Indoor Air

Ventilation is one of the most critical components in a layered approach toward reducing the spread of airborne infectious diseases in indoor spaces. However, building ventilation systems act together with natural ventilation, local filtration systems and other aerosol removal processes to remove infectious aerosols from an occupied space. Airflow-based determinations of ACH do not account for the full range of aerosol removal processes; however understanding the effective aerosol removal rate is critical to providing airborne infection control. In this study, we investigated the relationship between the calculated air change rate of a space (i.e. volumetric airflow based) and the effective air change rate for aerosol particle removal within the breathing zone based on direct measurements of the rate of change in tracer particle concentrations at representative occupant locations in a room. Further, we examined positional effects under well mixed and non-well mixed conditions. Our results demonstrate that tracer particles combined with real-time sensors can be used to make rapid, accurate measurements of the effective air change rate (eACH) for respiratory aerosols within the breathing zone of non-well mixed rooms. We used two experimental test beds for these analyses. First, numerical simulation (computational fluid dynamic simulation, CFD) was conducted to visualize airflow and particle removal paths within a realistic large room. Here, simulated sensors were placed in concentric zones around a nebulizer providing test-particle releases. This CFD model allowed a direct comparison of the differences between eACH and airflow ACH values under varying levels of mixing and airflow, in a fully controlled system. We then recapitulated this system in physical space to validate the CFD results under real-world conditions that include all mechanisms of particle removal that contribute to true aerosol clearance rates, including deposition and leakage. Here, we measured eACH using the decay of DNA tracer aerosols nebulized and monitored in real-time. We find that a standard sampling time of 15 minutes from the end of nebulization is sufficient to produce an accurate eACH value under non-well mixed conditions. The availability of a rapid direct test for eACH will enable empirical optimization of a wide range of ventilation and filtration mechanisms to reach and maintain target aerosol clearance rates that deliver reliable airborne infection control in typical indoor environments.

The SARS-CoV-2 pandemic has resulted in many live events being canceled or held without spectator participation. It is therefore necessary to develop strategies to determine the conditions under which cultural activities can be maintained. In this study the results from available literature were combined with findings, guidelines and regulations for other indoor environments and recommendations were derived. In the cultural sector, the number of experimental investigations, surveys and simulations is comparatively small. This is probably due to the complexity of the events in terms of location and visitor flow, so the respective conditions under which they take place can be very different. It is therefore practically impossible to predict the risk of infection for a specific situation with potential virus spreaders attending or to derive general rules that go beyond the known measures of vaccination, testing, masks and distance. Cultural events can be held under pandemic conditions, provided certain conditions are met. Most study results agree on this. However, any recommendations for hygiene, safety and ventilation measures in cultural institutions can only minimize the risk of infection, but cannot completely rule it out. It is also of considerable importance that visitors protect themselves individually and act responsibly. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=68 SRC="FIGDIR/small/22281932v1_ufig1.gif" ALT="Figure 1"> View larger version (10K): org.highwire.dtl.DTLVardef@13b4f8dorg.highwire.dtl.DTLVardef@e3b4a9org.highwire.dtl.DTLVardef@16a0140org.highwire.dtl.DTLVardef@78280b_HPS_FORMAT_FIGEXP M_FIG C_FIG

We experimentally investigated indoor air ventilation using the CO2 tracer technique to verify the infection cluster of SARS-CoV-2 that erupted at an office space. Multi-placed observations revealed extremely low air change rates (0.1/h) at the site. The local infection clusters were observed several meters away from a door that is the only ventilation in the office, which suggests a negative effect of plastic sheeting shielding. The thermo-fluid simulation showed that the plastic sheet blocked the airflow and trapped the exhaled air in each partition cell. As risk suppression methods, improving air ventilation by opening windows and using fans were verified, and significant improvements (10-28/h) were observed for each partition cells.

