Hydrogen is the most abundant element in the universe. In diatomic form, hydrogen is an odorless, colorless gas with an exceptionally wide range of flammability in air. The lower explosive limit (LEL) for hydrogen is 4.0 % volume in air. The upper explosive limit is 75.0 % volume in air. This wide range of flammability poses a tremendous explosive hazard. It is easily ignited in the presence of low concentrations of oxygen under normal ambient conditions. Due to lack of odor, color or taste associated with a leak condition, early detection, at ppm levels, is a prudent safety measure.