Summary Analyses of breath and limited blood samples from 30 pairs of taxi drivers and panelists who drove in New York City traffic for 8 hours on two consecutive days indicated that both panelists and drivers attained similar COHb (blood carboxyhemoglobin) levels. This was true for both smokers and non-smokers had significantly higher concentrations of COHb than non-smokers. There was no consistent deference between the first and second day of driving in the levels of alveolar carbon monoxide.