Characterizing Ultrafine Particle Exposures in Two Types of Indoor Environments: San Francisco Bay Area Classrooms and Beijing High-Rise Apartments
Ultrafine particles are defined as those particles having a diameter of 100 nm or less. They are emitted by both indoor and outdoor sources and are ubiquitous in the environment. Epidemiological studies have indicated that ultrafine particle (UFP) exposures are associated with adverse health outcomes, and toxicological studies have suggested that this is mechanistically possible. At present, the mass concentrations of fine particles having a diameter of 2.5 micrometers or less (i.e. PM2.5) are monitored and regulated in most developed countries in the world. However, UFP concentrations correlate poorly with PM2.5 concentrations; thus, the extensive outdoor PM2.5 data available cannot be used to draw inferences regarding UFP exposure concentrations, and efforts to reduce PM2.5 levels cannot be expected to reduce UFP levels. While a growing number of studies have investigated UFP concentrations both indoors and outdoors over the last two decades, there remain many microenvironments in which UFP exposures have not been characterized. In this dissertation, UFP exposure concentrations are characterized and the factors influencing those concentrations are explored, within two microenvironments that had hitherto not been investigated: San Francisco Bay Area elementary school classrooms and Beijing high-rise apartments.
Children between the ages of 6 and 11 years old living in California spend an average of 10% of their time in school, second only to the amount of time spent at home (53%). In addition, children are considered to be more susceptible to some health effects resulting from pollutant exposures than are adults. To contribute towards a characterization of children's exposure to ultrafine particles, a field study was conducted in six classrooms in the San Francisco Bay Area. The purpose of this study was to provide data regarding children's UFP exposures in school classrooms, the contributions of indoor and outdoor sources to those exposures, and the influence of building parameters and occupant behaviors on those exposures. Additional aims were to characterize the classroom ventilation rates, and to explore the balance between maintaining adequately high ventilation for the removal of bioeffluents and other indoor emitted pollutants, while also seeking to limit the indoor proportion of outdoor particles (IPOP). The data collection phase of this study involved monitoring particle number (PN) concentrations and the concentrations of three gaseous co-pollutants (CO2, NO, O3) for two to four school days in each classroom. Time-resolved data on classroom ventilation characteristics and occupant activities were recorded using temperature and state-change sensing data loggers, and by a researcher who was present in the classroom for the duration of the school day. In all, 18 days of data were collected from June to December 2008.
The average indoor PN concentration during periods of student occupancy in the six classrooms ranged from 5.2 x 103 to 16.5 x 103 cm-3. Indoor sources had a relatively small influence on classroom PN concentrations, with only three significant source events detected during periods of student occupancy across the six classrooms. For this small sample of admittedly limited scope, the classrooms monitored in warmer months (i.e., June through early November) had both a higher outdoor and indoor average PN concentration during periods of student occupancy than those monitored during colder months (i.e., late November and early December). This higher exposure to outdoor generated particles during warm months was influenced by more frequent opening of doors and windows for the purpose of maintaining a comfortable temperature in the classroom. The mean daily-integrated UFP exposures of the students while in their classrooms was 50,000 cm-3 h d-1, which was approximately a factor of 6 less than the mean exposure calculated in a parallel study for a sample of children in San Francisco Bay Area homes. The higher daily-integrated exposure experienced by children in homes is partly attributable to the higher PN concentrations measured in homes during hours of occupancy than in schools, and partly a result of the greater time that children spend in their home on a daily basis as compared to their classrooms. For these classrooms, outdoor PN concentrations measured on-site appear to be a good indicator of the relative exposure concentrations encountered by students within their classrooms. The utility of outdoor data for predicting exposures indoors depends critically on the dominance of outdoor air as the source of indoor PN levels.
The time-weighted average air-exchange rate for the six classrooms ranged from 1.1 to 10.8 h-1, and the accompanying range for the rate of ventilation per person was 4 to 27 L/s. Two of the classrooms utilized mechanical ventilation systems, while four were ventilated by means of doors and windows. In the case of the naturally ventilated classrooms, the ventilation rate generally exceeded the standard specified by the American Society of Heating Refrigerating and Air-Conditioning Engineers (ASHRAE) when doors and/or windows were in an open state, but often fell below the standard otherwise. For the mechanically ventilated classrooms, the air-exchange rate appeared unnecessarily high in one case and too low in the other. Results from five of the six sites were analyzed to see if an increase in the air-exchange rate was accompanied by an increase in the IPOP; for four of the classrooms the data were so correlated. However, reducing the air-exchange rate as a strategy for decreasing the indoor level of outdoor generated particles is not recommended, and instead strategies were investigated for reducing the IPOP using active filtration.
