Residential and occupational exposure to radon is the second leading cause of lung cancer after cigarette smoking. As many as eight million homes in the US have elevated radon levels according to Environmental Protection Agency estimates. High exposure levels in homes are largely a result of radon-contaminated gas rising from the soil. This makes it an unusual indoor air pollutant in that it has a natural source. This study examines the synergism between smoking and radon, what levels are considered safe, and what to do to safeguard against overexposure to radon.