Recent Advances in Indoor Chemistry

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.

Superflux of an organic adlayer towards its local reactive immobilization

On-surface mass transport is the key process determining the kinetics and dynamics of on-surface reactions, including the formation of nanostructures, catalysis, or surface cleaning. Volatile organic compounds (VOC) localized on a majority of surfaces dramatically change their properties and act as reactants in many surface reactions. However, the fundamental question "How far and how fast can the molecules travel on the surface to react?" remains open. Here we show that isoprene, the natural VOC, can travel ~1 μm s-1, i.e., centimeters per day, quickly filling low-concentration areas if they become locally depleted. We show that VOC have high surface adhesion on ceramic surfaces and simultaneously high mobility providing a steady flow of resource material for focused electron beam synthesis, which is applicable also on rough or porous surfaces. Our work established the mass transport of reactants on solid surfaces and explored a route for nanofabrication using the natural VOC layer.

Reactions and Products of Squalene and Ozone: A Review

This critical review describes the squalene-ozone (SqOz) reaction, or squalene ozonolysis. Ambient ozone penetrates indoors and drives indoor air chemistry. Squalene, a component of human skin oil, contains six carbon-carbon double bonds and is very reactive with ozone. Bioeffluents from people contribute to indoor air chemistry and affect the indoor air quality, resulting in exposures because people spend the majority of their time indoors. The SqOz reaction proceeds through various formation pathways and produces compounds that include aldehydes, ketones, carboxylic acids, and dicarbonyl species, which have a range of volatilities. In this critical review of SqOz chemistry, information on the mechanism of reaction, reaction probability, rate constants, and reaction kinetics are compiled. Characterizations of SqOz reaction products have been done in laboratory experiments and real-world settings. The effect of multiple environmental parameters (ozone concentration, air exchange rate (AER), temperature, and relative humidity (RH)) in indoor settings are summarized. This critical review concludes by identifying the paucity of available exposure, health, and toxicological data for known reaction products. Key knowledge gaps about SqOz reactions leading to indoor exposures and adverse health outcomes are provided as well as an outlook on where the field is headed.

Indoor heterogeneous photochemistry of molds and their contribution to HONO formation

To better understand the impact of molds on indoor air quality, we studied the photochemistry of microbial films made by Aspergillus niger species, a common indoor mold. Specifically, we investigated their implication in the conversion of adsorbed nitrate anions into gaseous nitrous acid (HONO) and nitrogen oxides (NO_x ), as well as the related VOC emissions under different indoor conditions, using a high-resolution proton transfer reaction-time of flight-mass spectrometer (PTR-TOF-MS) and a long path absorption photometer (LOPAP). The different mold preparations were characterized by the means of direct injection into an Orbitrap high-resolution mass spectrometer with a heated electrospray ionization (ESI-Orbitrap-MS). The formation of a wide range of VOCs, having emission profiles sensitive to the types of films (either doped by potassium nitrate or not), cultivation time, UV-light irradiation, potassium nitrate concentration and relative humidity was observed. The formation of nitrous acid from these films was also determined and found to be dependent on light and relative humidity. Finally, the reaction paths for the NO_x and HONO production are proposed. This work helps to better understand the implication of microbial surfaces as a new indoor source for HONO emission.

Indoor heterogeneous photochemistry of furfural drives emissions of nitrous acid

People spend approximately 80% of their time indoor, making the understanding of the indoor chemistry an important task for safety. The high surface-area-to-volume ratio characteristic of indoor environments leads the semi-volatile organic compounds (sVOCs) to deposit on the surfaces. Using a long path absorption photometer (LOPAP), this work investigates the formation of nitrous acid (HONO) through the photochemistry of adsorbed nitrate anions and its enhancement by the presence of furfural. Using a high-resolution proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS), this work also investigates the surface emissions of VOCs from irradiated films of furfural and a mix of furfural and nitrate anions. Among the emitted VOCs, 2(5H)-furanone/2-Butenedial was observed at high concentrations, leading to maleic anhydride formation after UV irradiation. Moreover, the addition of potassium nitrate to the film formed NO_x and HONO concentrations up to 10 ppb, which scales to ca. 4 ppb for realistic indoor conditions. This work helps to understand the high levels of HONO and NO_x measured indoors.

Emerging investigator series: chemical and physical properties of organic mixtures on indoor surfaces during HOMEChem

Organic films on indoor surfaces serve as a medium for reactions and for partitioning of semi-volatile organic compounds and thus play an important role in indoor chemistry. However, the chemical and physical properties of these films are poorly characterized. Here, we investigate the chemical composition of an organic film collected during the HOMEChem campaign, over three cumulative weeks in the kitchen, using both Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) and offline Aerosol Mass Spectrometry (AMS). We also characterize the viscosity of this film using a model based on molecular formulas as well as poke-flow measurements. We find that the film contains organic material similar to cooking organic aerosol (COA) measured during the campaign using on-line AMS. However, the average molecular formula observed using FT-ICR MS is ∼C50H90O11, which is larger and more oxidized than fresh COA. Solvent extracted film material is a low viscous semisolid, with a measured viscosity <104 Pa s. This is much lower than the viscosity model predicts, which is parametrized with atmospherically relevant organic molecules, but sensitivity tests demonstrate that including unsaturation can explain the differences. The presence of unsaturation is supported by reactions of film material with ozone. In contrast to the solvent extract, manually removed material appears to be highly viscous, highlighting the need for continued work understanding both viscosity measurements as well as parameterizations for modeled viscosity of indoor organic films.

