Indoor Air

BackgroundThe ongoing outbreak of COVID-19 has spread rapidly and sparked global concern. While the transmission of SARS-CoV-2 through human respiratory droplets and contact with infected persons is clear, the aerosol transmission of SARS-CoV-2 has been little studied. MethodsThirty-five aerosol samples of three different types (total suspended particle, size segregated and deposition aerosol) were collected in Patient Areas (PAA) and Medical Staff Areas (MSA) of Renmin Hospital of Wuhan University (Renmin) and Wuchang Fangcang Field Hospital (Fangcang), and Public Areas (PUA) in Wuhan, China during COVID-19 outbreak. A robust droplet digital polymerase chain reaction (ddPCR) method was employed to quantitate the viral SARS-CoV-2 RNA genome and determine aerosol RNA concentration. ResultsThe ICU, CCU and general patient rooms inside Renmin, patient hall inside Fangcang had undetectable or low airborne SARS-CoV-2 concentration but deposition samples inside ICU and air sample in Fangcang patient toilet tested positive. The airborne SARS-CoV-2 in Fangcang MSA had bimodal distribution with higher concentration than those in Renmin during the outbreak but turned negative after patients number reduced and rigorous sanitization implemented. PUA had undetectable airborne SARS-CoV-2 concentration but obviously increased with accumulating crowd flow. ConclusionsRoom ventilation, open space, proper use and disinfection of toilet can effectively limit aerosol transmission of SARS-CoV-2. Gathering of crowds with asymptomatic carriers is a potential source of airborne SARS-CoV-2. The virus aerosol deposition on protective apparel or floor surface and their subsequent resuspension is a potential transmission pathway and effective sanitization is critical in minimizing aerosol transmission of SARS-CoV-2.

Grignard Pure (GP) is a unique and proprietary blend of Triethylene Glycol (TEG) and inert ingredients designed for continuous antimicrobial treatment of air. GP received approval from the US EPA under its Section 18 Public Health Emergency Exemption program for use in seven states. This study characterizes the efficacy of GP for inactivating MS2 bacteriophage - a non-enveloped virus widely used as a surrogate for SARs-CoV-2. Experiments measured the decrease in the airborne viable MS2 concentration in the presence of different concentrations of GP from 60 to 90 minutes, accounting for both natural die-off and settling of MS2. Experiments were conducted both by introducing GP aerosol into air containing MS2 and by introducing airborne MS2 into air containing GP aerosol. GP is consistently able to rapidly reduce viable MS2 bacteriophage concentration by 2-3 logs at GP concentrations of 0.02 mg/m3 to 0.5 mg/m3 (corresponding to TEG concentrations of 0.012 mg/m3 to 0.287 mg/m3). Related GP efficacy experiments by the US EPA, as well as GP (TEG) safety and toxicology, are also discussed. SynopsisLimited research on the germicidal properties of triethylene glycol against airborne pathogens was conducted during the 1940s and 50s. This paper investigates the inactivation rate of airborne bacteriophage MS2 by Grignard Pure product, containing a unique and proprietary blend of Triethylene Glycol (TEG) and inert ingredients.

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.

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.

Executive SummaryPrior literature documents cases of airborne infectious disease transmission at distances ranging from [≥] 2 m to inter-continental in scale. Physics- and biology- based models describe the key aspects of these airborne disease transmission events, but important gaps remain. This report extends current approaches by developing a new, single-particle based theory that (a) assesses the likelihood of rare airborne infections (where individuals inhale either one or no infectious particles) and (b) explicitly accounts for the variability in airborne exposures and population susceptibilities within a geographic region of interest. For these hazards, airborne particle fate and transport is independent of particulate concentration, and so results for complex releases can be determined from the results of many single-particle releases. This work is intended to provide context for both (a) the initial stages of a disease outbreak and (b) larger scale ([≥] 2 m) disease spread, including distant disease "sparks" (low probability, unexpected disease transmission events that infect remote, susceptible populations). The physics of airborne particulate dispersion inherently constrains airborne disease transmission. As such, this work suggests results that, a priori, may be applicable to many airborne diseases. Model PredictionsO_LIModeling predictions of the single-particle transmission kernel suggest that outdoor airborne disease transmission events may occur episodically as the infection probabilities can vary over many orders of magnitude depending on the distance downwind; specific virus, prion, or microorganism; and meteorological conditions. C_LIO_LIModel results suggest that, under the right conditions, an indoor infected person could spread disease to a similar, or greater, number of people downwind than in the building they occupy. However, the downwind, per-person infection probability is predicted to be lower than the within-building, per-person infection probability. This finding is limited to airborne transmission considerations. C_LIO_LIThis work suggests a new relative disease probability metric for airborne transmitted diseases. This metric, which is distinct from the traditional relative risk metric, is applicable when the rate at which the infectious agent losses infectivity in the atmosphere is [lsim] 1 h-1. C_LI

