questionnaires to adults from the metropolitan city of Naples (Italy). Three Multiple Linear Regression Analyses (MLRA) were developed, including Knowledge, Attitudes, and Behavior regarding household chemical air pollution and the related risks. One thousand six hundred seventy subjects received a questionnaire to be filled out and collected anonymously. The mean age of the sample was 44.68 years, ranging from 21-78 years. Most of the people interviewed (76.13%) had good attitudes toward house cleaning, and 56.69% stated paying attention to cleaning products. Results of the regression analysis indicated that positive attitudes were significantly higher among subjects who graduated, with older age, male and non-smokers, but they were correlated with lower knowledge. In conclusion, a behavioral and attitudinal program targeted those with knowledge, such as younger subjects with high educational levels, but do not engage in correct practices towards household indoor chemical pollution.

Elevated levels of chloramines and chlorine detected near an indoor sports complex

Chloramines (NH₂Cl, NHCl₂, and NCl₃) are toxic compounds that can be created during the use of bleach-based disinfectants that contain hypochlorous acid (HOCl) and the hypochlorite ion (OCl^-) as their active ingredients. Chloramines can then readily transfer from the aqueous-phase to the gas-phase. Atmospheric chemical ionization mass spectrometry using iodide adduct chemistry (I-CIMS) made observations across two periods (2014 and 2016) at an urban background site on the University of Leicester campus (Leicester, UK). Both monochloramine (NH₂Cl) and molecular chlorine (Cl₂) were detected and positively identified from calibrated mass spectra during both sampling periods and to our knowledge, this is the first detection of NH₂Cl outdoors. Mixing ratios of NH₂Cl reached up to 2.2 and 4.0 parts per billion by volume (ppbv), with median mixing ratios of 30 and 120 parts per trillion by volume (pptv) during the 2014 and 2016 sampling periods, respectively. Levels of Cl₂ were observed to reach up to 220 and 320 pptv. Analysis of the NH₂Cl and Cl₂ data pointed to the same local source, a nearby indoor sports complex with a swimming pool and a cleaning product storage shed. No appreciable levels of NHCl₂ and NCl₃ were observed outdoors, suggesting the indoor pool was not likely to be the primary source of the observed ambient chloramines, as prior measurements made in indoor pool atmospheres indicate that NCl₃ would be expected to dominate. Instead, these observations point to indoor cleaning and/or cleaning product emissions as the probable source of NH₂Cl and Cl₂ where the measured levels provide indirect evidence for substantial amounts transported from indoors to outdoors. Our upper estimate for total NH₂Cl emissions from the University of Leicester indoor sports complexes scaled for similar sports complexes across the UK is 3.4 × 10⁵ ± 1.1 × 10⁵ μg h^-1 and 0.0017 ± 0.00034 Gg yr^-1, respectively. The Cl-equivalent emissions in HCl are only an order of magnitude less to those from hazardous waste incineration and iron and steel sinter production in the UK National Atmospheric Emissions Inventory (NAEI).

Near-source hypochlorous acid emissions from indoor bleach cleaning

Cleaning surfaces with sodium hypochlorite (NaOCl) bleach can lead to high levels of gaseous chlorine (Cl₂) and hypochlorous acid (HOCl); these have high oxidative capacities and are linked to respiratory issues. We developed a novel spectral analysis procedure for a cavity ring-down spectroscopy (CRDS) hydrogen peroxide (H₂O₂) analyzer to enable time-resolved (3 s) HOCl quantification. We measured HOCl levels in a residential bathroom while disinfecting a bathtub and sink, with a focus on spatial and temporal trends to improve our understanding of exposure risks during bleach use. Very high (>10 ppmv) HOCl levels were detected near the bathtub, with lower levels detected further away. Hypochlorous acid concentrations plateaued in the room at a level that depended on distance from the bathtub. This steady-state concentration was maintained until the product was removed by rinsing. Mobile experiments with the analyzer inlet secured to the researcher's face were conducted to mimic potential human exposure to bleach emissions. The findings from mobile experiments were consistent with the spatial and temporal trends observed in the experiments with fixed inlet locations. This work provides insight on effective strategies to reduce exposure risk to emissions from bleach and other cleaning products.

