Fine and ultrafine particle removal efficiency of new residential HVAC filters

Impact of Wildfire Smoke PM2.5 on Indoor Air Quality of Public Buildings on a University Campus

With increasing wildfire events impacting many regions worldwide, understanding and mitigating the effects of wildfire smoke on indoor air quality (IAQ) in public buildings are essential for protecting occupant health. This study investigated the impact of wildfire smoke on the IAQ across 24 campus buildings in Alberta, Canada, representing public spaces with varied ventilation systems. Using a network of low-cost sensors to monitor indoor PM2.5, the study identified significant spikes during wildfire smoke events, with 71% of buildings exceeding the Canadian Ambient Air Quality Standards daily limit of 27 μg/m3. The buildings had mechanical ventilation systems with filters with different Minimum Efficiency Reporting Value (MERV) ratings. MERV13 filters were found to be more efficient at capturing PM2.5 particles, resulting in lower indoor/outdoor PM2.5 ratios (0.12 ± 0.07) compared to MERV8 filters (0.28 ± 0.14). Buildings with air change rates (ACH) ranging from 5 to 15 per hour exhibited different infiltration patterns, with higher ACH generally leading to elevated indoor PM2.5 concentrations during wildfire events. This highlights the need to balance ventilation and pollutant infiltration by optimizing ACH rates and filtration efficiency to reduce indoor PM2.5. The trajectory-fire interception method, combined with satellite data, enhanced the identification of wildfire-influenced periods, contributing to a better understanding of smoke infiltration dynamics. These findings underscore that even advanced filtration and ventilation systems alone may not ensure a healthy IAQ during extreme pollution. Real-time pollutant measurements are crucial for effective IAQ management. The findings offer valuable insights for building administrators and policymakers, helping them develop strategies to mitigate the effects of wildfire smoke and to support healthier indoor environments during wildfire seasons.

Practical considerations for using low-cost sensors to assess wildfire smoke exposure in school and childcare settings

BACKGROUND

More frequent and intense wildfires will increase concentrations of smoke in schools and childcare settings. Low-cost sensors can assess fine particulate matter (PM2.5) concentrations with high spatial and temporal resolution.

OBJECTIVE

We sought to optimize the use of sensors for decision-making in schools and childcare settings during wildfire smoke to reduce children's exposure to PM2.5.

METHODS

We measured PM2.5 concentrations indoors and outdoors at four schools in Washington State during wildfire smoke in 2020-2021 using low-cost sensors and gravimetric samplers. We randomly sampled 5-min segments of low-cost sensor data to create simulations of brief portable handheld measurements.

RESULTS

During wildfire smoke episodes (lasting 4-19 days), median hourly PM2.5 concentrations at different locations inside a single facility varied by up to 49.6 µg/m3 (maximum difference) during school hours. Median hourly indoor/outdoor ratios across schools ranged from 0.22 to 0.91. Within-school differences in concentrations indicated that it is important to collect measurements throughout a facility. Simulation results suggested that making handheld measurements more often and over multiple days better approximates indoor/outdoor ratios for wildfire smoke. During a period of unstable air quality, PM2.5 over the next hour indoors was more highly correlated with the last 10-min of data (mean R2 = 0.94) compared with the last 3-h (mean R2 = 0.60), indicating that higher temporal resolution data is most informative for decisions about near-term activities indoors.

IMPACT STATEMENT

As wildfires continue to increase in frequency and severity, staff at schools and childcare facilities are increasingly faced with decisions around youth activities, building use, and air filtration needs during wildfire smoke episodes. Staff are increasingly using low-cost sensors for localized outdoor and indoor PM2.5 measurements, but guidance in using and interpreting low-cost sensor data is lacking. This paper provides relevant information applicable for guidance in using low-cost sensors for wildfire smoke response.

Spatiotemporal Mapping of Ultrafine Particle Fluxes in an Office HVAC System with a Diffusion Charger Sensor Array

