AbstractIn November 1979, the derailment of a train passing through Mississauga, Ontario, caused the explosion of tank cars containing liquid propane and the leakage of chlorine through a hole in another tank car, Officials evacuated more than 200,000 people from the area, but firemen stayed, exposing themselves to noxious fumes from the explosions and fires. When the crisis was over, health officials administered health tests and questionnaires to the affected men and to a control group of unaffected firefighters. Health information was gathered again one and two years later. In this study, two independent sets of analysts examine the health data to determine whether exposure to hazardous chemicals at the derailment site had any lasting effects on the lung function of the Mississauga firefighters.