Complex sentence: One of the major hazards associated with firefighting operations is the toxic environment created by combustible materials, the four major hazards being smoke, the oxygen deficient atmosphere, elevated temperatures, and toxic atmospheres.
Split into simpler sentences: One of the major hazards associated with firefighting operations is the toxic environment created by combusting materials. The four major hazards associated with these situations are smoke, the oxygen deficient atmosphere, elevated temperatures, and toxic atmospheres.