Investigation on the effectiveness of ventilation dilution on mitigating COVID-19 patients' secondary airway damage due to exposure to disinfectants

Chlorine-containing disinfectants are widely used in hospitals to prevent hospital-acquired severe acute respiratory syndrome coronavirus 2 infection. Meanwhile, ventilation is a simple but effective means to maintain clean air. It is essential to explore the exposure level and health effects of coronavirus disease 2019 patients' inhalation exposure to by-products of chloride-containing disinfectants under frequent surface disinfection and understand the role of ventilation in mitigating subsequent airway damage. We determined ventilation dilution performance and indoor air quality of two intensive care unit wards of the largest temporary hospital constructed in China, Leishenshan Hospital. The chloride inhalation exposure levels, and health risks indicated by interleukin-6 and D-dimer test results of 32 patients were analysed. The mean ± standard deviation values of the outdoor air change rate in the two intensive care unit wards were 8.8 ± 1.5 h-1 (Intensive care unit 1) and 4.1 ± 1.4 h-1 (Intensive care unit 2). The median carbon dioxide and fine particulate matter concentrations were 480 ppm and 19 μg/m3 for intensive care unit 1, and 567 ppm and 21 μg/m3 for intensive care unit 2, all of which were around the average levels of those in permanent hospitals (579 ppm and 21 μg/m3). Of these patients, the median (lower quartile, upper quartile) chloride exposure time and calculated dose were 26.66 (2.89, 57.21) h and 0.357 (0.008, 1.317) mg, respectively. A statistically significant positive correlation was observed between interleukin-6 and D-dimer concentrations. To conclude, ventilation helped maintain ward air cleanliness and health risks were not observed.

Reaction of HOCl with Wood Smoke Aerosol: Impacts on Indoor Air Quality and Outdoor Reactive Chlorine

High loadings of biomass burning (BB) aerosol particles from wildfire or residential heating sources can be present in both outdoor and indoor environments, where they deposit onto surfaces such as walls and furniture. These pollutants can interact with oxidants in both the aerosol and deposited forms. Hypochlorous acid (HOCl), a strong oxidant emitted during cleaning with chlorine-cleaning agents such as bleach, can attain mixing ratios of hundreds of ppbv indoors; moreover, lower mixing ratios are naturally present outdoors. Here, we report the heterogeneous reactivity of HOCl with wood smoke aerosol particles. After exposure to gas-phase HOCl, the particle chlorine content increased reaching chlorine-to-organic mass ratios of 0.07 with the chlorine covalently bound as organochlorine species, many of which are aromatic. Investigating individual potential BB components, we observed that unsaturated species such as coniferaldehyde and furfural react efficiently with HOCl. These observations indicate that organochlorine pollutants will form indoors when bleach cleaning a wildfire impacted space. The chlorine component of particles internally mixed with BB material and chloride initially increased, upon HOCl exposure, indicating that active chlorine recycling in the outdoor environment will be suppressed in the presence of BB emissions.

Antimicrobial efficacy, mode of action and in vivo use of hypochlorous acid (HOCl) for prevention or therapeutic support of infections

The objective is to provide a comprehensive overview of the rapidly developing field of the current state of research on in vivo use of hypochlorous acid (HOCl) to aid infection prevention and control, including naso-pharyngeal, alveolar, topical, and systemic HOCl applications. Also, examples are provided of dedicated applications in COVID-19. A brief background of HOCl's biological and chemical specifics and its physiological role in the innate immune system is provided to understand the effect of in vivo applications in the context of the body's own physiological defense mechanisms.

A review of indoor Gaseous organic compounds and human chemical Exposure: Insights from Real-time measurements

Gaseous organic compounds, mainly volatile organic compounds (VOCs), have become a wide concern in various indoor environments where we spend the majority of our daily time. The sources, compositions, variations, and sinks of indoor VOCs are extremely complex, and their potential impacts on human health are less understood. Owing to the deployment of the state-of-the-art real-time mass spectrometry during the last two decades, our understanding of the sources, dynamic changes and chemical transformations of VOCs indoors has been significantly improved. This review aims to summarize the key findings from mass spectrometry measurements in recent indoor studies including residence, classroom, office, sports center, etc. The sources and sinks, compositions and distributions of indoor VOCs, and the factors (e.g., human activities, air exchange rate, temperature and humidity) driving the changes in indoor VOCs are discussed. The physical and chemical processes of gas-particle partitioning and secondary oxidation processes of VOCs, and their impacts on human health are summarized. Finally, the recommendations for future research directions on indoor VOCs measurements and indoor chemistry are proposed.

