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.

A survey of street sellers' exposure to carbon monoxide in Mexico City.

The aim of this study was to explore the CO exposure experienced by street sellers working on busy roads in Mexico City. In January 1991, CO measurements were taken at street level of selected avenues in the city centre. A survey of the numbers of street sellers working on the monitored streets, and their working schedules, was also undertaken. Pavement CO levels were compared with the concurrent concentrations measured at the nearest fixed-site monitoring station. Short term pavement CO concentrations ranged from 2.0 to 70.0 ppm with a mean concentration of 26 ppm. There was a significant positive correlation between pavement and fixed-site CO concentrations. CO concentrations at the pavements were consistently higher than the concurrent fixed-site monitor levels; the average ratio of street/fixed-site concentrations was 2.2. There were more than 1000 street sellers working in the surveyed avenues. More than 80% of the sellers reported that they work at least 6 days a week, with an average working shift of 10 hours per day. The findings of this study suggest that street sellers in Mexico City are exposed to CO concentrations well above national and international air quality criteria. Further research is needed to evaluate the health effects caused by CO exposures among such occupationally exposed groups.