Indoor and outdoor carbon monoxide (CO) concentrations were measured in 68 Ghalyun cafes in Ardabil City of Iran. One hour sampling was performed in each location and hourly mean concentrations were reported. Respective mean concentrations of CO in indoor and outdoor air were 24.75±17.17 and 2.65±1.33 ppm with an average indoor to outdoor air concentration ratio of 13.3±13.1. Within the variables studied; type of the tobacco smoked, location of the cafe inside the building, and utilizing mechanical ventilator, respectively, were recognized as the most influential factors controlling CO concentrations in Ghalyun cafes (p<0.05). 73.5% of workers (27.3 and 95.7% of cafes located on ground floors and basements, respectively) were exposed to higher levels of CO than the corrected OEL–TWA (Occupational Exposure Limit) of 10.4 ppm