Impact of percentile computation method on PM 24-h air quality standard.

In 1997, the US Environmental Protection Agency (US EPA) introduced a percentile form of the National Ambient Air Quality Standard (NAAQS) for particulate matter (PM). Studies had shown that a specified percentile in the frequency distribution of measured values of PM increased the probability of detecting non-attainment areas (power) and decreased the likelihood of misclassification of attainment areas as being non-attainment (type 2 error). However, this new NAAQS used a percentile form that was different from a standard percentile in a distribution. Instead of taking the percentile of the distribution of the required 3 years of measurements, the PM(2.5) values for the selected percentile for each year were determined and the average of these 3 values was used as the NAAQS indicator value. However, no studies have been made of this average of the 3 years method and compared to a standard percentile in the multiyear data. The relationships between the values obtained using these two approaches have been explored. PM data measured at selected US EPA Aerometric Information Retrieval System (AIRS) website from January 2004 to December 2008 at 20 sites in 20 different states in United States were utilized. PM samples were collected for 24-h periods from midnight to midnight every third day for PM(2.5) and every sixth day for PM(10). At some sites, continuous measurements of PM(2.5) were made and averaged to provide 24-hr values. Using these data, the NAAQS percentile values were compared with the actual 98th percentile values of the three years of data. Regression and t-test analyses were used to compare these two methods and found high correlation coefficients and no significant difference in most cases. Overall, the two methods showed substantial agreement such that either of the two approaches could serve as the statistical form of the 24-h standard. In exploring the PM(10) standard, an arbitrarily chosen standard value of 85 μg/m(3) was used to explore the development of more stable implementation programs and provide better protection to public health against short term exposures.