Based on studies carried out by the National Academy of Sciences in the United States, radon would thus be the second leading cause of lung cancer after smoking, and accounts for 15,000 to 22,000 cancer deaths per year in the US alone.[93] The United States Environmental Protection Agency (EPA) says that radon is the number one cause of lung cancer among non-smokers.[94] The general population is exposed to small amounts of polonium as a radon daughter in indoor air; the isotopes 214Po and 218Po are thought to cause the majority[95] of the estimated 15,000–22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.[96] The Surgeon General of the United States has reported that over 20,000 Americans die each year of radon-related lung cancer.[97]