Aerosols and droplets generated from expiratory events play a critical role in the transmission of infectious respiratory viruses. Increasingly robust evidence has suggested the crucial role of fine aerosols in airborne transmission of respiratory diseases, which is now widely regarded as an important transmission path of COVID-19. In this report, we used CFD modelling to investigate the efficiency of using portable air purifiers containing HEPA filters to reduce airborne aerosols in hospitals and serve as a potential retrofit mitigation strategy. We used a consulting room to set up our simulations because currently the clearance time between consultations is the controlling factor that limits the patient turnover rate. The results suggest the inlet/suction of the air purifier unit should be lifted above the floor to achieve better clearance efficiency, with up to 40% improvement possible. If multiple air purifiers are used, the combined efficiency can increase to 62%. This work provides practical guidance on a mitigation strategy that can be easily implemented in an expedient, cost-effective and rapid manner, and paves the way for developing more science-informed strategies to mitigate the airborne transmission of respiratory infections in hospitals.

COVID-19 pandemic has caused a severe demand for facemasks, and this has resulted in the use of those made from alternate media. As SARS-CoV-2 spreads primarily due to airborne droplets, it is critical to verify the filtration efficiency of these alternate media based facemasks. While several media are being tested and used, commercially available dust cleaners have shown reasonable filtration efficiency. This may also be due to the potential electrostatic charge on the surface which enhances capture of the fine particles. In this manuscript, we report the size dependent filtration efficiency studied systematically in a filter holder-based system as 47 mm punches; and test results on a mannequin that was 3D printed wearing a bandana mask that was placed in a chamber.

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.

COVID-19 is a life-threatening respiratory infection that has had a profound impact on indoor air quality awareness. Ultraviolet-C (UV-C) is a physical disinfection process that triggers microbial inactivation through creating irreversible genetic material damage. An upper room device equipped with germicidal UV-C (UR GUV) was evaluated against airborne SARS-CoV-2 for antimicrobial efficacy using a robust aerosol testing protocol. In 30 minutes, it led to a virucidal efficacy of 99.994 % in a large, room-sized chamber. UR GUV is a promising mitigation strategy for airborne pathogens.

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.

Open schools in winter in highly epidemic areas pose a controversial issue: ventilation of classrooms (an essential mitigation factor for airborne transmission) is expected to sensibly decrease due to outdoor temperatures getting colder and regulators going to allow less restrictive policies on windows closure. Fundamental questions to be addressed are therefore: to which extent can we contain airborne transmission risk in schools? what would be the optimal ventilation strategy during the cold season considering the fact that most schools are not provided with mechanical ventilation systems? To try answering these questions a risk model for airborne transmission of covid-19 in classrooms has been develped based on previous models for tubercolosis and influenza. The separate cases of infective student and infective teacher, as well as infective teacher with microphone are investigated. We explored 3500 different air ventilation cycles for different lesson+break times and carried out a numerical optimization of the risk function. Safety risk-zones for breaks and lessons durations were estimated combining the effect of surgical masks and optimal windows opening cycles.

