Carbon monoxide (CO) is one of the more toxic agents present in the gas phase of second-hand tobacco smoke. There is sufficient evidence suggesting that passive smokers are involuntarily poisoned by low CO concentrations. At lower doses, CO affects the central nervous system leading to deterioration in visual perception, manual dexterity, learning, driving performance, and attention level. The effects of chronic inhalation of CO at doses corresponding to tobacco smoking on the cardiovascular system are not well investigated but might involve myocardial hypertrophy and arrhythmias. In people with pre-existing disease, CO pollution alone may result in increased morbidity and mortality. In the study CO levels were monitored in 22 Polish pubs. The temporary CO concentration varied in examined pubs from 0 to 33.11 ppm. The average 8-hours CO concentration varied from 0.21 to 10.20 ppm. Nine percent of pubs exceeded the WHO or EU limit value at some point during the monitoring process. The average weekly CO concentration in all examined microenvironments varied from 0 to 4.80 ppm. The most important factor influencing CO concentration was air-exchange through open doors and windows. In pubs where doors and windows were closed, the following statistical important factors influencing CO concentration were found: 1. the number of smokers present in the pub, 2. the pub's capaciousness, and 3. and the pub's location. The results of the study show that second-hand tobacco smoke is a significant source of CO in Polish pubs. Passive smokers in Polish pubs might be exposed to very high CO concentration exceeding EU reference value.