Final answer:
Firefighters may face health risks from exposure to carbon monoxide, formaldehyde, and volatile organic compounds among other toxic substances during their work.
Step-by-step explanation:
Firefighters are exposed to a variety of hazardous substances that can lead to adverse health effects. Three substances that are most likely to result in such effects include carbon monoxide (CO), which is a toxic gas released by the combustion of fossil fuels; formaldehyde, a toxic gas emitted from various building materials; and volatile organic compounds (VOCs) which are given off by paint and solvents as they dry. These substances can be present in the environments where firefighters work and pose a risk of cancer, respiratory issues, and other long-term health effects.