Wildfire smoke (WFS) events are an important public health concern for communities in the Pacific Northwest of the United States. Previous studies of portable air cleaners, including high efficiency particulate air (HEPA) filtration and do-it-yourself (DIY) box fan filters built with MERV 13-rated filters, have indicated that their use in residential settings may be an effective way to reduce indoor exposures to fine particulate matter during WFS episodes. The lower-cost, easy to build instructions and availability of materials of DIY box fan filters have made their distribution by both public health agencies and community groups an attractive approach to improve community preparedness. Here, we describe a low-cost, easy-to-assemble, portable exposure chamber system that can be used to support a variety of community-engaged demonstrations of WFS removal efficiency as well as provide a mechanism to estimate the efficiency of filtration systems in a controlled environment. We conducted experiments using the portable chamber to assess the clean air delivery rate (CADR) of a MERV 13-rated DIY box fan filter, which was found to be 92.2 and 145.2 cfm at low and high fan speeds, respectively. In addition to using the chamber system to evaluate the CADR of DIY box fan filters, we also provide a case-study example, working with a tribal community in Central Washington, who used the tent system for a live demonstration of a DIY box fan filter experiment during their community gathering to promote WFS and air quality intervention knowledge and distribution of box fan filters.

We measured the compartmental air change per hour (ACH) using a CO2 sensor network in an office space where a cluster of COVID-19 infections attributed to aerosol transmission occurred. Generalized linear mixed models and dynamic time warping were used for a time series data analysis, and the results indicated that the ventilation conditions were poor at the time of the cluster outbreak, and that the low ACH in the room likely contributed to the outbreak. In addition, the adverse effects of inappropriate partitions and the effectiveness of ventilation improvements were investigated in detail. ACH of less than 2 /h was considered a main contributor for the formation of the COVID-19 cluster in the studied facility. Practical ImplicationsA systematic method for measuring and evaluating indoor ventilation to prevent the spread of infectious diseases caused by aerosols is presented. Ventilation bias caused by ventilation pathways and inappropriate use of plastic sheeting can be detected by a CO2 sensor network and time series data analysis. Estimated ventilation rate will be a good index to suppress the formation of the COVID-19 cluster.

Poor indoor air quality has been demonstrated to increase the risk of transmitting infectious agents and to expose individuals to the phenomenon of sick building syndrome. In light of these findings, most countries have established specific guidelines regarding indoor air quality in university rooms. In France, for instance, the maximum permissible concentration of carbon dioxide (CO2) in university rooms without mechanical ventilation is set at 1,300 parts per million (ppm), and the minimum volume per occupant is 15 m3. For rooms with mechanical ventilation, the minimum clean air flow rate is 25 m3/h/occupant. The primary objective of this study was to design and demonstrate the feasibility of a simple, cost-effective method for comparing the reality of indoor air quality in all university rooms with legal requirements. The secondary objectives of the present study were to demonstrate the efficacy of the proposed method in identifying and reporting problematic situations, and in issuing practical recommendations. Mobile CO2 sensors (Aranet4) were provided to volunteer lecturers to measure the CO2 concentration during and after classes. The number of occupants and the condition of openings were also recorded. These data were supplemented by measurements from 117 fixed Carbon Nexelec sensors. The data were then fitted to a model, which enabled the characterization of air quality and the estimation of the gauge reduction required to comply with the law. None of the 14 rooms without mechanical ventilation complied with the legal minimum of 15 m3/occupant. 75% of the third quartiles of CO2 concentrations during classes exceeded 2692 ppm. In rooms with mechanical ventilation, median clean air flow was 15 m3/h/occupant at 100% occupancy (9 m3/h/occupant for the first quartile). In 32 out of 41 rooms with mechanical ventilation (78%), the clean air flow was estimated to be below the legal minimum of 25 m3/h/occupant at 100% occupancy. Concentrations in excess of 5,000 ppm were observed in 23 of the 101 rooms equipped with fixed sensors. The proposed method has demonstrated its feasibility in real-life conditions. For the purpose of evaluating the air quality of all rooms affiliated with universities, it is recommended that this method be used in a systematic manner. The findings of this study indicate that a significant proportion of the examined rooms may not be in accordance with the relevant legislation, thereby jeopardizing the health of the occupants. In order to comply with the law, the method proposed here to estimate gauge reduction should be applied.

