Introduction: Smoking is the major preventable risk of morbidity and mortality. However, its prevalence is high in developed countries and increasing in developing countries, even though its effects are now better known. The purpose of this study was to compare the exhaled carbon monoxide concentration (exCO) between smokers and nonsmokers, evaluate the factors that influence this parameter among smokers and the potential influence of passive smoking by measuring exCO in workers and patients of Instituto do Coração HC-FMUSP. Materials and methods: This cross study included 256 volunteers who responded to a questionnaire and were submitted to exCO measuring with the MicroCo meter device. Results: There were 106 males and 150 females. Mean age was 43.4 years (Vmin-max: 15-83). There were 107 smokers, 118 nonsmokers and 31 passive smokers. Mean exCO was 14.01 ppm (Vmin-max: 1-44) among smokers, 2.03 ppm (Vmin-max: 0-5) among passive smokers and 2.50 ppm (Vmin-max: 0-9) among nonsmokers. Significant statistical difference was observed between smokers and the other groups (p < 0.001), but not between nonsmokers and passive smokers. A positive correlation was found between the number of cigarettes smoked per day and exCO values while there was a negative correlation between the exCO values and the timing of the last cigarette. For a reference limit value of 6 ppm, sensitivity was 70% and specificity was 96%. Conclusion: exCO metering is easy to perform, low-cost, noninvasive and allows the obtention of immediate results and the reference limit value of 6 ppm has good specificity to evaluate the smoking habit.

Keywords: Smoking. Carbon monoxide. Tobacco smoke pollution. Brazil.