Behavioral and medical control measures are not effective in containing the spread of SARS-CoV-2. Here we report on the effectiveness of a preemptive environmental strategy using UV-C light to prevent airborne transmission of the virus in a hamster model and show that UV-C exposure completely prevents airborne transmission between individuals

This paper presents a proof-of-concept study establishing effectiveness of the Active Plasma Sterilizer (APS) for sterilization in planetary protection. The APS uses Compact Portable Plasma Reactors (CPPRs) to produce surface dielectric barrier discharge, a type of cold plasma, using ambient air to generate and distribute reactive species like ozone used for decontamination. Sterilization tests were performed with pathogenic bacteria (Escherichia coli and Bacillus subtilis) on materials (Aluminum, Polycarbonate, Kevlar and Orthofabric) relevant to space missions. Results show that the APS can achieve 4 to 5 log reductions of pathogenic bacteria on four selected materials, simultaneously at 11 points within 30 minutes, using power of 13.2 {+/-} 2.22 W. Spatial sterilization data shows the APS can uniformly sterilize several areas of a contaminated surface within 30 minutes. Ozone penetration through Kevlar and Orthofabric layers was achieved using the CPPR with no external agent assisting penetration. Preliminary material compatibility tests with SEM analysis of the APS exposed materials showed no significant material damage. Thus, this study shows the potential of the APS as a light-weight sustainable sterilization technology for planetary protection with advantages of uniform spatial decontamination, low processing temperatures, low exposure times, material compatibility and the ability to disinfect porous surfaces.

Airborne transmission of respiratory pathogens depends on their ability to remain viable in drying respiratory droplets, yet the physicochemical drivers of bacterial inactivation during droplet evaporation remain poorly quantified. This study combines controlled droplet experiments with physicochemical modeling to investigate how osmotic pressure dynamics influence bacterial survival. Using Escherichia coli and Staphylococcus epidermidis as Gram-negative and Gram-positive surrogates, respectively, we measured viability loss in artificial saliva droplets dried at multiple relative humidities and reconstructed the time-resolved osmotic pressure using the Respiratory Aerosol Model (ResAM). Both organisms remained stable while droplets were liquid but lost viability following efflorescence, when rapid solute concentration changes produced sharp osmotic pressure increases. The extent of inactivation scales log-linearly with the rate of osmotic pressure change around efflorescence: E. coli decays faster than S. epidermidis, and relationships derived in artificial saliva predict survival in independent phosphate-buffered saline experiments. A more rapid drop in humidity led to more severe osmotic shocks and greater inactivation. These results identify the rate of osmotic pressure change during efflorescence as a quantitative, medium-independent predictor of bacterial survival in drying respiratory droplets. ImportanceAirborne infection risk depends on how long microorganisms remain viable in respiratory particles after exhalation, yet the physical mechanisms controlling bacterial survival during droplet drying are not well defined. Evaporation of respiratory droplets concentrates salts and can impose sudden and extreme osmotic stress on microbes, but this process has been difficult to quantify because osmotic pressure cannot be measured directly inside microscopic droplets. Integration of droplet experiments with a physicochemical aerosol model shows that bacterial inactivation is governed primarily by the rate of osmotic pressure increase during droplet efflorescence rather than by static values of humidity or solute concentration alone. This mechanism explains why rapid drying may produce strong inactivation.

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.

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.

The COVID-19 pandemic demonstrated that reliable risk assessment of venues is still challenging and resulted in the indiscriminate closure of many venues worldwide. Therefore, this study used an experimental, numerical and analytical approach to investigate the airborne transmission risk potential of differently ventilated, sized and shaped venues. The data were used to assess the effect size of different mitigation measures and to develop recommendations. In general, positions in the near field of an emission source were at high risk in all ventilation systems studied, while the risk of infection from positions in the far field varied depending on the ventilation strategy. Occupancy rate, airflow rate, residence time, SARS-CoV-2 virus variants, a high activity level and face masks affected the individual and total infection risk in all venues. The total infection risk was lowest for the displacement ventilation case and highest for the naturally ventilated venue. Therefore, in our study, a properly designed displacement ventilation system is the most effective ventilation strategy to keep airborne transmission and the number of secondary cases low, compared to mixing or natural ventilation.

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.

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.

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.