 While the average rate of production of 220Rn (from the thorium decay series) is about the same as that of 222Rn, the amount of 220Rn in the environment is much less than that of 222Rn because of the short half-life of 220Rn (55 seconds, versus 3.8 days respectively).Radon concentration in the atmosphere is usually measured in becquerel per cubic meter (Bq/m3), the SI derived unit.Another unit of measurement common in the US is picocuries per liter (pCi/L); 1 pCi/L=37 Bq/m3.