Hydrogen, the lightest element, is a colorless, odorless, tasteless and flammable gas found at concentrations of about 100 ppm (0.01%) in the air. Sources for this gas are numerous, including dissociation of ammonia, electrolysis of water, and extraction from by-product streams of chemical and petroleum processing.

Hydrogen is widely used in the petroleum refinery processes of hydro treating and hydrocracking, and serves the food industry by turning inedible grease into soaps and animal feeds.

In the welding industry, Hydrogen is used as a fuel in underwater oxy-hydrogen torches, and for metal welding and brazing. The space program also uses hydrogen as a rocket fuel when weight and energy considerations are paramount.