BACKGROUND: Recently several herbal cigarettes are marketed in Korea as an aid to help smokers stop smoking. However, there is no firm evidence that these products are safe to use. This study was to examine the safety of a herbal cigarette(Simsimcho) by measuring its carbon monoxide(CO) exposure as a result of consuming such cigarettes. METHODS: CO concentrations in expired air were measured in six male smokers with the Bedfont Micro Smokerlyzer. Baseline CO concentrations were initially measured in all individuals. Two design models were used. In the first model, concentrations after the first and second herbal cigarette consumed were measured. In the second model, the course of CO concentrations over 1 hour after the consumption of one herbal cigarette was assessed. Also, CO concentrations were measured after inhaling ordinary cigarettes in the same manner for each model. RESUITS: In the first model, CO concentrations after the two herbal cigarettes rose from 14 ppm to 39 ppm (Smoker 1) and from 7 ppm to 22 ppm (Smoker 2), respectively. In the same smokers, CO concentrations after the two ordinary cigarettes rose from 17 ppm to 28 ppm (Smoker 1) and from 12 ppm to 24 ppm (Smoker 2), respectively. In the second model, CO concentrations rose from 28 ppm to 42 ppm (Smoker 3), 16 ppm to 36 ppm (Smoker 4), 16 ppm to 23 ppm (Smoker 5), and 8 ppm and 15 ppm (Smoker 6), respectively. After inhaling an ordinary cigarette, CO concentrations rose from 26 ppm to 32 ppm (Smoker 1) and from 17 ppm to 33 ppm (Smoker 2). CONCLUSION: Our data showed that smoking these herbal cigarettes led to a similar degree of exposure to CO as smoking ordinary tobacco cigarettes, and may even exceed the latter. Thus, the product was shown to be a potential hazard to health.