Analysis of indoor particles and gases and their evolution with natural ventilation

The air composition and reactivity from outdoor and indoor mixing field campaign was conducted to investigate the impacts of natural ventilation (ie, window opening and closing) on indoor air quality. In this study, a thermal desorption aerosol gas chromatograph (TAG) obtained measurements of indoor particle- and gas-phase semi- and intermediately volatile organic compounds both inside and outside a single-family test home. Together with measurements from a suite of instruments, we use TAG data to evaluate changes in indoor particles and gases at three natural ventilation periods. Positive matrix factorization was performed on TAG and adsorbent tube data to explore five distinct chemical and physical processes occurring in the indoor environment. Outdoor-to-indoor transport is observed for sulfate, isoprene epoxydiols, polycyclic aromatic hydrocarbons, and heavy alkanes. Dilution of indoor species is observed for volatile, non-reactive species including methylcyclohexane and decamethylcyclopentasiloxane. Window opening drives enhanced emissions of semi- and intermediately volatile species including TXIB, DEET, diethyl phthalate, and carvone from indoor surfaces. Formation via enhanced oxidation was observed for nonanal and 2-decanone when outdoor oxidants entered the home. Finally, oxidative depletion of gas-phase terpenes (eg, limonene and α-pinene) was anticipated but not observed due to limited measurement resolution and dynamically changing conditions.

Emerging investigator series: primary emissions, ozone reactivity, and byproduct emissions from building insulation materials

Building insulation materials can affect indoor air by (i) releasing primary volatile organic compounds (VOCs) from building enclosure cavities to the interior space, (ii) mitigating exposure to outdoor pollutants through reactive deposition (of oxidants, e.g., ozone) or filtration (of particles) in infiltration air, and (iii) generating secondary VOCs and other gas-phase byproducts resulting from oxidant reactions. This study reports primary VOC emission fluxes, ozone (O3) reaction probabilities (γ), and O3 reaction byproduct yields for eight common, commercially available insulation materials. Fluxes of primary VOCs from the materials, measured in a continuous flow reactor using proton transfer reaction-time of flight-mass spectrometry, ranged from 3 (polystyrene with thermal backing) to 61 (cellulose) μmol m-2 h-1 (with total VOC mass emission rates estimated to be between ∼0.3 and ∼3.3 mg m-2 h-1). Major primary VOC fluxes from cellulose were tentatively identified as compounds likely associated with cellulose chemical and thermal decomposition products. Ozone-material γ ranged from ∼1 × 10-6 to ∼30 × 10-6. Polystyrene with thermal backing and polyisocyanurate had the lowest γ, while cellulose and fiberglass had the highest. In the presence of O3, total observed volatile byproduct yields ranged from 0.25 (polystyrene) to 0.85 (recycled denim) moles of VOCs produced per mole of O3 consumed, or equivalent to secondary fluxes that range from 0.71 (polystyrene) to 10 (recycled denim) μmol m-2 h-1. Major emitted products in the presence of O3 were generally different from primary emissions and were characterized by yields of aldehydes and acetone. This work provides new data that can be used to evaluate and eventually model the impact of "hidden" materials (i.e., those present inside wall cavities) on indoor air quality. The data may also guide building enclosure material selection, especially for buildings in areas of high outdoor O3.

Modelling consortium for chemistry of indoor environments (MOCCIE): integrating chemical processes from molecular to room scales

We report on the development of a modelling consortium for chemistry in indoor environments that connects models over a range of spatial and temporal scales, from molecular to room scales and from sub-nanosecond to days, respectively. Our modeling approaches include molecular dynamics (MD) simulations, kinetic process modeling, gas-phase chemistry modeling, organic aerosol modeling, and computational fluid dynamics (CFD) simulations. These models are applied to investigate ozone reactions with skin and clothing, oxidation of volatile organic compounds and formation of secondary organic aerosols, and mass transport and partitioning of indoor species to surfaces. MD simulations provide molecular pictures of limonene adsorption on SiO2 and ozone interactions with the skin lipid squalene, providing kinetic parameters such as surface accommodation coefficient, desorption lifetime, and bulk diffusivity. These parameters then constrain kinetic process models, which resolve mass transport and chemical reactions in gas and condensed phases for analysis of experimental data. A detailed indoor chemical box model is applied to simulate α-pinene ozonolysis with improved representation of gas-particle partitioning. Application of 2D-volatility basis set reveals that OH-induced aging sometimes drives increases in indoor organic aerosol concentrations, due to organic mass functionalization and enhanced partitioning. CFD simulations show that concentrations of ozone and primary product change near the human surface rapidly, indicating non-uniform spatial distributions from the occupant surface to ambient air, while secondary ozone product is relatively well-mixed throughout the room. This development establishes a framework to integrate different modeling tools and experimental measurements, opening up an avenue for development of comprehensive and integrated models with representations of various chemistry in indoor environments.