Contaminated aerosols in room air are one of the transmission routes of the coronavirus. The amount of contaminated aerosols in the room seems to play an important role for the infection risk. In rooms without technical air refreshing systems, the aerosol concentration can be reduced with simple natural ventilation activity. Instrumental and vocal lessons and rehearsals take place in closed indoor rooms. Therefore it is important to optimize the necessary ventilation activity in order to keep the infection risk for musicians low. Therefore, knowledge about the maximum duration of the lesson or rehearsal for ventilation intervals are necessary. In this study, carbon dioxide concentration (CO2) as an indicator of the indoor air quality (IAQ) was measured during 47 music lessons and rehearsals at a university of music including 141 persons. From these measurements, the air exchange rates of the rooms and the CO2 emission rates per person were extracted. The results show that the CO2 emission in musical activities can be assigned to light and moderate activities between 28 l/h and 39 l/h. Wind instruments had the highest CO2 emissions. Singers showed low CO2 emission rates comparable to the control group which only spoke and listened. Recommendations for the frequency of ventilation breaks were derived from empirical data and allow for an individual risk assessment of instrumental and vocal lessons and rehearsals depending on room size and number of musicians.

On May 12, 2023 CDC recommended 5 air changes per hour (5 ACH) and in July 2021 California (CDPH) recommended 6 to 12 ACH to reduce long-range, aerosol transmission of COVID-19 and other pathogens in classrooms. EPA recommends MERV 13+ DIY air cleaners for temporary use during wildfires, and a recent EPA study reported inconsistent usage in homes due to excessive noise generated by the DIY air cleaners. Questions also remain about wear and tear including how long filters retain their filtration properties and need to be replaced. Herein we report real-world experience from daily usage of 47 HEPA and 60 MERV 16 DIY air cleaners in a California elementary school during two academic school years from spring 2021 through fall of 2023 across 16 classrooms, a library, an auditorium, a lunchroom, and in a hallway. Three to six purifiers were needed in classrooms to meet 6 to 12 ACH. Teachers reported noise generated by MERV 16 DIY purifiers on lowest fan speed as acceptable for classroom use. Filtration efficiency at 0.3 m (most penetrating particle size) for DIY air cleaners with 5" MERV 16 filters used in the classrooms averaged 77% after six months (1st batch in February 2022) compared to 92% for newly installed filters (2nd batch in October 2022). Follow up testing on both batches of filters after an additional academic year (June 2023) showed only slight changes in average filtration efficiency. Portable air cleaners (HEPA and DIY) averaged and estimated 10 ACH (6-15 ACH) across the 16 classrooms demonstrating feasibility and unit economics of meeting CDPH targets per classroom for $200-$650 with DIY versus $600-$12,000 with the HEPA models used. In one 9000 cubic foot classroom with 7 air purifiers, air exchange rate was measured using ambient aerosols at 18 ACH from air purifiers (within 20% of ACH estimated based on CADR of purifiers) and 7 ACH from HVAC for a combined total of 25 ACH. Based on this long-term experience, specific recommendations for enhancing and improving CDCs web page "Ventilation in Buildings" include: (1) recommended operation of MERV 13+ DIY at their low speed for low noise, cost-effective air cleaning (2) electro-mechanical safety especially in relation to power outlets (3) an open-source procedure known as the "spike test" using ambient aerosols to verify ACH in a room, like the Portacount for mask fit testing. Spike testing can become the basis for ACH certification or verification in any room without generating aerosol contaminants (e.g. salt water, smoke, tracers which may be unsafe or disallowed indoors).

Restaurants present an especial challenge in the prevention of the spread of COVID-19 via exhalatory bioaerosols because customers are unprotected by facemasks while eating, so that ventilation protocols in such establishments become particularly important. However, despite the fact that this pandemic airborne disease has been with us for two full years, many restaurants are still not successfully prioritising air renovation as a key tool for reducing infection risk. We demonstrate this in the run-up to the 2021 Christmas celebrations by reporting on CO2 concentration data obtained from a hotel breakfast room and restaurants during the 5-day Spanish holiday period of 4th-8th December. In the case of the breakfast room, inadequate ventilation resulted in average CO2 levels ranging from 868 to 1237ppm on five consecutive days, with the highest levels coinciding with highest occupancy numbers. Inside the five restaurants, three of these were well ventilated, maintaining stable average CO2 concentrations below 700ppm. In contrast, two restaurants failed to keep average CO2 levels below 1000ppm, despite sporadic, but ineffective, attempts by one of them to ventilate the establishment. More effort needs to be made to foster in both restaurant managers and the general public an improved awareness of the value of CO2 concentrations as an infection risk proxy and the relevance of ventilation issues to the propagation of respiratory diseases.

