Helium is much less dense than air, and its chemical properties are extremely inert, so it is safer than hydrogen (hydrogen can burn in air and may cause explosion). Helium is usually used to fill the air in spacecraft or advertising balloons.

Helium is a colorless and odorless gas at room temperature and atmospheric pressure. The volume content of helium in dry air is 5.24× 10-6. As a substance with the lowest critical temperature found by human beings. Dark yellow when discharged at low voltage.