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The utilization of portable air cleaners (PACs) is a recommended supplemental approach to help remove airborne pathogens and mitigate disease transmission in learning environments. To improve PAC effectiveness, science-based information is needed to optimize their implementation strategies such as the deployment location, height, and number of PACs. In this study, we developed a Computational Fluid Dynamics (CFD) model to assess how PACs perform in occupied classrooms equipped with displacement and mixing ventilation systems. The results show that PACs with a flow rate of 2.6 h -1 reduce the mean aerosol intake of all students by up to 66%. A key benefit of using PACs is to facilitate air mixing and movement in indoor environments with inadequate ventilation, thereby effectively reducing high aerosol concentrations near the infector. Furthermore, our results highlight the impact of PAC location on its performance. PACs achieve the best effectiveness when placed closed to the infector (within a distance <3 m). In the absence of knowing who is infected, deploying a PAC at the center of the room is recommended. Moreover, adjusting PAC flow discharge height to the breathing height of occupants ( e.g. , 0.9-1.2 m for seated people) can enhance their effectiveness in spaces with poor air mixing.

Providing sufficient amounts of outdoor air to occupants is a critical building function for supporting occupant health, well-being and productivity. In tropical climates, high ventilation rates require substantial amounts of energy to cool and dehumidify supply air. This study evaluates the energy consumption and associated cost for thermally conditioning outdoor air provided for building ventilation in tropical climates, considering Singapore as an example locale. We investigated the influence on energy consumption and cost of the following factors: outdoor air temperature and humidity, ventilation rate (L/s per person), indoor air temperature and humidity, air conditioning system coefficient of performance (COP), and cost of electricity. Results show that dehumidification of outdoor air accounts for more than 80% of the energy needed for building ventilation in Singapore's tropical climate. Improved system performance and/or a small increase in the indoor temperature set point would permit relatively large ventilation rates (such as 25 L/s per person) at modest or no cost increment. Overall, even in a thermally demanding tropical climate, the energy cost associated with increasing ventilation rate up to 25 L/s per person is less than 1% of the wages of an office worker in an advanced economy like Singapore's. This result implies that the benefits of increasing outdoor air ventilation rate up to 25 L/s per person--which is suggested to provide for productivity increases, lower sick building syndrome symptom prevalence, and reduced sick leave--can be much larger than the incremental cost of ventilation.

Inhalation exposure to ambient ultrafine particles (UFP) has been shown to induce adverse health effects such as respiratory and cardiovascular mortality. Human exposure to particles of outdoor origin often occurs indoors due to entry of UFP into buildings. The objective of the present study is to investigate entry of UFP into a building considering building operational characteristics and their size-dependent effects on UFP concentrations. Indoor and outdoor UFP concentrations along with air change rates were continuously measured in a full-scale test building. Estimates of infiltration factor, penetration coefficient, and deposition rate have been made for a range of particle sizes from 4 to 100 nm. The results show that UFP infiltration factor varies with particle diameter, window position, air change rate, and central fan operation. When the central fan was on continuously, the average infiltration factor ranged from 0.26 (particles <10 nm) to 0.82 (particles >90 nm) for two large window openings, and from 0.07 to 0.60 for two small window openings. Under the central fan-off condition, the average infiltration factor ranged from 0.25 (particles <10 nm) to 0.72 (particles >90 nm) for two small window openings, while it ranged from 0.01 to 0.48 with all windows closed. Larger window openings led to higher infiltration factors due to the larger extent of particle penetration into the building. The fan operation mode (on vs off) also has a strong impact, as the infiltration factor was consistently lower (up to 40%) when the fan was on due to additional particle deposition loss to the furnace filter and duct surfaces.

