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BackgroundSeveral studies suggest that far-field transmission (>6 ft) explains a significant number of COVID-19 superspreading outbreaks.ObjectiveTherefore, quantifying the ratio of near- and far-field exposure to emissions from a source is key to better understanding human-to-human airborne infectious disease transmission and associated risks.MethodsIn this study, we used an environmentally-controlled chamber to measure volatile organic compounds (VOCs) released from a healthy participant who consumed breath mints, which contained unique tracer compounds. Tracer measurements were made at 0.76 m (2.5 ft), 1.52 m (5 ft), 2.28 m (7.5 ft) from the participant, as well as in the exhaust plenum of the chamber.ResultsWe observed that 0.76 m (2.5 ft) trials had ~36-44% higher concentrations than other distances during the first 20 minutes of experiments, highlighting the importance of the near-field exposure relative to the far-field before virus-laden respiratory aerosol plumes are continuously mixed into the far-field. However, for the conditions studied, the concentrations of human-sourced tracers after 20 minutes and approaching the end of the 60-minute trials at 0.76 m, 1.52 m, and 2.28 m were only ~18%, ~11%, and ~7.5% higher than volume-averaged concentrations, respectively.SignificanceThis study suggests that for rooms with similar airflow parameters disease transmission risk is dominated by near-field exposures for shorter event durations (e.g., initial 20-25-minutes of event) whereas far-field exposures are critical throughout the entire event and are increasingly more important for longer event durations.Impact statementWe offer a novel methodology for studying the fate and transport of airborne bioaerosols in indoor spaces using VOCs as unique proxies for bioaerosols. We provide evidence that real-time measurement of VOCs can be applied in settings with human subjects to estimate the concentration of bioaerosol at different distances from the emitter. We also improve upon the conventional assumption that a well-mixed room exhibits instantaneous and perfect mixing by addressing spatial distances and mixing over time. We quantitatively assessed the exposure levels to breath tracers at alternate distances and provided more insights into the changes on "near-field to far-field" ratios over time. This method can be used in future to estimate the benefits of alternate environmental conditions and occupant behaviors.

The human body emits a wide range of chemicals, including CO2 and isoprene. To examine the impact of cognitive tasks on human emission rates of CO2 and isoprene, we conducted an across-subject, counterbalanced study in a controlled chamber involving 16 adults. The chamber replicated an office environment. In groups of four, participants engaged in 30 min each of cognitive tasks (stressed activity) and watching nature documentaries (relaxed activity). Measured biomarkers indicated higher stress levels were achieved during the stressed activity. Per-person CO2 emission rates were greater for stressed than relaxed activity (30.3 ± 2.1 vs 27.0 ± 1.7 g/h/p, p = 0.0044, mean ± standard deviation). Isoprene emission rates were also elevated under stressed versus relaxed activity (154 ± 25 μg/h/p vs 116 ± 20 μg/h/p, p = 0.041). The chamber temperature was held constant at 26.2 ± 0.49 °C; incidental variation in temperature did not explain the variance in emission rates. Isoprene emission rates increased linearly with salivary α-amylase levels (r2 = 0.6, p = 0.02). These results imply the possibility of considering cognitive tasks when determining building ventilation rates. They also present the possibility of monitoring indicators of cognitive tasks of occupants through measurement of air quality.

Air cleaning reduces indoor exposure to fine particulate matter (PM2.5) during wildfire smoke events. However, resource and cost constraints may limit access to air cleaning during such an event, as both commercial devices and the higher-rated MERV filters that do-it-yourself (DIY) assemblies typically rely upon tend to be expensive and in short supply. With these constraints in mind, we developed and evaluated several configurations of a novel, DIY air cleaner that uses common household fabrics as filtration media. Clean air delivery rates (CADRs) of the devices were experimentally evaluated in two ways: first, with independent measurements of flowrates and single pass removal efficiencies, and second, via pull-down testing in a large chamber. With two layers of cotton batting fabric and a flowrate-increasing cardboard shroud attached, the device achieved particulate matter CADRs of 162, 134, and 206 m3/h in 0.02–0.3, 0.3–1, and 1–2.5 µm particle diameter bins, respectively, during chamber testing. Results indicate that these simple, inexpensive, fabric configurations can meaningfully reduce PM2.5 levels in smaller zones of a home, and thus represent a viable option for improving indoor air quality during rapid-onset air pollution events, such as wildfires. [ABSTRACT FROM AUTHOR]

