Because the level of radioactivity is directly related to the number and type of radioactive atoms present, radon and all other radioactive atoms are measured in picocuries. For instance, a house having 4 picocuries of radon per liter of air (4 pCi/L) has about eight or nine atoms of radon decaying every minute in every liter of air inside the house. A 1,000-square-foot house with 4 pCi/L of radon has nearly 2 million radon atoms decaying inside it every minute.

Picocurie (pCi): one pCi is one-trillionth of a curie, 0.037 disintegrations per second, or 2.22 disintegrations per minute.

Curie (Ci): a standard measurement for radioactivity, specifically, the rate of decay for a gram of radium at 37 billion decays per second; a unit of radioactivity equal to 3.7 x 1010 disintegrations per second.

Radon is a worldwide health risk in homes. Most radon-induced lung cancers occur from low- and medium-dose exposures in people's homes. Radon is the second most important cause of lung cancer after smoking in many countries.

• Lung cancer kills thousands of Americans every year. Smoking, radon, and secondhand smoke are the leading causes of lung cancer. Although lung cancer can be treated, the survival rate is one of the lowest among all those with cancer. From the time of diagnosis, between 11 and 15% of those afflicted will live beyond five years, depending upon demographic factors. In many cases, lung cancer can be prevented.
  

• Smoking is the leading cause of lung cancer. Smoking causes an estimated 159,000 cancer deaths in the U.S. every year (American Cancer Society, 2014). And the rate among women is rising. On January 11, 1964, Dr. Luther L. Terry, then U.S. Surgeon General, issued the first warning on the link between smoking and lung cancer. Lung cancer now surpasses breast cancer as the number one cause of death among women. A smoker who is also exposed to radon has a much higher risk of lung cancer.
  

• Radon is the number-one cause of lung cancer among non-smokers, according to EPA estimates. Overall, radon is the second-leading cause of lung cancer. Radon is responsible for about 21,000 lung cancer deaths every year. About 2,900 of these deaths occur among people who have never smoked. On January 13, 2005, Dr. Richard H. Carmona, the U.S. Surgeon General, issued a national health advisory on radon.
  

• Secondhand smoke is the third-leading cause of lung cancer and responsible for an estimated 3,000 lung cancer deaths every year. Smoking affects non-smokers by exposing them to secondhand smoke. Exposure to secondhand smoke can have serious consequences for children’s health, including asthma attacks, effects on the respiratory tract (e.g., bronchitis, pneumonia), and possibly causing ear infections.

How does radon induce cancer?

If inhaled, radon decay products (polonium-218 and polonium-214, solid form), unattached or attached to the surface of aerosols, dusts and smoke particles become deeply lodged in the lungs, where they can radiate and penetrate the cells of mucous membranes, bronchi, and other pulmonary tissues. The ionizing radiation energy affecting the bronchial epithelial cells is believed to initiate the process of carcinogenesis. Although radon-related lung cancers are mainly seen in the upper airways, radon increases the incidence of all histological types of lung cancer, including small-cell carcinoma, Aden carcinoma, and squamous cell carcinoma. Lung cancer due to inhalation of radon decay products constitutes the only known risk associated with radon. In studies done on miners, variables such as age, duration of exposure, time since initiation of exposure, and especially the use of tobacco have been found to influence individual risk. In fact, the use of tobacco multiplies the risk of radon-induced lung cancer enormously.

What is the evidence?

More is known about the health risk of radon exposure to humans than about most other human carcinogens. This knowledge is based on extensive epidemiological studies of thousands of underground miners, carried out over more than 50 years worldwide, including on miners in the United States and Canada. In addition to the data on miners, experimental exposures on laboratory animals confirm that radon and its decay products can cause lung cancer.