We estimated the impact of a smoke-free workplace bylaw on non-smoking bar workers' health in Ontario, Canada. We measured bar workers' urine cotinine before (n = 99) and after (n = 91) a 2004 smoke-free workplace bylaw. Using pharmacokinetic and epidemiological models, we estimated workers' fine-particle (PM2.5 ) air pollution exposure and mortality risks from workplace secondhand smoke (SHS). workers' pre-law geometric mean cotinine was 10.3 ng/ml; post-law dose declined 70% to 3.10 ng/ml and reported work hours of exposure by 90%. Pre-law, 97% of workers' doses exceeded the 90th percentile for Canadians of working age. Pre-law-estimated 8-h average workplace PM2.5 exposure from SHS was 419 µg/m(3) or 'Very Poor' air quality, while outdoor PM2.5 levels averaged 7 µg/m(3) , 'Very Good' air quality by Canadian Air Quality Standards. We estimated that the bar workers' annual mortality rate from workplace SHS exposure was 102 deaths per 100000 persons. This was 2.4 times the occupational disease fatality rate for all Ontario workers. We estimated that half to two-thirds of the 10620 Ontario bar workers were non-smokers. Accordingly, Ontario's smoke-free law saved an estimated 5-7 non-smoking bar workers' lives annually, valued at CA $50 million to $68 million (US $49 million to $66 million).