Reactive Chlorine Emissions from Cleaning and Reactive Nitrogen Chemistry in an Indoor Athletic Facility

Indoor gas-phase radical sources are poorly understood but expected to be much different from outdoors. Several potential radical sources were measured in a windowless, light-emitting diode (LED)-lit room in a college athletic facility over a 2 week period. Alternating measurements between the room air and the supply air of the heating, ventilation, and air-conditioning system allowed an assessment of sources. Use of a chlorine-based cleaner was a source of several photolabile reactive chlorine compounds, including ClNO₂ and Cl₂. During cleaning events, photolysis rates for these two compounds were up to 0.0023 pptv min^-1, acting as a source of chlorine atoms even in this low-light indoor environment. Unrelated to cleaning events, elevated ClNO₂ was often observed during daytime and lost to ventilation. The nitrate radical (NO₃), which is rapidly photolyzed outdoors during daytime, may persist in low-light indoor environments. With negligible photolysis, loss rates of NO₃ indoors were dominated by bimolecular reactions. At times with high NO₂ and O₃ ventilated from outdoors, N₂O₅ was observed. Elevated ClNO₂ measured concurrently suggests the formation through heterogeneous reactions, acting as an additional source of reactive chlorine within the athletic facility and outdoors.

A review of critical residential buildings parameters and activities when investigating indoor air quality and pollutants

Indoor air in residential dwellings can contain a variety of chemicals, sometimes present at concentrations or in combinations which can have a negative impact on human health. Indoor Air Quality (IAQ) surveys are often required to characterize human exposure or to investigate IAQ concerns and complaints. Such surveys should include sufficient contextual information to elucidate sources, pathways, and the magnitude of exposures. The aim of this review was to investigate and describe the parameters that affect IAQ in residential dwellings: building location, layout, and ventilation, finishing materials, occupant activities, and occupant demography. About 180 peer-reviewed articles, published from 01/2013 to 09/2021 (plus some important earlier publications), were reviewed. The importance of the building parameters largely depends on the study objectives and whether the focus is on a specific pollutant or to assess health risk. When considering classical pollutants such as particulate matter (PM) or volatile organic compounds (VOCs), the building parameters can have a significant impact on IAQ, and detailed information of these parameters needs to be reported in each study. Research gaps and suggestions for the future studies together with recommendation of where measurements should be done are also provided.

Comparison of Simultaneous Measurements of Indoor Nitrous Acid: Implications for the Spatial Distribution of Indoor HONO Emissions

Despite its importance as a radical precursor and a hazardous pollutant, the chemistry of nitrous acid (HONO) in the indoor environment is not fully understood. We present results from a comparison of HONO measurements from a time-of-flight chemical ionization mass spectrometer (ToF-CIMS) and a laser photofragmentation/laser-induced fluorescence (LP/LIF) instrument during the House Observations of Microbial and Environmental Chemistry (HOMEChem) campaign. Experiments during HOMEChem simulated typical household activities and provided a dynamic range of HONO mixing ratios. The instruments measured HONO at different locations in a house featuring a typical air change rate (ACR) (0.5 h-1) and an enhanced mixing rate (∼8 h-1). Despite the distance between the instruments, measurements from the two instruments agreed to within their respective uncertainties (slope = 0.85, R2 = 0.92), indicating that the lifetime of HONO is long enough for it to be quickly distributed indoors, although spatial gradients occurred during ventilation periods. This suggests that emissions of HONO from any source can mix throughout the house and can contribute to OH radical production in sunlit regions, enhancing the oxidative capacity indoors. Measurement discrepancies were likely due to interferences with the LP/LIF instrument as well as calibration uncertainties associated with both instruments.