Commercial HVAC systems intended to mitigate indoor air pollution are operated based on standards that exclude aerosols with smaller diameters, such as ultrafine particles (UFPs, Dp ≤ 100 nm), which dominate a large proportion of indoor and outdoor number-based particle size distributions. UFPs generated from occupant activities or infiltrating from the outdoors can be recirculated and accumulate indoors when they are not successfully filtered by an air handling unit. Monitoring UFPs in real occupied environments is vital to understanding these source and mitigation dynamics, but capturing their rapid transience across multiple locations can be challenging due to high-cost instrumentation. This 9-month field measurement campaign pairs four medium-cost diffusion charger sensors with volumetric airflow rates modulated and monitored in a cloud-based building automation system of an open-plan living laboratory office and dedicated air handling unit to evaluate spatiotemporal particle number and surface area concentrations and migration trends. Particle number flux rates reveal that an estimated daily median of 8 × 1013 UFPs enter the air handling unit from the outdoors. Switching from a MERV14 to a HEPA filter reduces the number of UFPs supplied to the room by tens of trillions of UFPs daily, increasing the median filtration efficiency from 40% to 96%. These results demonstrate the efficacy of an optimal air handling unit's performance to improve indoor air quality, while highlighting UFP dynamics that are not accounted for in current filtration standards nor in occupant-centered HVAC control. Scalable sensor development can popularize UFP monitoring and allow for future UFP integration within building control and automation platforms. The framework established for this campaign can be used to evaluate particle fluxes considering different analytes.

Phenomenology of ultrafine particle concentrations and size distribution across urban Europe

The 2017-2019 hourly particle number size distributions (PNSD) from 26 sites in Europe and 1 in the US were evaluated focusing on 16 urban background (UB) and 6 traffic (TR) sites in the framework of Research Infrastructures services reinforcing air quality monitoring capacities in European URBAN & industrial areaS (RI-URBANS) project. The main objective was to describe the phenomenology of urban ultrafine particles (UFP) in Europe with a significant air quality focus. The varying lower size detection limits made it difficult to compare PN concentrations (PNC), particularly PN_10-25, from different cities. PNCs follow a TR > UB > Suburban (SUB) order. PNC and Black Carbon (BC) progressively increase from Northern Europe to Southern Europe and from Western to Eastern Europe. At the UB sites, typical traffic rush hour PNC peaks are evident, many also showing midday-morning PNC peaks anti-correlated with BC. These peaks result from increased PN_10-25, suggesting significant PNC contributions from nucleation, fumigation and shipping. Site types to be identified by daily and seasonal PNC and BC patterns are: (i) PNC mainly driven by traffic emissions, with marked correlations with BC on different time scales; (ii) marked midday/morning PNC peaks and a seasonal anti-correlation with PNC/BC; (iii) both traffic peaks and midday peaks without marked seasonal patterns. Groups (ii) and (iii) included cities with high insolation. PNC, especially PN_25-800, was positively correlated with BC, NO₂, CO and PM for several sites. The variable correlation of PNSD with different urban pollutants demonstrates that these do not reflect the variability of UFP in urban environments. Specific monitoring of PNSD is needed if nanoparticles and their associated health impacts are to be assessed. Implementation of the CEN-ACTRIS recommendations for PNSD measurements would provide comparable measurements, and measurements of <10 nm PNC are needed for full evaluation of the health effects of this size fraction.

Filter evaluation and selection for heating, ventilation, and air conditioning systems during and beyond the COVID-19 pandemic

Particle size removal efficiencies for 0.1-1.0 μm ( PSE 0.1 - 1.0 $$ {PSE}_{0.1-1.0} $$ ) and 0.3-1.0 μm ( PSE 0.3 - 1.0 $$ {PSE}_{0.3-1.0} $$ ) diameter of Minimum Efficiency Reporting Value (MERV) filters, an electrostatic enhanced air filter (EEAF), and their two-stage filtration systems were evaluated. Considering the most penetrating particle size was 0.1-0.4 μm particulate matter (PM), the PSE 0.1 - 1.0 $$ {PSE}_{0.1-1.0} $$ as an evaluation parameter deserves more attention during the COVID-19 pandemic, compared to the PSE 0.3 - 1.0 $$ {PSE}_{0.3-1.0} $$ . The MERV 13 filters were recommended for a single-stage filtration system because of their superior quality factor (QF) compared to MERV 6, MERV 8, MERV 11 filters, and the EEAF. Combined MERV 8 + MERV 11 filters have the highest QF compared to MERV 6 + MERV 11 filters and EEAF + MERV 11 filters; regarding 50% of PSE 0.1 - 1.0 $$ {PSE}_{0.1-1.0} $$ as the filtration requirements of two-stage filtration systems, the MERV 8 + MERV 11 filtration system can achieve this value at 1.0 m/s air velocity, while PSE 0.1 - 1.0 $$ {PSE}_{0.1-1.0} $$ values were lower than 50% at 1.5 m/s and 2.0 m/s. EEAF obtained a better PSE 0.3 - 1.0 $$ {PSE}_{0.3-1.0} $$ in the full-recirculated test rig than in the single-pass mode owing to active ionization effects when EEAF was charged by alternating current.

