Re-circulating direct gas-fired industrial air heaters shall NOT ____________ in buildings that contain flammable solids, gases, explosive materials, or substances that may become toxic when exposed to flame.

Re-circulating direct gas-fired industrial air heaters should not re-circulate room air in environments where there are flammable solids, gases, explosive materials, or substances that may become toxic when exposed to flame because doing so poses a significant safety risk.

Re-circulating air from a room that could contain these hazardous materials can lead to the accumulation of flammable or toxic gases, creating a dangerous environment. If any of these materials are present and the heater re-circulates that air, it could ignite, potentially leading to explosions or the release of toxic substances.

In contrast, circulating filtered air or providing fresh air helps to mitigate these risks, as it ensures that the air supplied to the heater is free from hazardous materials. Heating air directly, while necessary for the operation of the heater, is also conducted in controlled conditions to ensure safety. Being aware of the distinctions and the implications of each choice is crucial for maintaining safety standards in industrial settings.