The presence of airborne pollutants in indoor environments has been associated with occupants' discomfort and/or adverse health effects. This study investigates occupational exposure in relation to indoor air mixing and source location relative to a human body. Experimental and computational methods were used to provide information about the pollutant distribution in the vicinity of the human body for different levels of room air mixing. Study results show that the often used assumption of uniform pollutant distribution in an occupied space is not always appropriate for estimation of inhalation exposure. Results also indicate that an occupant may experience very high acute exposure to airborne pollutants when little air mixing exists in a space and the pollutant source is in the vicinity of the occupant. The buoyancy-driven flow induced by the convective heat transfer from an occupant's body can transport pollutants in the occupant's vicinity to the breathing zone. Specific study results reveal that a source located in the occupant's front chest region makes a relatively large contribution to the breathing zone concentration compared with the other sources in the vicinity of the human body. With the source position in this region, exposure can be nine times greater than that calculated with the uniform mixing assumption. The buoyancy-driven convective plume around a body seems to have a significant influence on pollutant transport and human exposure, especially in the absence of room air mixing.

Providing sufficient amounts of outdoor air to occupants is a critical building function for supporting occupant health, well-being and productivity. In tropical climates, high ventilation rates require substantial amounts of energy to cool and dehumidify supply air. This studyevaluates the energy consumption and associated cost for thermally conditioning outdoor air provided for building ventilation in tropical climates, considering Singapore as an example locale. We investigated the influence on energy consumption and cost of the followingfactors: outdoor air temperature and humidity, ventilation rate (L/s per person), indoor airtemperature and humidity, air conditioning system coefficient of performance (COP), andcost of electricity. Results show that dehumidification of outdoor air accounts for more than 80% of the energy needed for building ventilation in Singapore’s tropical climate. Improved system performance and/or a small increase in the indoor temperature set point would permit relatively large ventilation rates (such as 25 L/s per person) at modest or no cost increment.Overall, even in a thermally demanding tropical climate, the energy cost associated with increasing ventilation rate up to 25 L/s per person is less than 1% of the wages of an office worker in an advanced economy like Singapore’s. This result implies that the benefits ofincreasing outdoor air ventilation rate up to 25 L/s per person—which is suggested to provide for productivity increases, lower sick building syndrome symptom prevalence, and reduced sick leave—can be much larger than the incremental cost of ventilation.

Airborne particle transport in indoor environments plays an important role in occupant exposure to aerosols and public health problems. Several studies have examined indoor airflow and particle transport using computational fluid dynamics models. For the Lagrangian particle tracking model, the minimum particle concentration necessary for accurate prediction may vary with the airflow regime and sampling volume. Nonetheless, only a few studies have systematically quantified suitable particle numbers and sampling volumes, according to indoor airflow and ventilation conditions. This study addresses this gap by exploring quality control strategies for a Lagrangian particle tracking model to reliably predict indoor particle transport. Based on transient simulations, we analyzed the spatiotemporal distributions of indoor particle trajectories while varying the number of particles, sampling volume, and ventilation strategy. The results indicate that in general a sampling volume of 5 L can predict the normalized mean concentrations better than a 1 L sampling volume, particularly when dealing with a smaller number of particles. Furthermore, the required particle number concentrations vary significantly depending on the chosen ventilation strategy. For instance, under the conditions of a 5 L sampling volume and an air exchange rate of 2.7 h−1, the minimum particle number concentrations for achieving reliable modeling predictions were observed to be 0.0075 cm−3 for displacement ventilation and 0.015 cm−3 for mixing ventilation. These results highlight the crucial role of the number of simulated particle trajectories in Lagrangian particle tracking models in determining prediction quality. The study findings suggest that quality control measures should acknowledge the significant variability in required particle numbers, which can often differ by an order of magnitude, contingent upon the specific combination of ventilation strategy and sampling volume. • Quality control strategies for Lagrangian simulation were evaluated. • At a ventilation rate of 2.7 h−1, minimum concentration ranged 0.0075–0.015 cm−3. • A 5 L sampling volume improves prediction reliability compared to 0.5 L and 1 L. • Minimum particle concentrations varied with ventilation strategy over 10 times. [ABSTRACT FROM AUTHOR]