Exploring the science, safety, and benefits of air care products: perspectives from the inaugural air care summit

Seventy-one percent of US households purchase air care products. Air care products span a diverse range of forms, including scented aerosol sprays, pump sprays, diffusers, gels, candles, and plug-ins. These products are used to eliminate indoor malodors and to provide pleasant scent experiences. The use of air care products can lead to significant benefits as studies have shown that indoor malodor can cause adverse effects, negatively impacting quality of life, hygiene, and the monetary value of homes and cars, while disproportionately affecting lower income populations. Additionally, studies have also shown that scent can have positive benefits related to mood, stress reduction, and memory enhancement among others. Despite the positive benefits associated with air care products, negative consumer perceptions regarding the safety of air care products can be a barrier to their use. During the inaugural Air Care Summit, held on 18 May 2018 in the Washington, DC, metropolitan area, multidisciplinary experts including industry stakeholders, academics, and scientific and medical experts were invited to share and assess the existing data related to air care products, focusing on ingredient and product safety and the benefits of malodor removal and scent. At the Summit's completion, a panel of independent experts representing the fields of pulmonary medicine, medical and clinical toxicology, pediatric toxicology, basic science toxicology, occupational dermatology and experimental psychology convened to review the data presented, identify potential knowledge gaps, and suggest future research directions to further assess the safety and benefits of air care products.

Exposure Assessment For Air-To-Skin Uptake of Semivolatile Organic Compounds (SVOCs) Indoors

Semivolatile organic compounds (SVOCs) are ubiquitous in the indoor environment and a priority for exposure assessment because of the environmental health concerns that they pose. Direct air-to-skin dermal uptake has been shown to be comparable to the inhalation intake for compounds with certain chemical properties. In this study, we aim to further understand the transport of these types of chemicals through the skin, specifically through the stratum corneum (SC). Our assessment is based on collecting three sequential forehead skin wipes, each hypothesized to remove pollutants from successively deeper skin layers, and using these wipe analyses to determine the skin concentration profiles. The removal of SVOCs with repeated wipes reveals the concentration profiles with depth and provides a way to characterize penetration efficiency and potential transfer to blood circulation. We used a diffusion model applied to surface skin to simulate concentration profiles of SVOCs and compared them with the measured values. We found that two phthalates, dimethyl and diethyl phthalates, penetrate deeper into skin with similar exposure compared to other phthalates and targeted SVOCs, an observation supported by the model results as well. We also report the presence of statistically significant declining patterns with skin depth for most SVOCs, indicating that their diffusion through the SC is relevant and eventually can reach the blood vessels in the vascularized dermis. Finally, using a nontarget approach, we identified skin oxidation products, linked to respiratory irritation symptoms, formed from the reaction between ozone and squalene.

Indoor Chemistry

This review aims to encapsulate the importance, ubiquity, and complexity of indoor chemistry. We discuss the many sources of indoor air pollutants and summarize their chemical reactions in the air and on surfaces. We also summarize some of the known impacts of human occupants, who act as sources and sinks of indoor chemicals, and whose activities (e.g., cooking, cleaning, smoking) can lead to extremely high pollutant concentrations. As we begin to use increasingly sensitive and selective instrumentation indoors, we are learning more about chemistry in this relatively understudied environment.

Connecting the Elementary Reaction Pathways of Criegee Intermediates to the Chemical Erosion of Squalene Interfaces during Ozonolysis

Criegee intermediates (CI), formed in alkene ozonolysis, are central for controlling the multiphase chemistry of organic molecules in both indoor and outdoor environments. Here, we examine the heterogeneous ozonolysis of squalene, a key species in indoor air chemistry. Aerosol mass spectrometry is used to investigate how the ozone (O₃) concentration, relative humidity (RH), and particle size control reaction rates and mechanisms. Although the reaction rate is found to be independent of RH, the reaction products and particle size depend upon H₂O. Under dry conditions (RH = 3%) the reaction produces high-molecular-weight secondary ozonides (SOZ), which are known skin irritants, and a modest change in particle size. Increasing the RH reduces the aerosol size by 30%, while producing mainly volatile aldehyde products, increases potential respiratory exposure. Chemical kinetics simulations link the elementary reactions steps of CI to the observed kinetics, product distributions, and changes in particle size. The simulations reveal that ozonolysis occurs near the surface and is O₃-transport limited. The observed secondary ozonides are consistent with the formation of mainly secondary CI, in contrast to gas-phase ozonolysis mechanisms.