Thermal acceptability (TA) and perceived air quality acceptability (PAQA) are typically analysed in climate chambers or cross-sectional field studies. Individual factors, such as expectations and perceived environment history, may influence the acceptability response. Longitudinal studies with multi-day design are absent in the literature. Fifteen Singaporean subjects participated in a 7-day longitudinal experiment in which they carried a portable sensor that continuously recorded personal air temperature, relative humidity and carbon dioxide concentration at 1-minute intervals. Instantaneous TA and PAQA were regularly sampled by survey for each subject. High acceptability was found at home, restaurants and workplaces, whereas low acceptability was found for outdoor and transport environments. The participants from Singapore’s modern tropical environment spent an average of 96% of their time indoors. Weak associations were reported between acceptabilities and measured physical parameters taken independently. Clustering data by location, subject’s sleeping ventilation habit, air-conditioning operation status and the changes in physical parameters over a designated time period enhanced the understanding of the acceptability results. In general, acceptability was lower for those who slept in air-conditioned environments than for those who slept without air-conditioning. The carbon dioxide mixing ratio was critical for PAQA predictions but not for TA. The Gaussian process (GP) had a better predictive power than a multiple linear regression approach. Using GP, we found that a general predictive model had comparable simulation performance as for individual predictive models. The longitudinal experiment has demonstrated effectiveness for TA and PAQA analysis, which could be beneficial to future studies in personal comfort prediction.

Indoor carbon dioxide (CO2) levels serve as an indicator of ventilation sufficiency in relation to metabolic effluents. Recent evidence suggests that elevated CO2 exposure (with or without other bioeffluents) may cause adverse cognitive effects. In shelter-in-place (SIP) facilities, indoor CO2 levels may become particularly elevated. This study evaluates four low-cost alkaline earth metal oxides and hydroxides as CO2 sorbents for potential use in indoor air cleaning applications. Sorbents studied were MgO, Mg(OH)2, Ca(OH)2 and commercially available soda lime. Uncarbonated sorbents characterized with nitrogen adsorption porosimetry showed BET surface areas in the 5.6–27 m2/g range. Microstructural analyses, including X-ray diffraction, thermogravimetric analysis and scanning electron microscopy confirmed the carbonation mechanisms and extent of sorption under environmental conditions typical of indoor spaces. Ca-based sorbents demonstrated higher extent of carbonation than Mg-based sorbents. Laboratory parameterizations, including rate constants (k) and carbonation yields (y), were applied in material balance models to assess the CO2 removal potential of Ca-based sorbents in three types of indoor environments. Soda lime (k = [2.2–3.6] × 10−3 m3 mol CO2−1 h−1, y = 0.49–0.51) showed potential for effective use in SIP facilities. For example, CO2 exposure in a modeled SIP facility could be reduced by 80% for an 8-h sheltering interval and to levels below 5000 ppm for an 8-h period with a practically sized air cleaner. Predicted effectiveness was more modest for bedrooms and classrooms.

This study is concerned with the role of a fin-and-tube heat exchanger in modifying microbial indoor air quality. Specifically, depositional losses of ambient bioaerosols and particles onto dry (not cooled) and wet (cool) coil surfaces were measured for different airspeeds passing through the test coil. Total, bacterial and fungal DNA concentrations in condensate water produced by a wet coil were also quantified by means of fluorescent dsDNA-binding dye and qPCR assays. Results revealed that the deposition of bioaerosols and total particles is substantial on coil surfaces, especially when wet and cool. The average deposition fraction was 0.14 for total DNA, 0.18 for bacterial DNA and 0.22 for fungal DNA on the dry coil, increasing to 0.51 for total DNA, 0.50 for bacterial DNA and 0.68 for fungal DNA on the wet coil. Overall, as expected, deposition fractions increased with increasing particle size and increasing airspeed. Deposited DNA was removed from the cooling coil surfaces through the flow of condensing water at a rate comparable to the rate of direct deposition from air. A downward trend of bacterial and fungal DNA measured in condensate water over time provides suggestive evidence of biological growth on heat exchangers during nonoperational times of a ventilation system. This investigation provides new information about bioaerosol deposition onto a conventional fin-and-tube cooling coil, a potentially important factor influencing indoor exposure to microbial aerosols in air-conditioned buildings.