1.Indoor airborne particulates are well-known health hazards and filtration is one common method of reducing exposures. Based on our previously developed Regional Shelter Analysis methodology and parameters that characterize the existing US building stock, we perform a high-level assessment of the potential benefits of upgrading existing filters in furnace and in heating, ventilation, and air conditioning systems using off-the-shelf filters. We use three metrics to assess the improvement: Building Transmission Factor (a measure of protection against outdoor airborne particles), Indoor Normalized Time and Space Integrated Air Concentration (a measure of indoor exposure to indoor-origin airborne particles), and Building Exit Fraction (fraction of indoor airborne particles that are released to the outdoor atmosphere). We also discuss the potential reduction in regional exposures due to particles exiting the building and exposing downwind building occupants. Our modeling indicates that while buildings provide their occupants passive protection against airborne particulate hazards, including but not limited to PM2.5, PM10, and wildfire smoke; improving particle filtration efficiency may further improve this protection. The degree of improvement varies with particle size and building type. Of the building types studied, apartments are predicted to benefit most, with greater than a factor of 2 improvement ([≥]50% reduction in exposures) for 1 {micro}m particle exposures when using MERV 7 to 12 rated filters. Non-residential buildings were notably less responsive to improved filtration but had the highest Building Exit Fractions with 30% to 40% of indoor airborne particles released to the outdoor atmosphere (apartment buildings only released 6% to 9%). Improvements predicted for single family homes were intermediate between apartments and non-residential buildings. Improvements in the Regional Exposure metric are larger, ranging from a factor of 2.5x to 10x for residences (when using MERV 7 to 12 rated filters) and up to 25x for large apartments with MERV 14 or 15 rated filters. The results of our modeling analysis are broadly consistent with the limited experimental data and modeling results available in the literature.

The COVID-19 pandemic has focused renewed attention on the ways in which building HVAC systems may be operated to mitigate the risk of airborne disease transmission. The most common suggestion is to increase outdoor-air ventilation rates so as to dilute the concentrations of infectious aerosol particles indoors. Although this strategy does reduce the likelihood of disease spread, it is often much more costly than other strategies that provide equivalent particle removal or deactivation. To address this tradeoff and arrive at practical recommendations, we explain how different mitigation strategies can be expressed in terms of equivalent outdoor air (EOA) to provide a common basis for energy analysis. We then show the effects of each strategy on EOA delivery and energy cost in simulations of realistic buildings in a variety of climates. Key findings are that in-duct filtration is often the most efficient mitigation strategy, while significant risk reduction generally requires increasing total airflow to the system, either through adjusted HVAC setpoints or standalone disinfection devices.

The objective was to evaluate the determination of biomarkers of air quality during a mass gathering event at a convention center using a novel air sampling device, AirAnswers(R). This sampler has previously only been used in smaller locations. Here it was run at five crowded locations within the exhibit area for the four days duration of a trade show. The AirAnswers(R) device uses electro-kinetic flow to sample air at high rates and capture bio-aerosols on grounded electrodes in assayable form. Cartridges were removed from the devices and immediately conveyed to the Inspirotec facility in North Chicago, where assays were performed. Biomarkers determined were for allergens and molds previously described for this system. Testing for a new marker, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA was also included. The method was validated by determination of capture efficiency with reference to an impinger sampler in a Class III controlled environment chamber. Average capture efficiency for triplicate runs was 14%. One SARS-CoV-2 positive sample as found at the registration area, which was physically separate from the main exhibit area. Cat allergen Fel d 1was found in four of the locations, dog allergen Can f 1 at two. The airborne biomarker of mold proliferation, (1[->]3)-{beta}-D-Glucan, was above the assay range in all locations. The widespread presence of this mold marker could be accounted for by signs of water leakage. A generic 18S RNA marker for mold was developed and similarly showed the presence of mold in all locations, as was a genus marker for penicillium. A species marker for Cladosporium cladosporioides was in two locations. Species markers for Eurotium amstelodami and Trichoderma viride were each in a single location. The main findings were of the widespread presence of mold markers, and the sporadic appearance of SARS-CoV-2. Masking was recommended but not enforced.

