A New Exposure Metric for the Cumulative Effect of Short-term Peaks of Exposure to Traffic-related Ultrafine Particles

Abstract

Introduction Traffic-related ultrafine particles (UFPs) can cause adverse health outcomes through inflammatory reactions. Current epidemiology studies focus on either short-term health outcomes associated with UFP levels averaged over days or weeks, or long-term outcomes associated with a long-term (annual or longer) average exposure to UFP. We hypothesize that frequent and repeated exposure to short-term UFP peaks that last for just one or a few hours could overwhelm human’s biological defenses, which may result in long-term health effects. We propose a new exposure metric for measuring the cumulative effect of the peaks. Method Our study uses data collected by the Community Assessment of Freeway Exposure and Health (CAFEH) project, which recruited 704 participants from six “near-highway” or “urban background” neighborhoods in the Greater Boston Area between 2009 and 2012. The CAFEH project developed land use regression models to estimate hourly averages of ambient UFP level throughout the study areas, and applied time-activity adjustment to calculate the adjusted hourly estimate. Our alternative metric is the cumulative peak exposure, determined by the intensity (the 99th percentile of an individual’s adjusted hourly UFP estimates) and the frequency (the number of hours with the UFP level higher than the 99th percentile of all adjusted hourly UFP estimates of all participants) of UFP peaks. Results The cumulative peak exposure does not always correlate well with the annual average, especially in certain near-highway neighborhoods. (Pearson’s R between the intensity of UFP peaks and the annual average of hourly UFP estimates ranges from 0.33 to 0.98.) Thus, it appears that there is still sufficient variation within the residents’ UFP peak exposure after holding their annual average constant, which suggests they are distinct measurements. Conclusion Measuring the cumulative effect of peak exposures may provide a new exposure assessment metric that could be tested for association with long-term health outcomes.