OBJECTIVES: To determine the normal range for carbon monoxide concentrations in the exhaled breath of subjects in the emergency department and to develop a protocol for the use of a breath analyser to detect abnormal carbon monoxide exposure. METHODS: A hand held breath analyser was used to measure end expiratory carbon monoxide concentrations in 382 consenting subjects. Questionnaire data were collected to assess the effect of common sources of carbon monoxide exposure on breath carbon monoxide levels. Smokers were used as a carbon monoxide exposed group for comparison with non-smokers. RESULTS: The range of carbon monoxide concentrations obtained in the non-smoking group was 0-6 ppm and in the smoking group was 1-68 ppm. Smokers had a mean breath carbon monoxide concentration of 16.4 ppm and non-smokers had a mean of 1.26 ppm (95% confidence interval (CI) for difference 13.6 to 16.8 ppm). Male sex and frequent motor vehicle use were associated with slightly higher carbon monoxide concentrations (by 0.40, 95% CI 0.18 to 0.63 ppm, and 0.38, 95% CI 0.13 to 0.63 ppm, respectively) in the non-smoking group. Mean breath carbon monoxide concentrations increased in direct proportion to the number of cigarettes smoked (p<0.001) and there was a negative correlation between carbon monoxide and time since last smoking a cigarette (p<0.001). Altogether 23% of smokers had breath carbon monoxide concentrations in the range 1-6 ppm. CONCLUSIONS: Breath analysis was rapid and results correlated well with carbon monoxide exposure. In this population subjects with breath carbon monoxide concentrations greater than 6 ppm should be assessed for the risk of carbon monoxide poisoning. However even carbon monoxide concentrations less than 6 ppm do not exclude carbon monoxide poisoning within the last 24 hours.