Radon is listed by the U.S. Environmental Protection Agency as a class A carcinogen because the agency considers the gas a proven cause of cancer in humans. According to the EPA, radon is the second leading cause of lung cancer in the U.S., next to smoking. Radon can penetrate into buildings through weakened structures, cracks and small openings. In enclosed spaces it can sometimes accumulate to levels that pose danger to humans, especially as the gas is invisible, odorless and tasteless. Radon in indoor air can come from both the soil and from the water supply, if the source is groundwater. Radon in water poses an inhalation risk and an ingestion risk, although the risk of lung cancer from airborne radon is much greater than the risk of cancers from ingested radon.
It is estimated that radon accounts for, on a global average, about half of the total natural radiation background. Radon and its decay products (progeny) are present in the oceans, lakes and rivers and other bodies of water. Radon levels in outdoor air, water and soil vary considerably from one place to another. The gas is released naturally from the ground into outdoor air where it is quickly diluted to harmless concentration. Natural bodies of water in areas where uranium is concentrated may contain levels of radon that pose a risk to health.
Many common food plants can absorb radioactive polonium 210 and other radon progeny. Polonium releases ionizing alpha radiation which is extremely harmful to internal organs. The widespread use of radioactive phosphate fertilizers may also be increasing levels of radioactive polonium in animal feeds. In the United States, where use of chemical fertilizers is prevalent, polonium in dairy and meat products may expose human organs to radiation levels equivalent to tobacco use, which are extremely high in heavy smokers. Small amounts of polonium have been found in fluoridated drinking water, possibly influenced by farm irrigation and run-off. It remains unclear how much radioactive content chemical fertilizers contribute to food and water and how much is naturally present.