The work presented here suggests that outdoor sources may be a more important contributor than indoor sources to UFP concentrations in Bay Area classrooms. Therefore, strategies to reduce classroom UFP concentrations may be most effective if focused on decreasing the IPOP. The classroom air-exchange rate results indicate that teachers in naturally ventilated classrooms should be encouraged to keep windows and/or doors in the open state during periods of student occupancy to maintain adequate ventilation. In classrooms with mechanical ventilation systems, more attention may need to go towards ensuring that the classroom ventilation rate is neither too high nor too low. Since the IPOP is expected to and seen to increase with an increase in the air-exchange rate, it is recommended that strategies to increase classroom ventilation be accompanied with active filtration, ether via portable fan-filter air cleaners or through use of high efficiency in-duct filters. The results presented here were collected from a relatively small sample of sites. Thus, to the extent that children's exposure to ultrafine particles is considered an issue of concern, these results should be augmented by further research conducted in a larger sample of Bay Area schools.
Roughly 20% of the world's population lives in China, and yet research groups have only recently begun to investigate UFP concentrations in this region of the world. Studies investigating UFP concentrations in mainland China have thus far focused on the outdoor environment. Since people generally spend the majority of their time indoors, data are needed on the UFP exposure concentrations encountered in indoor microenvironments in China, so that population exposures in mainland China can be accurately characterized. To contribute towards filling this research gap, a field study was conducted in a sample of high-rise apartments in Beijing, one of the largest cities in China, with a population of roughly 20 million. In the past three decades, newly constructed housing developments in Beijing have primarily taken the form of high-rise buildings. The data collection phase of this study involved monitoring PN within four high-rise apartments for two to four days each. For two apartments, outdoor PN data were also collected. Temperature and state-change data loggers were used to record when occupant activities involving heat (e.g., cooking) were conducted and when door and window positions were changed, respectively. The residents also maintained a journal of their activities and the hours they were present at home. In all, ~9 days of time-series data were collected.
Distinct indoor PN peaks independent of outdoor concentrations were observed on twenty-seven occasions during monitoring at the four apartments. Cooking was responsible for the majority of the observed indoor PN peaks. In one apartment, although the residents cooked infrequently themselves, a large number of indoor peaks appeared to result from the infiltration of emissions from cooking in neighboring apartments. The average indoor PN concentrations at the four apartments ranged from 2,800 to 29,100 cm-3. The apartment with the highest indoor concentration was influenced by the neighbors cooking, and the apartment with the lowest concentration only experienced two indoor PN peaks in two days and had two portable fan-filter air cleaners that operated almost continuously. For the apartments where outdoor PN data were also collected, 58% and 81% of the residents' total UFP exposure while at home was attributed to outdoor sources. Conversely, in a study of seven single-family homes in the San Francisco Bay Area, an average of 30% of the residents' exposure was attributed to outdoor sources. The greater indoor exposure to outdoor particles in the former case is expected to have resulted from the higher outdoor concentration during hours the residents spent asleep, the larger fraction of time the residents spent at home and the greater use of natural ventilation. Particle emission rates were calculated for some of the cooking events in the Beijing apartments, and the average was almost identical to the average calculated for natural gas cooking events in the study of Bay Area homes.
The results from this study indicate that cooking makes a significant contribution to exposure in some Beijing apartments. However, outdoor generated particles make a larger contribution overall. An issue that requires further attention is whether or not cross-contamination between apartments is common in Beijing high-rise buildings. This study provides initial results for UFP concentrations in Beijing residences, and elucidates some of the factors that influence exposures in high-rise apartments. It is important that this study be reinforced with further study of different aspects of UFP exposure both in high-rise buildings and other types of residential structures in Beijing and throughout China, so that the UFP exposures of population living in China can be characterized, and, if necessary, controlled.