An explosive atmosphere is defined as a mixture of flammable substances and oxygen. The oxygen is usually present as a component of the ambient air. Flammable substances may be gases, liquids or their vapors as wells as aerosols consisting of mists or dusts. In the context of explosion protection, they are considered under normal atmospheric conditions.
The explosiveness depends on the substance used, its flammability and the mixture with air/oxygen. If the mixture contains an atmospheric oxygen mix with a composition that differs from the air mixture normally present in the environment, the precise oxygen content must be taken into account. If the oxygen content falls below a certain substance-specific value, the so-called limiting oxygen concentration, this mixture cannot be ignited. This effect is utilized in what is known as inertization where oxygen is replaced by an inert gas – one which is not suited for oxidation. Moreover, the concentration of the flammable substance in the air mixture must lie between the lower and upper explosive limits.
An explosive atmosphere can be created intentionally or unintentionally. The former often occurs in production processes. Here, an explosive atmosphere is present wherever the relevant substance is used in the presence of ambient air. This can include areas in which adhesives or paints are dried or applied or areas where materials are milled.
Unintentional explosive atmospheres arise when faults occur. For example, this can be a leaking gas line, an unintentional opening of an outlet valve or leaks in boilers or lines due to insufficient maintenance.