Bioaerosol Deposition on an Air-Conditioning Cooling Coil

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

Exposure to particulate matter and ozone of outdoor origin in Singapore

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

Indoor and outdoor particles in an air-conditioned building during and after the 2013 haze in Singapore

Particles released from biomass burning can contribute to severe air pollution. We monitored indoor and outdoor particles in a mechanically ventilated and air-conditioned building during and after the 2013 haze event in Singapore. Continuous monitoring of time-and size-resolved particles in the diameter range 0.01-10 μm was conducted for two weeks in each sampling campaign. During the haze event, the averaged size-resolved outdoor particle volume concentrations (dV/d(logDp)) for diameters larger than 0.3 μm were considerably higher than those during the post-haze days (9-185 μm3 cm-3 versus 1-35 μm3 cm-3). However, the average number concentration of particles with diameters in the range 10-200 nm was substantially lower on the hazy days than on the post-haze days (11,400 to 14,300 particles cm-3 for hazy days, versus an average of 23,700 particles cm-3 on post-haze days). The building mechanical ventilation system, equipped with MERV 7 filters, attenuated the penetration and persistence of outdoor particles into the monitored building. Indoor particle concentrations, in the diameter ranges 0.3-1.0 μm and 1.0-2.5 μm, closely tracked the corresponding patterns of outdoor particle concentrations. For particles in the size range 0.01-1.0 μm, the size-resolved mean indoor/outdoor (I/O) ratios were in the range 0.12-0.65 with the highest mean I/O ratio at 0.3 μm (0.59 in AC on mode and 0.64 in AC off mode). The air conditioning and mechanical ventilation system with MERV 7 filters provided low single-pass removal efficiency (less than ~ 30%) for particles with diameters of 0.01-1.0 μm. During the haze, for particles larger than ~0.2 μm, lower I/O ratios and higher removal efficiencies occurred with the air conditioning operating as compared to with mechanical ventilation only. This observation suggests the possibility of particle loss to air conditioning system surfaces, possibly enhanced by thermophoretic or diffusiophoretic effects

Occurrence, human exposure and dynamics of phthalates in ACMV indoor environments : method development and field studies

Phthalates are a group of common indoor semi-volatile organic compounds (SVOCs) that can concurrently exist in gas-phase, airborne particle-phase and settled dust phases, due to their special physicochemical properties. Human exposure to phthalates in indoor environments has raised public concerns owing to the identified associations between phthalates and a number of adverse human health effects. This study first develops sampling and analytical methods for field studies of airborne phthalates. Based on the methods, the study investigates occurrence of typical phthalates in indoor air of air-conditioned environments and assesses the impacts of air-conditioning and mechanical ventilation (ACMV) system on indoor phthalate concentrations. The study further examines phthalate concentration differences in indoor air and in personal air. More importantly, the study investigates the impacts of human induced particle resuspension and generation on airborne phthalates concentrations. Meanwhile, it evaluates gas/particle partitioning of selected phthalates when there are different levels of particle concentrations in real indoor environments. In the development of sampling and extraction method, works mainly focus on testing homemade two-bed sorbent tubes with silanized glass wool and Tenax TA as sorbents for airborne phthalate sampling. The sorbent tubes can sample airborne phthalate at low air flow rate for shorter sampling times, improve detection limits of low-volatile phthalates, and are compatible with thermal desorption (TD) method. The development of analytical method tests the extraction efficiency of the tubes. The extraction procedures and parameters are optimized. Important issues including carry-over, recovery, repeatability and linearity are considered. The study investigates occurrence of five typical phthalates in 24 ACMV indoor environments that are covered with different flooring materials. The results show that bis(2-ethylhexyl) phthalate (DEHP) has higher airborne concentrations in the involved indoor environments when compared with the literature data. There is also evidence that the rooms covered by polyvinyl chloride have apparently higher DEHP concentrations than those covered by carpets. The calculated whole room emission rates suggest that there are strong source strengths of DEHP and DBP (di-n-butyl phthalate) in the indoor environments. The study evaluates the impacts of operations of the ACMV system on indoor concentrations of DEHP and DBP in a typical office. The results indicate that indoor DEHP and DBP concentrations accumulate when the ACMV system is off, but decrease continuously to certain quasi-stable levels when the ACMV system is on. The evidence demonstrates that the ACMV system does have certain level of impacts on the indoor concentrations of the both high-volatile and low-volatile SVOCs. The study compares personally sampled phthalates with the corresponding phthalates in the bulk indoor air in three rooms. The results show that there is no detectable difference between the personal concentrations and the environmental concentrations. The explanation may be that the sedentary subjects’ activities are not strong enough to resuspend settled particles, which carry significantly higher phthalate concentrations. This study conducts intentional particle resuspension and generation in well-controlled real rooms to understand the impacts of human activities on airborne phthalate concentrations as well as their gas/particle partitioning variations. The results suggest that increased airborne particle concentration due to both particle resuspension and generation can result in obvious increases in airborne concentrations of phthalates with low volatilities. The increases are because of either resuspension of phthalates bound to settled dust or partitioning of gas-phase phthalates to the airborne particles. The findings also demonstrate that the gas fractions of the low-volatile phthalates decrease when the airborne particle concentrations increase.Doctor of Philosophy (CEE

