A pCi is a measure of the rate of radioactive decay of radon. One pCi is one trillionth of a Curie, 0.037 disintegrations per second, or 2.22 disintegrations per minute. Therefore, at the EPA’s recommended action level of 4 pCi/L (picocuries per liter), there will be approximately 12,672 radioactive disintegrations in one liter of air during a 24-hour period.