Investigating the Influence of Metal-Organic Framework Loading on the Filtration Performance of Electrospun Nanofiber Air Filters

Integrating metal-organic frameworks (MOFs) into electrospun nanofiber filters has become an effective method for improving particle filtration efficiency. This study hypothesized that there is an optimal amount of MOFs that can be integrated into electrospun nanofiber filters to achieve the maximum particle removal efficiency while minimizing the corresponding MOF synthesis time. To test the hypothesis, this study systematically explored the influence of the time-dependent in situ growing process of zeolitic imidazolate framework-67 (ZIF-67), a typical type of MOFs, on the filtration performance of polyacrylonitrile (PAN) electrospun nanofibers. The results show that the surface morphology and chemical composition of the PAN/ZIF-67 hybrid nanofiber filters gradually changed with the reaction time. For PAN/ZIF-67 hybrid nanofiber filters with relatively low initial PM0.3-0.4 filtration efficiency, a reaction time of only 5 min was sufficient for the synthesis of the amount of ZIF-67 that maximized the PM0.3-0.4 filtration efficiency. However, for thick filters with high original PM0.3-0.4 filtration efficiency (>90%), the integration of ZIF-67 was not necessary, because the efficiency enhancement would not be significant. In addition, the enhancement of filtration efficiency for ultrafine particles was positively correlated with the amount of incorporated ZIF-67. In summary, this study shortened the synthesis time of the in situ incorporation of MOFs into electrospun nanofiber filters from more than 10 h (reported in the literature) to only 5 min.

Wind tunnel-based testing of a photoelectrochemical oxidative filter-based air purification unit in coronavirus and influenza aerosol removal and inactivation

Recirculating air purification technologies are employed as potential means of reducing exposure to aerosol particles and airborne viruses. Toward improved testing of recirculating air purification units, we developed and applied a medium-scale single-pass wind tunnel test to examine the size-dependent collection of particles and the collection and inactivation of viable bovine coronavirus (BCoV, a betacoronavirus), porcine respiratory coronavirus (PRCV, an alphacoronavirus), and influenza A virus (IAV), by a commercial air purification unit. The tested unit, the Molekule Air Mini, incorporates a MERV 16 filter as well as a photoelectrochemical oxidating layer. It was found to have a collection efficiency above 95.8% for all tested particle diameters and flow rates, with collection efficiencies above 99% for supermicrometer particles with the minimum collection efficiency for particles smaller than 100 nm. For all three tested viruses, the physical tracer-based log reduction was near 2.0 (99% removal). Conversely, the viable virus log reductions were found to be near 4.0 for IAV, 3.0 for BCoV, and 2.5 for PRCV, suggesting additional inactivation in a virus family- and genus-specific manner. In total, this work describes a suite of test methods which can be used to rigorously evaluate the efficacy of recirculating air purification technologies.

Ventilation Systems and Their Impact on Nanoparticle Concentrations in Office Buildings

Nanoparticles (NPs) can infiltrate indoor environments and have a large impact on human health when inhaled. Thus, indoor air quality is highly dependent on the outdoor air and on the filters used in the ventilation systems. In the NanoOffice study, the concentrations and the size distribution of NPs were measured with a five-minute time resolution in twelve office buildings in Umeå. Measurements were taken with an SMPS 3938 during a one-week period in the heating and nonheating seasons. Large differences in ventilation between buildings appeared, despite the fact that similar MVHR ventilation systems were used, and most of them were equipped with F7 filters. The NP concentrations and the simultaneous ventilation flows were measured in buildings with a variable and a more constant ventilation flow. In some cases, an increase in NP concentration could be seen after ventilation turn-on or after an increase in the ventilation flow. There was also one case where the NP concentrations increased in connection with the ventilation being switched off or reducing its flow. However, variable NP concentrations were also shown in buildings with a fairly constant ventilation flow, which was prominent for the two buildings located closest to busy streets. The correlation coefficients between the ventilation flow and particles in different size classes were in general smallest for particles in the smallest size classes, indicating higher filtration efficiency.

