CARBON-MONOXIDE EXPOSURES INSIDE AN AUTOMOBILE TRAVELING ON AN URBAN ARTERIAL HIGHWAY

Carbon monoxide (CO) exposures were measured inside a motor vehicle du ring 88 standardized drives on a major urban arterial highway, El Cami no Real (traffic volume of 30,500-45,000 vehicles per day), over a 13- 1/2 month period. On each trip (lasting between 31 and 61 minutes), th e test vehicle drove the same 5.9-mile segment of roadway in both dire ctions, for a total of 11.8 miles, passing through 20 intersections wi th traffic lights (10 in each direction) in three California cities (M enlo Park, Palo Alto, and Los Altos). Earlier tests showed that the te st vehicle was free of CO intrusion. For the 88 trips, the mean CO con centration was 9.8 ppm, with a standard deviation of 5.8 ppm. Of nine covariates thal were examined to explain the variability in the mean C O exposures observed on the 88 trips (ambient CO at two fixed stations , atmospheric stability, seasonal trend function, time of day, average surrounding vehicle count, trip duration, proportion of time stopped at lights, and instrument type), a fairly strong seasonal trend was fo und. A model consisting of only a single measure of traffic volume and a seasonal trend component had substantial predictive power (R2 = 0.6 8); by contrast, the ambient CO levels, although partially correlated with average exposures, contributed comparatively little predictive po wer to the model. The CO exposures experienced while drivers waited at the red lights at an intersection ranged from 6.8 to 14.9 ppm and dif fered considerably from intersection to intersection. A model also was developed to relate the short-term variability of exposures to averag ing time for trip times ranging from 1 to 20 minutes using a variogram approach to deal with the serial autocorrelation. This study shows: ( 1) the mass balance equation can relate exterior CO concentrations as a function of time to interior CO concentrations; (2) CO exposures on urban arterial highways vary seasonally; (3) momentary CO exposures ex perienced behind red lights vary with the intersection; and (4) an ave raging time model can simulate exposures during short trips (20 minute s or less) on urban arterial highways.