Hydrogen is a colourless, odourless, flammable gas. It is supplied as a permanent gas at high pressure in metal cylinders. Hydrogen is the lightest substance known to man, and is non-toxic, although it does not support life. It also burns, having a flammable range of 4-75% in air. Hydrogen is useful to industry fo three main reasons: it is a reactive, it burns, and it has a very low boiling point.