EXPOSURE OF COMMUTERS TO CARBON-MONOXIDE IN MEXICO-CITY .3. COMPARISON OF IN-VEHICLE AND FIXED-SITE CONCENTRATIONS

The objective of this paper is to compare measurements of carbon monox ide taken concurrently inside vehicles and at fixed-site monitoring st ations (FSMs), in order to assess if the FSM stations can be used to e stimate commuters' exposure to this pollutant. During the study period ambient CO concentrations were very high. Five selected stations repo rted concentrations in excess of the Mexican (13 ppm) and United State s (9 ppm) 8-hour standards for CO. Since, for ail modes of transportat ion, the in-vehicle concentrations were always larger than the concurr ent ambient concentrations, the differences between them were always p ositive and the ratios were always greater than one. Average, in-vehic le/ambient ratios for each mode of transportation were: automobile, 5. 2; minivan, 5.2; minibus, 4.3; bus, 3.1; trolleybus 3.0; and metro, 2. 2. A series of simple regression models with a moderate predictive pow er (R(2) = 0.47 to 0.71) were developed for metro, bus, minibus, and a utomobile commuters. The models include the FSM measurements and also, depending on the mode of transportation, other variables, such as veh icular speed the route of travel and the wind speed. In the future, th e models should be validated in two ways to determine their predictive power. First, they should be verified against additional samples take n under similar conditions; and second, their applications under diffe rent conditions should be explored through sampling during a different season of the year or on other commuting routes.