Chlorination in the pandemic times: The current state of the art for monitoring chlorine residual in water and chlorine exposure in air

During the COVID-19 pandemic, the use of chlorine-based disinfectants has surged due to their excellent performance and cost-effectiveness in intercepting the spread of the virus and bacteria in water and air. Many authorities have demanded strict chlorine dosage for disinfection to ensure sufficient chlorine residual for inactivating viruses and bacteria while not posing harmful effects to humans as well as the environment. Reliable chlorine sensing techniques have therefore become the keys to ensure a balance between chlorine disinfection efficiency and disinfection safety. Up to now, there is still a lack of comprehensive review that collates and appraises the recently available techniques from a practical point of view. In this work, we intend to present a detailed overview of the recent advances in monitoring chlorine in both dissolved and gaseous forms aiming to present valuable information in terms of method accuracy, sensitivity, stability, reliability, and applicability, which in turn guides future sensor development. Data on the analytical performance of different techniques and environmental impacts associated with the dominated chemical-based techniques are thus discussed. Finally, this study concludes with highlights of gaps in knowledge and trends for future chlorine sensing development. Due to the increasing use of chlorine in disinfection and chemical synthesis, we believe the information present in this review is a relevant and timely resource for the water treatment industry, healthcare sector, and environmental organizations.

Impact of glycol-based solvents on indoor air quality-Artificial fog and exposure pathways of formaldehyde and various carbonyls

Artificial fog is commonly employed in the entertainment industry and indoor household celebrations. The fog is generated from glycol-based solvents, which can also be found in e-cigarettes and personal care products. Although potential health impacts of glycol inhalation are frequently cited by studies of e-cigarette smoking, the dynamics and the chemical composition of glycol-based aerosols have never been studied systematically. The objective of this work is to investigate the impact of glycol-based aerosol on indoor air quality. Specifically, we targeted artificial fogs generated with common glycols, including propylene glycol (PG) and triethylene glycol (TEG). With the aid of a novel aerosol collecting and monitoring instrument setup, we obtained time-resolved aerosol profiles and their chemical compositions in an experimental room. Artificial fog has given rise to a significant amount of ultra-fine particulate matter, demonstrating its negative impact on indoor air quality. Additionally, we found a high concentration (9.75 mM) of formaldehyde and other carbonyls in fog machine fluids stored for months. These compounds are introduced to the indoor air upon artificial fog application. We propose that carbonyls have accumulated from the oxidative decomposition of glycols, initiated by OH radicals and singlet oxygens (¹ O₂ ) and likely sustained by autooxidation. Oxidation of glycols by indoor oxidants has never been reported previously. Such chemical processes can represent an unrecognized source of toxic carbonyl compounds which is also applicable to other glycol-based solvents.

Effectiveness of air-purifying devices and measures to reduce the exposure to bioaerosols in school classrooms

The SARS-CoV-2 pandemic, which suddenly appeared at the beginning of 2020, revealed our knowledge deficits in terms of ventilation and air pollution control. It took many weeks to realize that aerosols are the main route of transmission. The initial attempt to hold back these aerosols through textile masks seemed almost helpless, although there is sufficient knowledge about the retention capacity of fabric filters for aerosols. In the absence of a sufficient number of permanently installed heating, ventilation, and air conditioning systems, three main approaches are pursued: (a) increasing the air exchange rate by supplying fresh air, (b) using mobile air purifiers, and (c) disinfection by introducing active substances into the room air. This article discusses the feasibility of these different approaches critically. It also provides experimental results of air exchange measurements in a school classroom that is equipped with a built-in fan for supplying fresh air. With such a fan and a window tilted at the appropriate distance, an air exchange rate of 5/h can be set at a low power level and without any significant noise pollution. Heat balance calculations show that no additional heat exchanger is necessary in a normal classroom with outside temperatures above 10°C. Furthermore, a commercial mobile air purifier is studied in a chamber and a test room setup in order to examine and evaluate the efficiency of such devices against viable viruses under controlled and realistic conditions. For this purpose, bacteriophages of the type MS2 are used. Both window ventilation and air purifiers were found to be suitable to reduce the concentration of phages in the room.

