Radon concentrations found in natural environments are much too low to be detected by chemical means: for example, a 1000 Bq/m3 (relatively high) concentration corresponds to 0.17 picogram per cubic meter. The average concentration of radon in the atmosphere is about 6×10−20 atoms of radon for each molecule in the air, or about 150 atoms in each ml of air.[7] The entire radon activity of the Earth's atmosphere at any one time is due to some tens of grams of radon, constantly being replaced by decay of larger amounts of radium and uranium.[8] Its concentration can vary greatly from place to place. In the open air, it ranges from 1 to 100 Bq/m3, even less (0.1 Bq/m3) above the ocean.