3He is also present in the Earth's atmosphere. The natural abundance of 3He in naturally occurring helium gas is 1.38 (1.38 parts per million). The partial pressure of helium in the Earth's atmosphere is about 0.52 Pa, and thus helium accounts for 5.2 parts per million of the total pressure (101325 Pa) in the Earth's atmosphere, and 3He thus accounts for 7.2 parts per trillion of the atmosphere. Since the atmosphere of the Earth has a mass of about 5.14 tonnes, the mass of 3He in the Earth's atmosphere is the product of these numbers, or about 37,000 tonnes of 3He. (In fact the effective figure is ten times smaller, since the above ppm are ppmv and not ppmw. One must multiply by 3 (the molecular mass of Helium-3) and divide by 29 (the mean molecular mass of the atmosphere), resulting in 3,828 tonnes of helium-3 in the earth's atmosphere.)