Frequency, duration and concentration of intermittent exposures to ambient air pollutants at elevated levels

摘要

Background: Various health effects are known to be affected by ambient air pollution. Despite continued efforts on improving ambient air quality, the anticipated benefit on health effects did not seem to coincide. Some of the adverse effects with air pollutants may be induced by acute exposure to high concentrations, but the occurrence and long-term trends of such exposure events have not been well characterized. Aims: To describe high air pollution events with respect to event frequency, duration and level for better exposure characterization. Methods: Historical data were obtained from a local air quality monitoring station, and daily concentrations for particulate matter (PM10) were calculated. Various screening values were used to define high pollutant levels. Any consecutive days with concentration above the screening level were considered the same event. For each event, the total length (in days), average concentration and maximum concentration were extracted. Probability distributions were calculated for event occurrence, event duration as well as cumulative exposure levels. Further, lag times between two air pollution events were recorded for comparison. Results: Six years of monitoring data were obtained for this analysis (n = 2,104). The daily PM10 concentration exceeded 125 mcg/m3 257 times, yielding occurrence probability of 0.12. The duration for each event ranged from 1 to 7 days and averaged 1.96 days. For each event, the cumulative exposure level ranged from 125.08 to 1187.88 mcg/m3-day. Substantial variation was also found in length between two air pollution events, ranging from 1 to 40 days in dry season and up to 199 days in rainy seasons. The annual average concentration decreased in the last two years, and air pollutant events occurred at a lower frequency and lasted for shorter time, however, the effects were more likely due to increase rain days. Conclusions: Exposure to elevated air pollutants may be considered intermittent events and should be considered when evaluating for potential impacts on human health.