Hydrogen is the most abundant element in the universe, constituting about 75 percent of all matter and the main element in the sun and other stars. It is also the lightest element and is colorless, odorless, tasteless, non-toxic, and highly combustible. Nearly all of the hydrogen on the earth exists locked in compounds, especially with oxygen as water and with carbon in hydrocarbons and other organic matter. Hydrogen is essential for life, and it is present in nearly all molecules in living organisms. But despite this great abundance and near ubiquity, it is extremely scarce as a gas, constituting less than one part per million in the earth's atmosphere by volume.