Elevated indoor CO2 levels are indicative of insufficient ventilation in occupied spaces and correlate with elevated concentrations of pollutants of indoor origin. Adverse health and well-being outcomes associated with elevated indoor CO2 levels are based on CO2 as a proxy, although some emerging evidence suggests CO2 itself may impact human cognition. Using portable monitors, we conducted an exposure study with 16 subjects in Singapore to understand the levels, dynamics and influencing factors of personal exposure to CO2. Participants carried a CO2 monitor continuously for 7-day periods recording their exposure levels at 1-min intervals. A recall diary was maintained of time-microenvironment-activity budget.  We found that the mode of bedroom ventilation was a major determinant of CO2 exposure. Approximately half of the participants slept in bedrooms employing ductless split air-conditioners (group “AC”); half slept in bedrooms naturally ventilated through operable windows (group “NV”).  Median CO2 exposure levels for AC vs. NV groups are significantly different (  = 650 ppm vs.  = 550 ppm, p < 0.001). Mean daily integrated exposures for group AC were statistically higher than for group NV: 22,800 ppm h/d vs. 16,000 ppm h/d (p < 0.005). Exposure events associated with potential adverse cognitive implications (duration > 2.5 h, average CO2 mixing ratio > 1000 ppm) occurred, on average, at frequencies of 0.5 d-1 across all participants, 0.6 d-1 for AC participants and 0.2 d-1 for NV participants. The majority of such events occurred in the home (86%), followed by work (9%) and transit (3%).

Intensive air quality measurements made from June 22-25, 2011 in the outflow of the Dallas-Fort Worth (DFW) metropolitan area are used to evaluate nitrous acid (HONO) sources and sinks. A two-layer box model was developed to assess the ability of established and recently identified HONO sources and sinks to reproduce observations of HONO mixing ratios. A baseline model scenario includes sources and sinks established in the literature and is compared to scenarios including three recently identified sources: volatile organic compound-mediated conversion of nitric acid to HONO (S1), biotic emission from the ground (S2), and re-emission from a surface nitrite reservoir (S3). For all mechanisms, ranges of parametric values span lower- and upper-limit values. Model outcomes for 'likely' estimates of sources and sinks generally show under-prediction of HONO observations, implying the need to evaluate additional sources and variability in estimates of parameterizations, particularly during daylight hours. Monte Carlo simulation is applied to model scenarios constructed with sources S1-S3 added independently and in combination, generally showing improved model outcomes. Adding sources S2 and S3 (scenario S2/S3) appears to best replicate observed HONO, as determined by the model coefficient of determination and residual sum of squared errors (r2 = 0.55 ± 0.03, SSE = 4.6 × 106 ± 7.6 × 105 ppt2). In scenario S2/S3, source S2 is shown to account for 25% and 6.7% of the nighttime and daytime budget, respectively, while source S3 accounts for 19% and 11% of the nighttime and daytime budget, respectively. However, despite improved model fit, there remains significant underestimation of daytime HONO; on average, a 0.15 ppt/s unknown daytime HONO source, or 67% of the total daytime source, is needed to bring scenario S2/S3 into agreement with observation. Estimates of 'best fit' parameterizations across lower to upper-limit values results in a moderate reduction of the unknown daytime source, from 0.15 to 0.10 ppt/s.

Elevated tropospheric ozone concentrations are associated with increased morbidity and mortality. Indoor ozone chemistry affects human exposure to ozone and reaction products that also may adversely affect health and comfort. Reactive uptake of ozone has been characterized for many building materials; however, scant information is available on how diurnal variation of ambient ozone influences ozone reaction with indoor surfaces. The primary objective of this study is to investigate ozone-surface reactions in response to a diurnally varying ozone exposure for three common building materials: ceiling tile, painted drywall, and carpet tile. A secondary objective is to examine the effects of air temperature and humidity. A third goal is to explore how conditioning of materials in an occupied office building might influence subsequent ozone-surface reactions. Experiments were performed at bench-scale with inlet ozone concentrations varied to simulate daytime (ozone elevated) and nighttime (ozone-free in these experiments) periods. To simulate office conditions, experiments were conducted at two temperatures (22 °C and 28 °C) and three relative humidity values (25%, 50%, 75%). Effects of indoor surface exposures were examined by placing material samples in an occupied office and repeating bench-scale characterization after exposure periods of 1 and 2 months. Deposition velocities were observed to be highest during the initial hour of ozone exposure with slow decrease in the subsequent hours of simulated daytime conditions. Daily-average ozone reaction probabilities for fresh materials are in the respective ranges of (1.7-2.7) × 10-5, (2.8-4.7) × 10-5, and (3.0-4.5) × 10-5 for ceiling tile, painted drywall, and carpet tile. The reaction probability decreases by 7%-47% across the three test materials after two 8-h periods of ozone exposure. Measurements with the samples from an occupied office reveal that deposition velocity can decrease or increase with time. Influence of temperature and humidity on ozone-surface reactivity was not strong.

