What is the second leading cause of lung cancer? Radon gas is the second leading cause of lung cancer. The leading cause of lung cancer is smoking. By ProMatcher Staff Updated: 08/08/2016 Next Question Source: National Safety Council Next Question Find Local Pros Free Actuaries Cost Estimates Related Questions Where is radon typically found in the U.S.? Which U.S. state has the greatest concentration of radon? How many homes have elevated radon levels? How many lung cancer deaths are related to radon? How Much Does It Cost to Get Rid of Radon?