Hydrogen:

Hydrogen (H₂) is the lightest of all components.  Its presence cannot be detected through any of the senses.  It is flammable within oxygen or air, or has a flammable range of from 4.1 % to 74.2 % through volume in air.  A combination of 10 to 65 % hydrogen through volume within air will explode if ignited.  Pure hydrogen burns quietly in air along with an almost invisible flame, and while burned within pure oxygen, an extremely high temperature might be reached.  Hydrogen will burn readily in chlorine gas, and under proper conditions, will merge along with nitrogen, creating ammonia.

A few chemical reactions generate hydrogen as a byproduct.  A lead-acid battery will generate hydrogen while it is being charged.  Potassium and Metallic sodium are instance of a few chemicals which react violently while exposed to water, producing hydrogen that might flame spontaneously because of the heat of the reaction. Several electroplating processes generate hydrogen. A few chemicals used to erase scale from the water side of boilers provide off hydrogen. Anything the operation that is significant to know whether hydrogen will be generates, and if so, precautions must be taken to avoid its ignition and accumulation.  The precautions to take involve adequate ventilation to avoid its accumulation and the elimination of probable sources of ignition.  Hydrogen is categorized as an asphyxiant.