Based on recent studies on indoor radon and lung cancer, the World Health Organization is recommending that homeowners remediate radon levels that exceed 2.7 picocuries per liter (pCi/L). WHO’s prior threshold — and the U.S. Environmental Protection Agency’s current recommended action level – is 4.0 pCi/L. According to WHO, radon is the second cause of lung cancer in the general population, after smoking, and epidemiological studies have provided convincing evidence of an association between indoor radon exposure and lung cancer, even at the relatively low radon levels commonly found in residential buildings. Based on the World Health Organizations new standard, millions of U.S. homes will require repairs to reduce the levels of the gas. In an effort to help develop radon prevention and mitigation programs WHO has published a handbook that contains its research, which is based on four years of work, compiled by 100 scientists from over 30 countries. The handbook focuses on residential radon exposure and provides detailed recommendations on reducing health risks from radon.