A study of airborne contaminant exposures experienced by firefighters was conducted as part of a larger study of the ability of self contained breathing apparatus (SCBA) to protect firefighters during actual firefighting conditions. Breathing zone samples were collected inside and outside SCBA masks during six training fires, 15 residential fires, and one automobile fire. They were usually collected from two engine company and two truck company firefighters using bag sampling techniques and solid sorbent samplers or impactors. During the knockdown phase of the fires, carbon-dioxide (124389) was the most important compound found in part per million (ppm) quantities, followed by carbon-monoxide (630080), ethylene (74851), methanol (67561), methane (74828), acetaldehyde (75070), acrolein (107028), benzene (71432), hydrogen-cyanide (74908), and hydrogen-chloride (7647010) in that order. Carbon-monoxide was the only combustion product that exceeded its immediately dangerous to life level (IDLH) of 1500ppm for 30 minutes. Approximately 10% of the knockdown samples exceeded the IDLH. Some of the carbon- monoxide, acrolein, formaldehyde (50000), hydrogen-cyanide, hydrogen- chloride, sulfuric-acid (7664939), and hydrogen-fluoride (7664393) exposures exceeded their short term exposure limits (STELs). Acrolein was the most significant in that approximately 50% of the samples exceeded its STEL. Particle concentrations during the knockdown and overhaul phases of the fires ranged up to 560 and 45mg/m3, respectively. Fourteen polycyclic aromatic hydrocarbons (PAHs) were detected during the knockdown phase of the fires. Only very low concentrations of five PAHs were found during the overhaul phase of the fires. Significant concentrations of carbon-monoxide, carbon-dioxide, benzene, and methane were detected inside the SCBA masks. The authors conclude that the in mask data suggest that exposure to some combustion products is occurring. This could reflect early SCBA removal or nonuse during the knockdown phase of the fires rather than actual leakage.