Human health and thermal comfort of office workers in Singapore

Poorly operated air conditioning and mechanical ventilation (ACMV) system might cause significant Sick Building Syndrome (SBS) symptoms and thermal discomfort in the hot and humid climate. This study presents our investigations on the prevalence of SBS symptoms and thermal comfort in offices in Singapore via two approaches including: (1) the onsite objective monitoring and questionnaire-based investigation under normal ACMV practices, and (2) the online survey with occupants in controlled indoor temperatures. The results indicate that the prevalence of individual SBS symptoms is lower than the similar studies in other geographic regions. Overcooling seems to be the domineering complaint in the local context and the occupants seem to prefer higher indoor temperature. As such, human behavioral adjustments such as adding clothing happen quite frequently. Moreover, the data also suggests that cultural traits might skew the survey results, especially in certain subjective aspects regarding the satisfactory level and comfort. To sum up, the prevalence of SBS symptoms are generally acceptable in current local context. However, due to the hot and humid ambient environment, traditional ACMV system with vapor compression refrigerant tends to trigger the overcooling issue in relation to the sensible and latent heat. It is invaluable to advance our understanding of the relationships between the ACMV system, human behavioral adjustments, and building energy consumptions in the tropical region

Indoor/outdoor particle relationships in tropical classroom

Particulate matter (PM) might cause various adverse human health effects, especially for susceptive populations such as children (Pope III, 2000). As children spend significant amount of their times in schools; it is desirable to understand their PM exposure levels in such environment.Accepted versio

Indoor and Outdoor Particulate Matter in Primary School Classrooms With Fan-Assisted Natural Ventilation in Singapore

We conducted multiday continuous monitoring of indoor and outdoor particulate matter (PM) in classrooms with fan-assisted natural ventilation (NV) at five primary schools in Singapore. We monitored size-resolved number concentration of PM with diameter 0.3–10 μm at all schools and alveolar deposited surface area concentrations of PM with diameter 0.01–1.0 μm (SA0.01–1.0) at two schools. Results show that, during the monitoring period, schools closer to expressways and in the downtown area had 2–3 times higher outdoor PM0.3–1.0 number concentrations than schools located in suburban areas. Average indoor SA0.01–1.0 was 115–118 μm2 cm−3 during periods of occupancy and 72–87 μm2 cm−3 during unoccupied periods. There were close indoor and outdoor correlations for fine PM during both occupied and unoccupied periods (Pearson’s r = 0.84–1.0) while the correlations for coarse PM were weak during the occupied periods (r = 0.13–0.74). Across all the schools, the size-resolved indoor/outdoor PM ratios (I/O ratios) were 0.81 to 1.58 and 0.61 to 0.95 during occupied and unoccupied periods, respectively, and average infiltration factors were 0.64 to 0.94. Average PM net emission rates, calculated during periods of occupancy in the classrooms, were lower than or in the lower range of emission rates reported in the literature. This study also reveals that indoor fine and submicron PM predominantly come from outdoor sources, while indoor sources associated with occupancy may be important for coarse PM even when the classrooms have high air exchange rates

Exposure to Particulate Matter and Ozone of Outdoor Origin in Singapore

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