The January 2025 Los Angeles urban wildfires caused extensive destruction and exposed millions to wildfire smoke containing hazardous volatile organic compounds (VOCs). To evaluate exposure risks, we conducted indoor and outdoor VOC sampling at 24 locations during three phases: active burning, smoldering, and off-gassing. Outdoor benzene concentrations peaked during active burning but remained below OEHHA health thresholds. In contrast, indoor BTEX concentrations increased during smoldering and remained elevated during the off-gassing phase, particularly in uninhabited homes inside burn zones, suggesting persistent emissions from smoke-impacted materials. These findings raise concerns about indoor air quality post-wildfire and the potential for prolonged exposure. We recommend ventilating homes and using HEPA and activated charcoal air purifiers before reoccupying fire-affected residences. Our results highlight the need for targeted mitigation and ongoing monitoring to protect public health during wildfire recovery.

There are a number of recent studies detailing the transmission of SARS-CoV-2 (Covid-19) via both Droplet and Aerosol airborne particle routes of infection. Because of this, it is necessary to understand the release of different sized particles in activities such as playing brass instruments in order for an analysis of risk to take place for such activities. In this investigation, the quantity and size of particles released by brass instruments while they are played was analysed for 7 different types of brass instrument. This was contrasted with the same individuals breathing as a comparison for more general activities as well as the effect of a mitigating polycotton barrier over the end of their instruments. To investigate the particles released, the particles were size sorted and counted with a six-channel laser particle counter. Multiple measurements were made by each individual in each condition investigated. The mean concentration exiting across all instruments measured was found to be 1.21x107 {+/-}1.03x106 Aerosol type particles/m3 and 1.43x104 {+/-}9.01x102 Droplet type particles/m3 per minute. When breathing, the mean count was 1.61x107 {+/-}1.33x106 Aerosol type particle/m3 and 5.45x103 {+/-}1.20x103 Droplet type particles/m3. When playing with a barrier cover, the mean number of particles emitted fell to 2.60x106 {+/-}2.11x105 Aerosol type particle/m3 and 5.20x103 {+/-}8.02x102 Droplet type particles/m3. This barrier represented an average 78.5% reduction for the number of respiratory Aerosol type particles and 63.8% reduction for Droplet type particles compared to playing an instrument without the barrier covering. It was investigated what effect playing for a more extended period of time had on the release of particles with comparisons made to singing, breathing and covering the instruments bell ends with a barrier cap. This showed that the mean number of Aerosol type particles produced while playing was 5.38x107 {+/-}3.15x106 Aerosol type particles/m3 produced and showed a significant drop in Aerosol type particle production when playing with a barrier used, with a mean average of 2.28x106{+/-}8.01x104 Aerosol type particles/m3. Both breathing and singing showed consistent numbers of Aerosol type particles produced with means of 6.59x107 {+/-}7.94x105 Aerosol type particles/m3 and 5.28x107 {+/-}5.36x105 Aerosol type particles/m3 respectively. This showed a drop in mean Aerosol type particles/m of 95.7% when using a barrier cap compared to playing without a barrier. It is concluded that, while playing a brass instrument, the propagation of respiratory Aerosols does occur and, to a smaller extent, so do Droplet size particles, but at a lower level than when the subject was breathing without an instrument. Finally, it was shown that the use of a barrier cap on the bell end of the instrument offers a significant reduction in the production of respiratory Aerosols into the immediate surroundings, which offers a possible mitigation method for playing in groups from the release of Aerosol type particles, especially in hard to ventilate spaces. FundingThis study was supported by funds from Arts Council England covering salary support for AP and KC. The cleanroom facility and particle counter were provisioned by Centre Stage Ltd. The funders did not have a role in the experimental design, data collection, analysis or decision to publish and content of the manuscript. Competing InterestsThe authors have declared working for Brass Bands England, which exists to support brass bands in England and the wider UK.