Transmission of SARS-CoV-2 by inhalation of respiratory aerosol in the Skagit Valley Chorale superspreading event

During the 2020 COVID-19 pandemic, an outbreak occurred following attendance of a symptomatic index case at a weekly rehearsal on 10 March of the Skagit Valley Chorale (SVC). After that rehearsal, 53 members of the SVC among 61 in attendance were confirmed or strongly suspected to have contracted COVID-19 and two died. Transmission by the aerosol route is likely; it appears unlikely that either fomite or ballistic droplet transmission could explain a substantial fraction of the cases. It is vital to identify features of cases such as this to better understand the factors that promote superspreading events. Based on a conditional assumption that transmission during this outbreak was dominated by inhalation of respiratory aerosol generated by one index case, we use the available evidence to infer the emission rate of aerosol infectious quanta. We explore how the risk of infection would vary with several influential factors: ventilation rate, duration of event, and deposition onto surfaces. The results indicate a best-estimate emission rate of 970 ± 390 quanta/h. Infection risk would be reduced by a factor of two by increasing the aerosol loss rate to 5 h^-1 and shortening the event duration from 2.5 to 1 h.

Assessment and mitigation of aerosol airborne SARS-CoV-2 transmission in laboratory and office environments

Bioaerosols are known to be an important transmission pathway for SARS-CoV-2. We report a framework for estimating the risk of transmitting SARS-CoV-2 via aerosols in laboratory and office settings, based on an exponential dose-response model and analysis of air flow and purification in typical heating, ventilation, and air conditioning (HVAC) systems. High-circulation HVAC systems with high-efficiency particulate air (HEPA) filtration dramatically reduce exposure to the virus in indoor settings, and surgical masks or N95 respirators further reduce exposure. As an example of our risk assessment model, we consider the precautions needed for a typical experimental physical science group to maintain a low risk of transmission over six months of operation. We recommend that, for environments where fewer than five individuals significantly overlap, work spaces should remain vacant for between one (high-circulation HVAC with HEPA filtration) to six (low-circulation HVAC with no filtration) air exchange times before a new worker enters in order to maintain no more than 1% chance of infection over six months of operation in the workplace. Our model is readily applied to similar settings that are not explicitly given here. We also provide a framework for evaluating infection mitigation through ventilation in multiple occupancy spaces.

Filtration performances of non-medical materials as candidates for manufacturing facemasks and respirators

The recent outbreak of the coronavirus disease (COVID-19) is causing a shortage of personal protective equipment (PPE) in different countries around the world. Because the coronavirus can transmit through droplets and aerosols, facemasks and N95 respirators that require complex certification, are urgently needed. Given the situation, the U.S. Centers for Disease Control and Prevention (CDC) recommends that “in settings where facemasks are not available, healthcare personnel might use homemade masks (e.g., bandana, scarf) for the care of patients with COVID-19 as a last resort.” Although aerosols and droplets can be removed through the fibers of fabrics through a series of filtration mechanisms, their filtration performances have not been evaluated in detail. Moreover, there are a series of non-medical materials available on the market, such as household air filters, coffee filters, and different types of fabrics, which may be useful when facemasks and respirators are not available. In this study, we comprehensively evaluated the overall and size-dependent filtration performances of non-medical materials. The experiments were conducted under different face velocities to study its influence on size-dependent filtration performances. The flow resistance across these filter materials is measured as an indicator of the breathability of the materials. The results illustrate that multiple layers of household air filters are able to achieve similar filtration efficiencies compared to the N95 material without causing a significant increase in flow resistance. Considering that these air filters may shed micrometer fibers during the cutting and folding processes, it is recommended that these filters should be inserted in multiple layers of fabrics when manufacturing facemasks or respirators.

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Agencies suggest the manufacturing of homemade face masks during COVID-19.

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This work examined a wide range of non-medical materials for their filtration performance.

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We studied the influences of face velocity, number of filter material layers, and the size-dependent filtration efficiency.

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Several layers of household air filters can achieve similar filtration performance compared to N95 materials.

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The information will be crucial for healthcare personnel and the general public in manufacturing homemade face masks.

In situ efficiency of filters in residential central HVAC systems

High-efficiency filtration in residential forced-air heating, ventilation, and air conditioning (HVAC) systems protects equipment and can reduce exposure to particulate matter. Laboratory tests provide a measure of the nominal efficiency, but they may not accurately reflect the in situ efficiency of the filters because of variations in system conditions and changes in filter performance over time. The primary focus of this paper is to evaluate the effective filtration efficiency, which is inclusive of any loading and system impacts, in 21 occupied residential homes through in-duct concentration measurements. We considered the role of filter media by testing both electret and non-electret media, as well as the role of loading by considering new and used filters. The results show that filters with higher nominal efficiency generally had higher effective filtration efficiency in the same home. In terms of performance change, there is no significant difference in efficiency between initial and 3-month non-electret filters, but the efficiency of electret filters generally decreased over time. However, both nominal efficiency and performance change were vastly overshadowed by the wide variety in loading and system conditions across homes, making it hard to predict filter efficiency in a given home without in situ measurements.