Ozone has adverse effects on human health. Skin oil on the human surface acts as an ozone sink indoors, producing oxidation products that can cause skin and respiratory irritations. Concentrations of ozone and oxidation products near human surfaces, including the breathing zone, can be modulated by indoor ventilation modes and human surface conditions. The objective of this study is to examine concentrations and spatial heterogeneity of ozone and ozonolysis products under representative ranges of indoor ventilation, clothing, and breathing conditions. Using computational fluid dynamics (CFD) simulation in conjunction with a chemical kinetic model, details of ozone reactions with the human surface and subsequent chemical reactions are examined. The results show that primary ozonolysis products are concentrated near the soiled clothing, while the secondary products are relatively well distributed throughout the room. Increasing indoor air mixing enhances the ozone deposition to the human surface, thereby resulting in higher emission rates of oxidation products in the room. Soiled clothing consumes more ozone than clean clothing and accordingly produces ~ 65% more primary products and ~15% more secondary products. The results also reveal that unsaturated hydrocarbons from the human breath, such as isoprene, contribute to only ~0.5% of ozone removal compared to ozone deposition to the human surface. [ABSTRACT FROM AUTHOR]

In indoor environments, accretion of mass to materials may provide sites for surface chemistry that differ from those of the original material. Since indoor surfaces are a major sink of oxidant gases, surface mass accretion may impact indoor O 3 chemistry. In this study, the effect of surface mass accretion on O 3 surface deposition was tested by deploying cleaned borosilicate glass plates in two types of indoor environments: a mechanically ventilated (MV) office and a naturally ventilated (NV) residence located in Singapore. In each environment, seven replicate glass plates and one field blank were deployed for between 7 and 56 days and examined in a laboratory chamber for O 3 deposition rate and surface reaction probability. Average mass accretion to plates, deployed in a horizontal position and including deposited particles, was 10.6 mg/(m 2 d) in the MV office vs. 18.5 mg/(m 2 d) in the NV residence and the comparison is at the threshold of statistical significance ( p  = 0.054). Ozone reactivity to the plates increased in magnitude and persistence with longer plate deployment. Ozone reaction probabilities to cleaned plates prior to deployment ranged [0.06–0.74] × 10 −6 for two hours of observable removal whereas plates deployed for 56 days ranged [0.15–1.2] × 10 −6 for four hours of observable removal. Regressions of cumulative O 3 removed during chamber tests vs. mass accreted show removal of 4.3 nmol O 3 /mg for the NV residence and 2.4 nmol O 3 /mg for the MV office. These results imply that accretion of mass to surfaces may alter indoor O 3 transformation pathways. [ABSTRACT FROM AUTHOR]

Heat recovery ventilation (HRV) system is emerging as a sustainable technology to reduce human exposure to air pollutants in residential buildings. However, there is a lack of information on the HRV system performance for controlling both indoor-generated and outdoor-generated particles in a residential building. To fill this knowledge gap, we conducted field monitoring of indoor airborne particles with two representative HRV system operating modes (ventilation and circulation modes) in a full-scale residential testbed. The results reveal that the filter efficiency, airflow rate, and filter bypass factor of the HRV system play important roles in indoor particle concentrations. The indoor/outdoor concentration ratio is reduced by 72%–92% when operating the HRV system. For both circulation and ventilation modes, the bypass factor is crucial for the particle removal performance of the HRV system. A 20% filter bypass factor leads to up to a 50% increase in indoor exposure to outdoor-originated and indoor-generated PM 2.5 compared to the no-bypass scenario. Although circulation mode performed better than ventilation mode, both operating modes with a minimal bypass factor can effectively remove particles generated from indoor emission sources (e.g., incense stick burning and bacon pan frying). The study results suggest that HRV system can reduce human exposure to indoor-generated particles by 56%–90% and particle control performance hinges upon minimizing HRV filter bypass. • We investigate indoor particle removal due to a heat recovery ventilation system. • The HRV system can reduce human exposure to indoor-generated particles by 50–90%. • The filter bypass factor is crucial for the HRV particle control performance. • A 20% filter bypass factor can lead up to 50% increase in indoor PM2.5 exposure. [ABSTRACT FROM AUTHOR]

