Which class includes substances whose fumes or vapors may be flammable?

The correct choice refers to Class 3 materials, which are classified as flammable liquids. This classification encompasses substances whose vapors or fumes can ignite easily when exposed to an open flame or spark. Flammable liquids are recognized for their low flash points, which means they can produce enough vapor at room temperature to form an ignitable mixture with air.

Class 3 includes many common substances used in various industries, such as gasoline, acetone, and alcohols. Understanding this classification is critical for firefighters and hazmat responders because it highlights the potential explosion and fire hazards associated with these materials.

The focus on the fumes and vapors being flammable underlines the importance of ventilation and proper handling of these substances to minimize risks during emergency response or clean-up operations. Being aware of the properties of Class 3 materials aids in developing effective safety protocols when dealing with hazardous situations involving these substances.