measurements of carbon monoxide and lead levels were made under two bridges, built to carry motorways over roads trafficked by 16000 and 6000 vehicles/day, in order to investigate whether pollutant levels were higher than those normally found at open sites. the monitoring equipment was located in the centre of both bridges and also near a gap in one bridge. the average carbon monoxide levels found were 3.8, 3.7, and 5.3 ppm and the average lead levels were 2.4, 2.7, and 3.9 mu g/m3. a detailed analysis was made of the carbon monoxide data. it was not possible with the lead data because of the much longer sampling period. it was found that the conditions at the more heavily trafficked site were comparable with those at an open site. at the less heavily trafficked site pollution levels were higher than expected probably due to the proximity of a roundabout, causing vehicles engines to operate in more polluting modes, and also to the reduced turbulence from lower traffic flows.(a)

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