A survey was carried out to measure commuters' exposure to PM2.5, CO, benzene, and the chemical composition of PM2.5on different routes and modes of transport in Mexico City. PM2.5(n=62), CO (n=54) and benzene (n=22) are presented from morning (6:30-9:30 a.m.) and evening (17:30-20:30) rush hours on minibuses, buses and Metro (underground or subway system). Three routes were selected from a previous commuters' exposure study covering some of the most important thoroughfares of the valley. For PM2.5, mass concentration was determined for all the samples. Nitrates, sulphates, inorganic elements and carbon fraction were analysed. CO was sampled using electrochemical sensors and 6-l canisters with flow controller devices were used to collect integrated samples for benzene. Minibuses had a slightly higher geometric mean PM2.5concentration in the morning than other modes of transport, but the ranking of geometric mean PM2.5by mode of transport is opposite in the evening and the variability within modes is approximately double the difference between modes. The highest single measurement was a concentration of 137μgm-3on a bus during an evening rush hour. The main component identified in PM2.5was carbon. Carbon monoxide levels in this study were approximately 3 times lower than those found in a commuter exposure study conducted in 1991. A strong association was shown between wind speed and PM2.5exposure in minibuses (r2=0.50) and buses (r2=0.54). The relationship between wind speed and CO exposure was strong only in minibuses (r2=0.52). © 2003 Elsevier Ltd. All rights reserved.
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