BACKGROUND Cigarette smoke is the major and most common indoor source of carbon monoxide. CO combines with haemoglobin to form carboxyhaemoglobin (COHb) which hinders oxygen transport and causes myocardial infarction and coronary heart disease. CO damages the endothelium of large and medium arteries and contributes to the development of atherosclerosis. METHODS Participants in the study included 148 active and 306 passive smokers. In both groups the largest proportion of respondents were aged 30-49. Numbers of male and female participants were identical among the active smokers, while the majority of the passive smokers were women. Majority of the participants in both groups lived in large towns (over 100,000 inhabitants). The levels of exhaled CO were measured with Micro+smokerlyzer distributed in Poland by Synecpol. Every participant was provided with relevant instructions and the test was conducted by trained researchers. RESULTS The mean level of exhaled carbon monoxide in active smokers was 12.57 ppm with higher levels found in men. The highest mean level of eCO was found in participants with body weight between 60 kg and 80 kg (mean eCO = 13.39 ppm). The highest levels were observed in participants living in towns with 51,000-100,000 inhabitants. In passive smokers, the mean level of exhaled carbon monoxide was 3.55 +/- 1.26 ppm with higher levels found in men. CONCLUSIONS For non-smokers, the study identified a significant relationship between the level of exhaled CO and the participants' gender (p < 0.05), weight (p = 0.003), and height (p = 0.0005). For smokers, there is a relationship between the level of eCO and the daily cigarette consumption (p = 0.01), the type of cigarettes most frequently smoked (p < 0.05) and the favoured cigarette brand (p = 0.005).