In which scenario is it acceptable to use re-circulating direct gas-fired industrial air heaters?

Using re-circulating direct gas-fired industrial air heaters is permissible in buildings that do not contain hazardous materials because these heaters can potentially release combustion byproducts into the space. In environments that hold hazardous materials, there is a risk that these byproducts can mix with harmful substances, leading to safety and health hazards, such as toxic chemical reactions or flammable mixtures.

Buildings that do not have hazardous materials can accommodate the operational characteristics of these heaters without introducing safety concerns related to combustion exhaust. This makes option C the appropriate choice for ensuring a safe and healthy working environment.

While adequate ventilation (as suggested by another choice) is important for any heating system to promote proper air circulation and to mitigate potential carbon monoxide build-up, it does not directly address the specific concern associated with materials that could lead to dangerous interactions with combustion products. Therefore, the emphasis is on the absence of hazardous materials when considering the safe use of re-circulating direct gas-fired industrial air heaters.