The National Institute of Occupational Safety and Health procedure No. TEB-APR-STP-0059 recommend of measuring the respirator filtration efficiency using sodium chloride aerosol with count median diameter of 75 nm {+/-} 20 nm and geometric standard deviation [≤]1.86. This study showed that this method would overestimate the respirators ability to protect against submicrometer particles. In this study, we converted both mobility diameter and equivalent volume diameter to aerodynamic diameter for comparison. The results showed that one unqualified KN95 respirator (with the filtration efficiency of 72%{+/-}3% for [≥]300 nm sodium chloride aerosol) still passed the test with a measured overall filtration efficiency of 98%{+/-}3%, due to its larger most penetrating particle size compared to the typical N95 respirator. In addition, after three cycle H2O2 plasma vaporous sterilizations, the most penetrating particle size for the N95 grade respirators also shifted to 250 nm - 500 nm, in which size the particles carried the peak concentration of the SARS-CoV-2 in hospitals. This size shift caused the significant difference between the size specific (250 nm - 500 nm) filtration efficiency and overall filtration efficiency using the same NaCl test aerosol. For example, after three cycle H2O2 plasma vaporous sterilizations, the size specific filtration efficiency of the N95 was 55%{+/-}2%, however, the measured overall filtration efficiency was still 86%{+/-}5%. The size Specific filtration efficiency of the KN95 was 69%{+/-}2%, but, the measured overall filtration efficiency was still 90%{+/-}3%. In order to protect health care personnel adequately, we recommend increasing the test aerosol size, and measuring the size specific filtration efficiency to evaluate the N95 alternatives (e.g. KN95), and the reuse of N95 level respirators. In addition, multi-cycle sterilization with ultraviolet germicidal irradiation appears to have fewer negative effects than H2O2.

We sought to assess whether HHFNC results in greater production of aerosolized particles than 6 liters per minute nasal cannula, using state-of-the-art techniques of aerosol measurement, in spontaneously breathing human volunteers in a simulated hospital room. For each volunteer, we first measured background aerosol levels in the room immediately prior to testing. We then measured aerosol levels while the healthy volunteer laid in bed - - with the head of bed at 30 degrees - - wearing the following oxygen delivery devices: (a) 6L/min nasal canula (NC) with humidification; (b) non-re-breather mask (NRB) with 15L/min gas flow, non-humidified; (c) HHFNC with 30L/min gas flow; (d) HHFNC with 60L/min gas flow. Two scanning mobility particle sizing (SMPS) systems (TSI 3080/3030, TSI 3080/3750) were used to measure aerosols 10 to 500 nanometer (nm) in size for each of the oxygen delivery devices. There was no variation in aerosol level within patients between room air, 6 L/min NC, 15 L/min NRB, 30 L/min HHFNC, and 60 L/min HHFNC, regardless of coughing.

BackgroundFor much of the SARS-CoV-2 (COVID-19) pandemic, many countries have struggled to offer definitive guidance on the wearing of masks or face coverings to reduce the highly infectious disease transmission resulting from a lack of compelling evidence on the effectiveness of communities wearing masks, and slow acceptance that aerosols are a primary SARS-CoV-2 disease transmission mechanism. Recent studies have shown that masks have been effective in several countries and populations, leaving only a lack of quantitative data on the control of airborne dispersion from human exhalation. This current study specifically has the objective to quantify the effectiveness of non-medical grade washable masks or face coverings in controlling airborne dispersion from exhalation (both droplet and aerosol) by measuring changes in direction, particle cloud velocities, and concentration. DesignThis randomized effectiveness study used a 10% NaCl nebulized polydisperse particle solution (0.3 m up to 10 m in size) delivered by an exhalation simulator to conduct 94 experiment runs with combinations of 8 different fabrics, 5 mask designs, and airflows for both talking and coughing. Multiple particle sensors were instrumented to measure reduction in aerosol dispersion. ResultsThree-way multivariate analysis of variance establishes that fabric, mask design, and exhalation breath level have a statistically significant effect on changing direction, reducing velocity, or concentrations of airborne particles (Fabric: P = < .001, Wilks {Lambda} = .000; Mask design: P = < .001, Wilks' {Lambda} = .000; Breath level: P = < .001, Wilks' {Lambda} = .004). There were also statistically significant interaction effects between combinations of all primary factors. Conclusions and RelevanceThe application of facial coverings or masks can significantly reduce the airborne dispersion of aerosolized particles from exhalation by diffusing the particle cloud direction and slow down its travel speed. Consequently, the results indicate that wearing masks when coupled with social distance can decrease the potentially inhaled dose of SARS-CoV-2 aerosols or droplets especially where infectious contaminants may exist in shared air spaces. The conclusion is well aligned with the concept of "time-distance-shielding" from hazardous materials emergency response. However, the effectiveness varies greatly between the specific fabrics and mask designs used.

