Which of the following should be controlled to prevent a fire hazard when working with combustible materials?

Proper ventilation is crucial in preventing fire hazards when working with combustible materials. Good ventilation helps to ensure that any flammable vapors or gases are dispersed and do not accumulate in the air. This is essential because combustible materials can ignite when they come into contact with a heat source, and the presence of adequate airflow reduces the risk of these materials reaching their ignition points.

Additionally, proper ventilation can help maintain safe oxygen levels, as excess oxygen can increase the risk of combustion. It allows for the removal of heat and prevents conditions that could lead to a fire. Ensuring that workspaces are well-ventilated is a critical component of fire safety practices, especially in industries that regularly handle flammable substances.

The other options, while they may play a role in the overall safety environment, do not specifically address the immediate concern of managing air quality and flammable vapor concentrations the way proper ventilation does. Thus, while increased humidity, constant lighting, and temperature changes can influence safety, they are not direct controls for preventing fire hazards when dealing with combustible materials.