Firefighters in the line of duty are exposed to many hazardous air toxics released from burning vegetation and other materials that may cause severe health risks. Current literature does not consider complex mixtures and cumulative impacts of these air toxics when assessing firefighter exposure risk. This study aims to determine if cumulative exposure to mixtures of air toxics will put firefighters at risk in the context of current firefighter exposure regulations. This was tested using CO exposure and air toxics data collected during prescribed burns. Personal CO monitors were deployed to 122 firefighters at 31 prescribed burns across Australian temperate forests. CO concentrations were used to determine equivalent exposures for air toxics using their emission ratio, which were then used to calculate cumulative exposure. Average personal monitor CO exposure was 9.2 ppm across all burns, with 1-min maximum ranging between 0.3 and 703 ppm. Work safe Australia exposure standards were exceeded for 6 firefighters out of 122. Five individual firefighters had average cumulative exposures exceeding 100% exposure limit. Six toxicological classes were identified to be the most hazardous, causing eye disorders, upper and lower respiratory disorders, skin disorders and cancer. Across these classes, three toxins contributed significantly to the cumulative exposure; respirable particles (< 3 μm), formaldehyde and acrolein. Using the cumulative exposure methodology as described here and a reduction in the CO exposure standards to account for the numerous air toxics emitted during a fire would supply decision makers with a tool to better identify exposure risks.