Water-soluble anionic polychloramide biocides based on maleic anhydride copolymers

Our goal was to develop film-forming polymers to extend the antimicrobial lifetimes of cleaned and disinfected surfaces. Antimicrobial polymers were prepared by first reacting poly(ethylene-alt-maleic anhydride) with isopropylamine, partially consuming the anhydride groups, followed by hydrolysis to give water-soluble, highly anionic polyamide PC3. Chlorination with NaOCl gave PC3Cl with oxidative chlorine contents up to 9 wt%. Dried, 5 µm thick, PC3Cl films, gave log 4 reductions in the concentration of Escherichia coli or Staphylococcus aureus exposed to films. A unique feature of the maleic anhydride copolymer platform was the ability to form covalent grafts to surfaces via anhydride reactions. PC3 solution was impregnated into cellulosic filter paper, heated to form ester linkages with cellulose, followed by chlorination with sodium dichloroisocyanurate dihydrate giving grafted PC3Cl. The treated paper (0.3 wt% PC3Cl) gave a log 4 reduction of E. coli concentration in 30 min.

Behavior of Isocyanic Acid and Other Nitrogen-Containing Volatile Organic Compounds in The Indoor Environment

Isocyanic acid (HNCO) and other nitrogen-containing volatile chemicals (organic isocyanates, hydrogen cyanide, nitriles, amines, amides) were measured during the House Observation of Microbial and Environmental Chemistry (HOMEChem) campaign. The indoor HNCO mean mixing ratio was 0.14 ± 0.30 ppb (range 0.012-6.1 ppb), higher than outdoor levels (mean 0.026 ± 0.15 ppb). From the month-long study, cooking and chlorine bleach cleaning are identified as the most important human-related sources of these nitrogen-containing gases. Gas oven cooking emits more isocyanates than stovetop cooking. The emission ratios HNCO/CO (ppb/ppm) during stovetop and oven cooking (mean 0.090 and 0.30) are lower than previously reported values during biomass burning (between 0.76 and 4.6) and cigarette smoking (mean 2.7). Bleach cleaning led to an increase of the HNCO mixing ratio of a factor of 3.5 per liter of cleaning solution used; laboratory studies indicate that isocyanates arise via reaction of nitrogen-containing precursors, such as indoor dust. Partitioned in a temperature-dependent manner to indoor surface reservoirs, HNCO was present at the beginning of HOMEChem, arising from an unidentified source. HNCO levels are higher at the end of the campaign than the beginning, indicative of occupant activities such as cleaning and cooking; however the direct emissions of humans are relatively minor.

Health effects of sodium hypochlorite: review of published case reports

Sodium hypochlorite is widely used as the main component of cleaners and has an excellent bleaching and sterilizing effect in living and medical environments. In addition to bleaching, it is used for wastewater treatment and for sterilization in food factories, and also for disinfectants during the COVID-19 pandemic. This study analyzed reports of the health effects of sodium hypochlorite and classified them by toxicity along the exposure pathway. Most case reports described the health effects of acute high-concentration exposure, with a common case being dental exposure, mainly during treatment.

Nanoscopic Study of Water Uptake on Glass Surfaces with Organic Thin Films and Particles from Exposure to Indoor Cooking Activities: Comparison to Model Systems

Water uptake by thin organic films and organic particles on glass substrates at 80% relative humidity was investigated using atomic force microscopy-infrared (AFM-IR) spectroscopy. Glass surfaces exposed to kitchen cooking activities show a wide variability of coverages from organic particles and organic thin films. Water uptake, as measured by changes in the volume of the films and particles, was also quite variable. A comparison of glass surfaces exposed to kitchen activities to model systems shows that they can be largely represented by oxidized oleic acid and carboxylate groups on long and medium hydrocarbon chains (i.e., fatty acids). Overall, we demonstrate that organic particles and thin films that cover glass surfaces can take up water under indoor-relevant conditions but that the water content is not uniform. The spatial heterogeneity of the changes in these aged glass surfaces under dry (5%) and wet (80%) conditions is quite marked, highlighting the need for studies at the nano- and microscale.

