Statistical Study of Variables Associated with Particulate Matter Exposure Levels at Bus Shelters

Abstract

This study expands on previous work that examined differences in exposure to particulate matter in and around bus stop shelters for passengers waiting along a busy urban corridor in Portland, Oregon. An extensive body of literature has demonstrated the negative health effects of exposure to particulate matter. Although concentrations of particulate matter were known to be greater near busy roadways, little research has been conducted on exposure in and around bus stop shelters. Two sizes of particulate matter were examined in this study: fine particulate matter of less than 2.5 microns in aerodynamic diameter (PM_2.5) and ultrafine particles. Pearson association tests were run between particulate concentrations and three categories of independent variables: location, traffic, and weather. Significant correlations were observed primarily between particulates and weather (temperature and relative humidity). With 1-min data intervals, a series of log-linear regression models with and without lagged variables was used to estimate the effects of location, traffic, and weather variables on particulate concentrations. The presence of a transit bus stopped at the shelter significantly increased both sizes of particulate matter concentrations. Wind, temperature, and shelter location also had significant effects on ultrafine and PM_2.5 levels. The estimated models for particulate concentrations inside and outside the bus stop shelters were compared to demonstrate differences in particulate behavior. Suggestions are made for shelter configuration given environmental and traffic considerations.