Exposure of commuters to carbon monoxide in Mexico City II. Comparison of in-vehicle and fixed-site concentrations.

The objective of this paper is to compare measurements of carbon monoxide taken concurrently inside vehicles and at fixed-site monitoring stations (FSMs), in order to assess if the FSM stations can be used to estimate commuters' exposure to this pollutant. During the study period ambient CO concentrations were very high. Five selected stations reported concentrations in excess of the Mexican (13 ppm) and United States (9 ppm) 8-hour standards for CO. Since, for all modes of transportation, the in-vehicle concentrations were always larger than the concurrent ambient concentrations, the differences between them were always positive and the ratios were always greater than one. Average, in-vehicle/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 power (R2 = 0.47 to 0.71) were developed for metro, bus, minibus, and automobile commuters. The models include the FSM measurements and also, depending on the mode of transportation, other variables, such as vehicular speed, the route of travel, and the wind speed. In the future, the models should be validated in two ways to determine their predictive power. First, they should be verified against additional samples taken under similar conditions; and second, their applications under different conditions should be explored through sampling during a different season of the year or on other commuting routes.