Spatiotemporal characterization of irradiance and photolysis rate constants of indoor gas-phase species in the UTest house during HOMEChem

We made intensive measurements of wavelength-resolved spectral irradiance in a test house during the HOMEChem campaign and report diurnal profiles and two-dimensional spatial distribution of photolysis rate constants (J) of several important indoor photolabile gases. Results show that sunlight entering through windows, which was the dominant source of ultraviolet (UV) light in this house, led to clear diurnal cycles, and large time- and location-dependent variations in local gas-phase photochemical activity. Local J values of several key indoor gases under direct solar illumination were 1.8-7.4 times larger-and more strongly dependent on time, solar zenith angle, and incident angle of sunlight relative to the window-than under diffuse sunlight. Photolysis rate constants were highly spatially heterogeneous and fast photochemical reactions in the gas phase were generally confined to within tens of cm of the region that were directly sunlit. Opening windows increased UV photon fluxes by 3 times and increased predicted local hydroxyl radical (OH) concentrations in the sunlit region by 4.5 times to 3.2 × 10⁷  molec cm^-3 due to higher J values and increased contribution from O₃ photolysis. These results can be used to improve the treatment of photochemistry in indoor chemistry models and are a valuable resource for future studies that use the publicly available HOMEChem measurements.

Hypochlorous acid initiated lipid chlorination at air-water interface

There has been a surge of interest in interfacial hypochlorous acid (HOCl) chemistry for indoor air quality and public health. Here we combined nanoelectrospray mass spectrometry (nESI-MS) and acoustic levitation (AL) techniques to study the chlorination chemistry of three model lipids (DPPE, POPG, DOPG) mediated by HOCl at the air-water interface of levitated water droplet. For DPPE with no CC double bonds, HOCl was insensitive to the alkane chains, and showed considerable delay directing to head amino groups compared to that in aqueous environment. Chlorination chemistry, for POPG and DOPG with CC double bonds, preferentially reacted with double bonds of one chain. The mechanism was discussed in light of these observations, and it is concluded that the increased hydrophilicity of the chlorinated chain disturbed the lipid packing and attracted it toward the water phase. In addition, the reaction rate constant and reactive uptake coefficient suggested that the chlorination of lipids exposed to HOCl at the air-water interface is likely to occur rapidly. These results gain the knowledge of HOCl mediated lipid interface reaction at the molecule level, and would better understand the adverse health effects associated with elevated indoor pollutants.

Volatile organic compound emissions during HOMEChem

Quantifying speciated concentrations and emissions of volatile organic compounds (VOCs) is critical to understanding the processes that control indoor VOC dynamics, airborne chemistry, and human exposures. Here, we present source strength profiles from the HOMEChem study, quantifying speciated VOC emissions from scripted experiments (with multiple replicates) of cooking, cleaning, and human occupancy and from unperturbed baseline measurements of the building and its contents. Measurements using a proton transfer reaction time-of-flight mass spectrometer were combined with tracer-based determinations of air-change rates to enable mass-balance-based calculations of speciated, time-resolved VOC source strengths. The building and its contents were the dominant emission source into the house, with large emissions of acetic acid, methanol, and formic acid. Cooking emissions were greater than cleaning emissions and were dominated by ethanol. Bleach cleaning generated high emissions of chlorinated compounds, whereas natural product cleaning emitted predominantly terpenoids. Occupancy experiments showed large emissions of siloxanes from personal care products in the morning, with much lower emissions in the afternoon. From these results, VOC emissions were simulated for a hypothetical 24-h period, showing that emissions from the house and its contents make up nearly half of total indoor VOC emissions.

Unintended Consequences of Air Cleaning Chemistry

Amplified interest in maintaining clean indoor air associated with the airborne transmission risks of SARS-CoV-2 have led to an expansion in the market for commercially available air cleaning systems. While the optimal way to mitigate indoor air pollutants or contaminants is to control (remove) the source, air cleaners are a tool for use when absolute source control is not possible. Interventions for indoor air quality management include physical removal of pollutants through ventilation or collection on filters and sorbent materials, along with chemically reactive processes that transform pollutants or seek to deactivate biological entities. This perspective intends to highlight the perhaps unintended consequences of various air cleaning approaches via indoor air chemistry. Introduction of new chemical agents or reactive processes can initiate complex chemistry that results in the release of reactive intermediates and/or byproducts into the indoor environment. Since air cleaning systems are often continuously running to maximize their effectiveness and most people spend a vast majority of their time indoors, human exposure to both primary and secondary products from air cleaners may represent significant exposure risk. This Perspective highlights the need for further study of chemically reactive air cleaning and disinfection methods before broader adoption.

