What percent of Earth's atmosphere is hydrogen?

Although there is an enormous amount of hydrogen contained in Earth's crust and in the waters of the oceans, the free hydrogen in the air is only about 55 parts per million by volume. Because it readily combines with a number of other elements, concentrations are quickly dissipated. Any free hydrogen will rise rapidly into the atmosphere, to be ionized into space : the lighter elements such as hydrogen and helium readily escape the pull of gravity. (Nitrogen and oxygen are heavy enough to be held more readily.)