Estimates of population exposure to ambient air pollution traditionally rely on concentrations measured at central-site monitors as a surrogate for concentrations to which people are exposed. In this study of Singapore, we estimate population-averaged exposure concentrations for PM2.5, PM10, and O3 by applying a model and data that account for age and gender demographics, intraurban regional variability, and microenvironmental effects with age- and gender-stratified time-activity budgets. The study addresses exposure only to air pollutants of outdoor origin. Spatially averaged midpoint estimates of lifetime ambient exposure concentrations are 59%, 52%, and 47% of outdoor concentrations for PM2.5, PM10, and O3, respectively. Utilizing ambient data for calendar year 2007, we estimate that intraurban variability in ambient concentration results in lifetime-integrated exposure concentrations in the respective ranges of 10e14 mg m_3 for PM2.5, 14e18 mg m_3 for PM10, and 7.5e15 mg m_3 for O3. Uncertainty in estimates of the indoor proportion of outdoor pollutants, which are input to the model, results in greater variability than do intraurban differences in ambient concentrations, resulting in respective ranges of 6.6e15 mg m_3 for PM2.5, 8.1e21 mg m_3 for PM10 and 6.8e16 mg m_3 for O3. Estimates of time spent in naturally ventilated (NV) homes are in the range 10e13 h/d across the population and exposures in NV homes contribute 49%, 53%, and 56% of total exposure for PM2.5, PM10 and O3, of outdoor origin, respectively. Results illustrate the importance of accurately characterizing climates specificIndoor-outdoor pollutant relationships to better quantify human exposure to air pollutants.

We develop an ozone transport and reaction model to determine reaction probabilities and assess the importance of physical properties such as porosity, porediameter, and material thickness on reactive uptake of ozone to five materials. The one-dimensional model accounts formolecular diffusion from bulk air to the air−material interface, reaction at the interface, and diffusive transport and reaction through material pore volumes. Material-ozone reaction probabilities that account for internal transport and internalpore area, γipa, are determined by a minimization of residuals between predicted and experimentally derived ozone concentrations. Values of γipa are generally less than effective reaction probabilities (γeff) determined previously, likely because of the inclusion of diffusion into substrates and reaction with internal surface area (rather than the use of the horizontally projected external material areas). Estimates of γipa average 1 × 10−7, 2 × 10−7, 4 × 10−5, 2 × 10−5, and 4 × 10−7 for two types of cellulose paper, pervious pavement, Portland cement concrete, and an activated carbon cloth, respectively. The transport and reaction model developed here accounts for observed differences in ozone removal to varying thicknesses of the cellulose paper, and estimates a near constant γipa as material thickness increases from 0.02 to 0.16 cm.

Background: High-flow nasal cannula therapy (HFNC) may increase aerosol generation, putting healthcare workers at risk, including from SARS-CoV-2.Aim: To examine whether use of HFNC increases near-field aerosols and whether there is an association with flow rate.Methods: Subjects aged four weeks to 24 months were recruited. Each child received HFNC therapy at different flow rates. Three stations with particle counters were deployed to measure particle concentrations and dispersion in the room: station 1 within 0.5 m, station 2 at 2 m, and station 3 on the other side of the room. Carbon dioxide (CO2) and relative humidity were measured. Far-field measurements were used to adjust the near-field measurements.Findings: Ten children were enrolled, aged from 6 to 24 months (median: 9). Elevated CO2 indicated that the near-field measurements were in the breathing plane. Near-field breathing plane concentrations of aerosols with diameter 0.3-10 μm were elevated by the presence of the patient with no HFNC flow, relative to the room far-field, by 0.45 particles/cm3. Whereas variability between subjects in their emission and dispersion of particles was observed, no association was found between HFNC use, at any flow rate, and near-field particle counts.Conclusion: This method of particle sampling is feasible in hospital settings; correcting the near-patient aerosol and CO2 levels for the room far-field may provide proxies of exposure risk to pathogens generated. In this pilot, near-patient levels of particles with a diameter between 0.3 and 10 μm and CO2 were not affected by the use of HFNC. [ABSTRACT FROM AUTHOR]