Increased disinfection byproducts in the air resulting from intensified disinfection during the COVID-19 pandemic

Intensified use of disinfectants to control COVID-19 could unintentionally increase the disinfection byproducts (DBPs) in the environment. In indoor spaces, it is critical to determine the optimal disinfection practice to prevent the spread of the virus while keeping DBPs at relatively low levels in the air. The formation of DBPs exceed 0.1 μg/mg while hypochlorite dosed at >10 mg/m3. The total DBP concentrations in highly disinfected places (100-200 mg/m3 hypochlorite) were as high as 66.8 μg/m3, and the Hazard Index (HI) was up to 0.84, and both values were much higher than those in less disinfected places (<10 mg/m3 hypochlorite). Taking into account the HI, formation yields and the origin of the DBPs, we recommended 10 mg/m3 as the suggested hypochlorite dose to minimize DBPs generation during routine disinfection for controlling the coronavirus. DBPs in indoor air could be eliminated by ventilation, reducing the usage of personal care products, and wiping the solid surface with water before or after disinfection. These results highlighted the necessity to control air-borne DBPs and their associated health risks arising from intensified disinfection, and will guide the further development of evidence-based regulation on DBP exposure during disinfection and improve public health protection.

Real-time characterization of aerosol particle composition, sources and influences of increased ventilation and humidity in an office

Most of human exposure to atmospheric pollutants occurs indoors, and the components of outdoor aerosols may have been changed in the way before reaching indoor spaces. Here we conducted real-time online measurements of mass concentrations and chemical composition of black carbon and the non-refractory species in PM_2.5 in an occupied office for approximately one month. The open-close windows and controlled dampness experiments were also performed. Our results show that indoor aerosol species primarily originate from outdoors with indoor/outdoor ratio of these species typically less than unity except for certain organic aerosol (OA) factors. All aerosol species went through filtration upon transport indoors. Ammonium nitrate and fossil fuel OA underwent evaporation or particle-to-gas partitioning, while less oxidized secondary OA (SOA) underwent secondary formation and cooking OA might have indoor sources. With higher particulate matter (PM) mass concentration outdoors than in the office, elevated natural ventilation increased PM exposure indoors and this increased exposure was prolonged when outdoor PM was scavenged. We found that increasing humidity in the office led to higher indoor PM mass concentration particularly more oxidized SOA. Overall, our results highlight that indoor exposure of occupants is substantially different from outdoor in terms of mass concentrations and chemical species.

Quantification and source characterization of volatile organic compounds from exercising and application of chlorine-based cleaning products in a university athletic center

Humans spend approximately 90% of their time indoors, impacting their own air quality through occupancy and activities. Human VOC emissions indoors from exercise are still relatively uncertain, and questions remain about emissions from chlorine-based cleaners. To investigate these and other issues, the ATHLETic center study of Indoor Chemistry (ATHLETIC) campaign was conducted in the weight room of the Dal Ward Athletic Center at the University of Colorado Boulder. Using a Vocus Proton-Transfer-Reaction Time-of-Flight Mass Spectrometer (Vocus PTR-TOF), an Aerodyne Gas Chromatograph (GC), an Iodide-Chemical Ionization Time-of-Flight Mass Spectrometer (I-CIMS), and Picarro cavity ringdown spectrometers, we alternated measurements between the weight room and supply air, allowing for determination of VOC, NH₃ , H₂ O, and CO₂ emission rates per person (emission factors). Human-derived emission factors were higher than previous studies of measuring indoor air quality in rooms with individuals at rest and correlated with increased CO₂ emission factors. Emission factors from personal care products (PCPs) were consistent with previous studies and typically decreased throughout the day. In addition, N-chloraldimines were observed in the gas phase after the exercise equipment was cleaned with a dichlor solution. The chloraldimines likely originated from reactions of free amino acids with HOCl on gym surfaces.