Public health experts have confirmed that airborne transmission of SARS-CoV-2 (COVID-19) is one of the primary mechanisms of infection (CDC, 2020). In addition to social distancing, mask wearing and hand washing, experts now recommend increasing the ventilation and filtration of indoor air. While there is widespread consensus on this general approach, to date there are no published guidelines for the levels of ventilation, filtration, etc. that are required to control the pandemic. This is an urgent concern because colder weather in the Northern Hemisphere has moved social activity indoors where the risk of infection is higher.

Mobile monitoring campaigns to estimate long-term air pollution levels are becoming increasingly common. Still, many campaigns have not conducted temporally-balanced sampling, and few have looked at the implications of such study designs for epidemiologic exposure assessment. We carried out a simulation study of fixed-site air quality monitors to better understand how different mobile monitoring designs involving short-term stationary measurements at fixed locations impact the resulting exposure surfaces. We used Monte Carlo resampling to simulate three archetypal monitoring designs using oxides of nitrogen (NOx) monitoring data from 69 regulatory sites in California: a year-around Balanced Design that sampled during all seasons of the year, days of the week, and all or various hours of the day; a temporally reduced Rush Hours Design; and a temporally reduced Business Hours Design. We evaluated the performance of each designs land use regression prediction model. The Balanced Design consistently yielded the most accurate annual averages; while the reduced Rush Hours and Business Hours Designs generally produced more biased results. A temporally-balanced sampling design is crucial for mobile monitoring campaigns aiming to assess accurate long-term exposure in epidemiologic cohorts. SynopsisAir pollution mobile monitoring campaigns rarely conduct temporally balanced sampling. We show that this results in biased annual average exposure estimates. O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=104 SRC="FIGDIR/small/21255641v2_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@126c8ddorg.highwire.dtl.DTLVardef@14d52e5org.highwire.dtl.DTLVardef@17d390dorg.highwire.dtl.DTLVardef@2cc3d1_HPS_FORMAT_FIGEXP M_FIG C_FIG

HypothesisAerosols are the principal cause of airborne infections and respiratory diseases. Droplets ejected from the host can evaporate and form a precipitate in the air (aerosol mode), or evaporate for some time, and fall on the ground (mixed mode) or directly fall on the ground and evaporate as sessile mode. Different evaporation modes, stages of evaporation and the relative humidity (RH) conditions affect the survival and infectivity of the bacteria in the precipitate. ExperimentsWe have investigated three droplet diameter reduction ratio-based stages of evaporation of a bacteria-laden levitated droplet at two different RH settings and evaporation modes (aerosol and mixed) mimicking real-life scenarios. The low RH condition mimics evaporation in arid regions. e.g., Delhi and the high RH conditions imitate cold areas like London. The study analyses the mass transport, micro-characterizes the samples, and investigates the survival and infectivity of bacteria in the sample. FindingsThe bacteria survive more in the high RH condition than in the low RH condition for all diameter reduction ratio-based stages and modes of evaporation. For the aerosol mode, at a fixed RH condition, the evaporation time plays a vital role as the bacteria in early-stage partially dried samples are more viable than the full precipitate. The evaporation rate, and the generation of reactive oxygen species (ROS) cause a remarkable difference in the viability and infectivity of the bacterial samples. Therefore, our findings report that the evaporation history of an infected droplet is an indispensable factor in determining bacterial viability and subsequent infectivity.