The objective of this study is to evaluate how the removal and dispersion of the cooking-generated particles in a residential unit vary with ventilation strategies. Computational fluid dynamics (CFD) simulations were performed to evaluate the particle removal and dispersion performance of six ventilation strategies with three make-up air supply scenarios and two kitchen range hood designs. The results showed that the particle capture efficiency ranged from 0.58 to 1.00 for 2.5 μm particles, and from 0.62 to 0.96 for 10 μm particles depending on the ventilation strategies. The peak particle number concentration of the breathing zone in the kitchen was the lowest when supplying the make-up air from the whole house ventilation system, which was only 0.07% of the peak concentration with supplying natural make-up air through a kitchen window. When using the make-up air from the whole house ventilation system, 1-h integrated exposure was decreased by 92.1% for the kitchen, 89.4% for the living room, and 98.7% for the bedroom, compared to the natural make-up air supply scenario. The results reveal that supplying make-up air from the living room and bedrooms while operating a typical range hood yields the largest particle removal, suggesting effective control of residential exposure to cooking-generated particles. • Particle dispersion and removal of six ventilation strategies were explored. • 1-h integrated exposure reduced by 92.1% depending on the make-up air scenario. • Make-up air from diffusers of each room is effective for removal and dispersion. • Utilizing the whole house ventilation system as a make-up air supply has benefits. [ABSTRACT FROM AUTHOR]

Abstract: Airborne particle entry into buildings is important for human exposure to particles and associated health effects. The present study investigated the entry of size-resolved outdoor ultrafine particles into a test building under three different ventilation scenarios using a multi-zone airflow and contaminant transport model. Simulations of the entry of outdoor ultrafine particles into a residential test building were performed and validated with measurement data. The study results show that accurate particle deposition and penetration inputs are required to predict the time-varying particle concentrations in buildings. For closed-window conditions, both deposition and penetration have significant effects on modeling UFP transport, while deposition is more important than penetration for open-window conditions. As the window opening area increases, the filtering effect of the building envelope decreases and more outdoor particles enter the building through window openings. The study results also show that the indoor–outdoor (I–O) concentration ratio is a strong function of particle size and building operating conditions. The comparison between measurements and prediction suggests that a multi-zone particle transport model can provide insight into particle entry into buildings under various weather and building operating scenarios. [Copyright &y& Elsevier]

Indoor ultrafine particles (UFP, <100 nm) undergo aerosol processes such as coagulation and deposition, which alter UFP size distribution and accordingly the level of exposure to UFP of different sizes. This study investigates the decay of indoor UFP originated from five different sources: a gas stove and an electric stove, a candle, a hair dryer, and power tools in a residential test building. An indoor aerosol model was developed to investigate differential effects of coagulation, deposition, and ventilation. The coagulation model includes Brownian, van der Waals, and viscosity forces, and also fractal geometry for particles of >24 nm. The model was parameterized using different values of the Hamaker constant for predicting the coagulation rate. Deposition was determined for two different conditions: central fan on versus central fan off. For the case of a central fan running, deposition rates were measured by using a nonlinear solution to the mass balance equation for the whole building. For the central fan off case, an empirical model was used to estimate deposition rates. Ventilation was measured continuously using an automated tracer gas injection and sampling system. The study results show that coagulation is a significant aerosol process for UFP dynamics and the primary cause for the shift of particle size distribution following an episodic high-concentration UFP release with no fans operating. However, with the central mechanical fan on, UFP deposition loss is substantial and comparable to the coagulation loss. These results suggest that coagulation should be considered during high concentration periods (>20,000 cm−3), while particle deposition should be treated as a major loss mechanism when air recirculates through ductwork or mechanical systems. Copyright 2012 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Abstract: Ventilation effectiveness is an indicator of the quality of supply air distribution in ventilated rooms. It is a representation of how well a considered space is ventilated compared to a perfect air mixing condition. Depending on pollutant properties and source position relative to the airflow, ventilation effectiveness can more or less successfully be used as an indicator of air quality and human exposure. This paper presents an experimentally and numerically based study that examines the relationship between ventilation effectiveness and particle concentration in typical indoor environments. The results show that the relationship varies predominantly with airflow pattern and particle properties. Fine particles (1μm) follow the airflow pattern more strictly than coarse particles (7μm), and the high ventilation effectiveness indicates better removal of fine particles than coarse particles. When a ventilation system provides high mixing in the space and ventilation effectiveness is close to one, particle sizes and source location have a relatively small effect on particle concentration in the breathing zone. However, when the supply air is short circuited and large stagnation zones exist within the space, the particle concentration in the breathing zone varies with particle size, source location, and airflow pattern. Generally, the results show that for fine particles (1μm), increase of ventilation effectiveness reduces occupant exposure; while for coarser particles (7μm), source location and airflow around the pollutant source are the major variables that affect human exposure. [Copyright &y& Elsevier]