Personal Passive Air Samplers for Chlorinated Gases Generated from the Use of Consumer Products

Various chlorine-based disinfectants are being used during the COVID-19 pandemic; however, only a few studies on exposure to harmful gases resulting from the use of these disinfectants exist. Previously, we developed a personal passive air sampler (PPAS) to estimate the exposure level to chlorine gas while using chlorinated disinfectants. Herein, we investigated the color development of the passive sampler corresponding to chlorine exposure concentration and time, which allows the general population to easily estimate their gas exposure levels. The uptake and reaction rate of PPAS are also explained, and the maximum capacity of the sampler was determined as 1.8 mol of chlorine per unit volume (m³) of the passive sampler. Additionally, the effects of disinfectant types on the gas exposure level were successfully assessed using passive samplers deployed in a closed chamber. It is noteworthy that the same level of chlorine gas is generated from liquid household bleach regardless of dilution ratios, and we confirmed that the chlorine gas can diffuse out from a gel-type disinfectant. Considering that this PPAS reflects reactive gas removal, individual working patterns, and environmental conditions, this sampler can be successfully used to estimate personal exposure levels of chlorinated gases generated from disinfectants.

Spatial and temporal scales of variability for indoor air constituents

Historically air constituents have been assumed to be well mixed in indoor environments, with single point measurements and box modeling representing a room or a house. Here we demonstrate that this fundamental assumption needs to be revisited through advanced model simulations and extensive measurements of bleach cleaning. We show that inorganic chlorinated products, such as hypochlorous acid and chloramines generated via multiphase reactions, exhibit spatial and vertical concentration gradients in a room, with short-lived ⋅OH radicals confined to sunlit zones, close to windows. Spatial and temporal scales of indoor constituents are modulated by rates of chemical reactions, surface interactions and building ventilation, providing critical insights for better assessments of human exposure to hazardous pollutants, as well as the transport of indoor chemicals outdoors.

Experimental evidence that halogen bonding catalyzes the heterogeneous chlorination of alkenes in submicron liquid droplets

A key challenge in predicting the multiphase chemistry of aerosols and droplets is connecting reaction probabilities, observed in an experiment, with the kinetics of individual elementary steps that control the chemistry that occurs across a gas/liquid interface. Here we report evidence that oxygenated molecules accelerate the heterogeneous reaction rate of chlorine gas with an alkene (squalene, Sqe) in submicron droplets. The effective reaction probability for Sqe is sensitive to both the aerosol composition and gas phase environment. In binary aerosol mixtures with 2-decyl-1-tetradecanol, linoleic acid and oleic acid, Sqe reacts 12-23× more rapidly than in a pure aerosol. In contrast, the reactivity of Sqe is diminished by 3× when mixed with an alkane. Additionally, small oxygenated molecules in the gas phase (water, ethanol, acetone, and acetic acid) accelerate (up to 10×) the heterogeneous chlorination rate of Sqe. The overall reaction mechanism is not altered by the presence of these aerosol and gas phase additives, suggesting instead that they act as catalysts. Since the largest rate acceleration occurs in the presence of oxygenated molecules, we conclude that halogen bonding enhances reactivity by slowing the desorption kinetics of Cl2 at the interface, in a way that is analogous to decreasing temperature. These results highlight the importance of relatively weak interactions in controlling the speed of multiphase reactions important for atmospheric and indoor environments.

Development of a personal passive air sampler for estimating exposure to effective chlorine while using chlorine-based disinfectants

With an increasing use of indoor disinfectants such as chlorine (Cl₂ ) and hypochlorous acid, a convenient sampler for estimating exposure to oxidants, such as effective chlorine, is necessary. Here, we developed a personal passive air sampler (PPAS) composed of a redox dye, o-dianisidine, in a polydimethylsiloxane (PDMS) sheet. o-Dianisidine readily reacts with gaseous oxidants generated by bleach usage, and its color changes as the reaction progresses; hence, personal exposure to effective chlorine could be easily detected by the naked eye, while cumulative exposure could be determined by measuring concentrations of o-dianisidine reacting with it. The PPAS was calibrated, and a sampling rate of 0.00253 m³ /h was obtained using a small test chamber. The PPAS was tested with the help of ten volunteers whose personal exposure to Cl₂ -equivalent gas was estimated after bathrooms were cleaned using spray and liquid-type household disinfection products, and the accumulated exposure-gas concentrations ranged from 69 to 408 ppbv and 148 to 435 ppbv, respectively. These PPAS-derived exposure concentrations were approximately two orders lower than those estimated using ConsExpo, suggesting a significant overestimation by prevailing screening models, possibly due to the ignorance of transformation reactions.