Flammable atmospheres are generally caused by
o Evaporation of flammable liquids (e.g., gasoline)
o By-products of chemical reactions (e.g., decomposition of organic
matter to form methane).
Explosive atmospheres are those in which a flammable gas or vapour is present in quantities between the Lower Explosive Limit (LEL) and the Upper Explosive Limit (UEL). These limits define the “Explosive Range” which varies from one substance to another. Refer to the section “Fire and Explosion Hazard” from your SDS (Safety Data Sheet)
for fire and explosion-related information.

*Remember these terms. *

The LEL is the lowest, and the UEL the highest concentration of gas or vapour that will support combustion. For example, gasoline has an LEL of 1.4% and a UEL of 7.6%. Below 1.4% there is not enough fuel to burn, while above 7.6% there is too much fuel and not enough oxygen to burn. The flash point is between the LEL Lower Explosive Limit and the UEL Upper Explosive Limit (See photo above)

The most common explosive gas likely to be encountered in sewers and other underground structures is methane or “natural gas” produced by decaying garbage and sewage.
Other explosive gases and vapours may be present in confined spaces depending on previous contents or accidental spills and leaks (e.g., leaking fuel-storage tanks near service stations).

Explosive ranges for common gases and vapours are listed in
Table 1. (Attached to your paperclip at the top, have a look at the chart to see explosive ranges.)

These values must be considered when selecting and operating gas testing equipment. Combustible atmospheres can arise in grain elevators, feed mills, and some industrial settings such as bag houses, because of the large quantities of dust generated. The most common combustible dust is grain or flour dust, there have been several explosions in grain elevators. You need to address this issue whenever you’re working in these settings.