A uniform pollutant concentration in indoor environments can be an inappropriate representation of breathing concentration. This is especially true when local airflow in the vicinity of an occupant is dominant in transporting pollutants. The present study investigates the airflow in the vicinity of a human body, effects of respiration on breathing concentration of particulate and gaseous pollutants, and inhalation exposure in relation to source position and overall airflow patterns. It is based on experiments with a human simulator in a full-scale environmental chamber. Airflow and pollutant concentrations in the vicinity of a thermal manikin are monitored, while varying parameters including breathing, arm/hand movements, and ventilation system. Results show that breathing of a sedentary manikin has a measurable influence on the airflow in breathing zone, whereas it has very small impacts on occupant thermal plume. Also, localized hand motions have insignificant effects on the thermal plume. The results indicate that overall airflow pattern affect the inhaled particle concentrations. With highly mixed airflow in the space, relatively uniform concentration patterns occur in the occupant vicinity. However, with stratified airflow patterns, non-uniform concentration patterns are observed due to the occupant thermal plume. With a particle source at floor level and in near proximity to an occupant, inhaled particle concentrations are up to four times higher than the ambient concentrations. This finding implies that occupant thermal plume may play a significant role in transporting pollutants from floor level to the breathing zone. The non-uniform concentration observed with stratified flow also suggests caution in estimating inhalation exposure using a “well-mixed” mass balance model. [Copyright &y& Elsevier]

• We examine impacts of ventilation rate and filter efficiency on indoor air pollution. • In polluted cities, high ventilation rates severely affect indoor air quality. • Adding an outdoor air intake filter is effective to control PM 2.5 of outdoor origin. • High efficiency filters (>MERV 11) are needed in extreme outdoor pollution. Building systems play a critical role in determining indoor concentrations of pollutants of outdoor origin in polluted cities. The objective of this study is to investigate impacts of outdoor ventilation flow rates and filter efficiencies on indoor concentrations of PM 2.5 and ozone for office buildings in two mega-cities: Los Angeles and Beijing. Using the U.S. Department of Energy medium office building model, parametric analysis was performed to examine indoor pollutant concentrations under varied filter configurations, ventilation modes and outdoor concentrations. The results show that for office buildings located in polluted cities, two-filter (air handling unit filter and outdoor air filter) systems are more resilient to outdoor PM 2.5 pollution than single filter (air handling unit filter only) systems, especially when outdoor ventilation air flow rate is high due to economizer ventilation or dedicated outdoor air systems. However, for ozone removal, adding an outdoor filter makes a marginal difference. The study results also suggest that during extreme outdoor pollution events in mega-cities such as Los Angeles and Beijing, high efficiency filters (>MERV 11) and reduced flow rates (<8.5 L/s per person) should be used to limit indoor PM 2.5 concentration below 35 μg/m3. [ABSTRACT FROM AUTHOR]

Hydroxyl (OH) radicals are highly reactive species that can oxidize most pollutant gases. In this study, high concentrations of OH radicals were found when people were exposed to ozone in a climate-controlled chamber. OH concentrations calculated by two methods using measurements of total OH reactivity, speciated alkenes, and oxidation products were consistent with those obtained from a chemically explicit model. Key to establishing this human-induced oxidation field is 6-methyl-5-hepten-2-one (6-MHO), which forms when ozone reacts with the skin-oil squalene and subsequently generates OH efficiently through gas-phase reaction with ozone. A dynamic model was used to show the spatial extent of the human-generated OH oxidation field and its dependency on ozone influx through ventilation. This finding has implications for the oxidation, lifetime, and perception of chemicals indoors and, ultimately, human health. [ABSTRACT FROM AUTHOR]