Radon is a radioactive gas and there is no safe level. However, in the U.S. indoor average radon level is 1.3 picocuries per liter of air. The EPA Action Level of 4 picocuries/per liter of air (pCi/L) was based on the achievability of early radon mitigation technology from three decades ago.The EPA Action Level of 4 picocuries/per liter of air (pCi/L) was based on the achievability of early radon mitigation technology from three decades ago. Annual exposure to 4 pCi/l is the equivalent of 200 chest x-rays per year. The World Health Organization recommends indoor radon should not exceed 100 Bq/m3 (Becquerel per cubic meter) which equates to 2.7 pCi/L. Lung cancer risk rises 16% per 2.7 pCi/L increase in radon exposure.