What is meant by "explosive atmosphere"?

An "explosive atmosphere" refers specifically to a mixture of air with flammable substances that can ignite under certain conditions. This definition encompasses gases, vapors, mists, or dusts that, when combined with air, create an environment where ignition could lead to an explosion or fire. Such an atmosphere is hazardous, particularly in industrial settings where combustible materials may be present alongside air.

The essence of the term lies in the potential for ignition and subsequent explosion if an ignition source—like a spark, flame, or even superheated surfaces—intersects with the volatile mixture of air and flammable substances. Understanding this concept is crucial for implementing safety measures and ensuring appropriate handling, storage, and use of materials that could contribute to the formation of an explosive atmosphere.

The other options describe conditions that do not align with the definition of an explosive atmosphere. A safe environment with no flammable substances inherently does not create the risk of explosions; the absence of ignition sources suggests a safer condition but does not define the mixture itself; and controlled temperature conditions pertain to environmental management rather than indicating the presence of a hazardous mixture.