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Radon
What is radon?
Radon is a colorless, odorless, radioactive gas. It forms naturally from the decay of radioactive elements, such as uranium, which are found at different levels in soil and rock throughout the world. Radon gas in the soil and rock can move into the air and into ground water and surface water.
Radon is present outdoors and indoors. It is normally found at very low levels in outdoor air and in drinking water from rivers and lakes. It can be found at higher levels in the air in houses and other buildings, as well as in water from underground sources, such as well water.
Radon breaks down (decays) into solid radioactive elements called
radon progeny (such as polonium-218, polonium-214, and lead-214). Radon progeny can attach to dust and other particles and can be breathed into the lungs. As radon and radon progeny in the air break down, they give off alpha particles, a form of high-energy radiation that can damage the DNA inside the body's cells.
How are people exposed to radon?
At home and in other buildings
For both adults and children, most exposure to radon comes from being indoors in homes, commercial buildings, schools, and other places. The levels of radon in homes and other buildings depend on the characteristics of the rock and soil in the area. As a result, radon levels vary greatly in different parts of the United States, even within neighborhoods. Elevated radon levels have been found in every state.
The radon gas given off by soil or rock can enter buildings through cracks in floors or walls; construction joints; or gaps in foundations around pipes, wires, or pumps. Radon levels are usually highest in the basement or crawl space. This level is closest to the soil or rock that is the source of the radon. Therefore, people who spend much of their time in basement rooms at home or at work have a greater risk for being exposed.
Small amounts of radon can also be released from the water supply into the air, especially if the water source is underground. As the radon moves from the water to air, it can be inhaled. Water that comes from deep, underground wells in rock may have higher levels of radon, whereas surface water (drawn from lakes or rivers) usually has very low radon levels. For the most part, water does not contribute much to overall exposure to radon.
Radon exposure can also occur from some building materials if they are made from radon-containing substances. Almost any building material made from natural substances, including concrete and wallboard, may give off some level of radon. In most cases these levels are very low, but in a few instances these materials may contribute significantly to radon exposure.
Some granite countertops may expose people to different levels of radon. Most health and radiation experts agree that while a small portion of granite countertops may give off increased levels of radon, most countertops give off extremely low levels. People concerned about radon from countertops and from other household sources can test these levels using home detection kits or by hiring a professional to do the testing (see the section "How can I avoid exposure to radon?"below).
Radon levels in the air are measured by units of radioactivity per volume of air. The most common measure used is picocuries per liter (pCi/L). According to the US Environmental Protection Agency (EPA), the average indoor radon level is about 1.3 pCi/L. People should take action to lower radon levels in the home if the level is 4.0 pCi/L or higher. The EPA estimates that nearly 1 out of every 15 homes in the United States may have elevated radon levels.
Outdoors, radon generally disperses and does not reach high levels. Average levels of radon outdoors, according to the EPA, are about 0.4 pCi/L.
At certain jobs
In the workplace, people working underground, such as some types of miners, are among the most likely to be exposed to high levels of radon. High death rates from lung problems among miners in some parts of the world were first noted hundreds of years ago, long before people knew what radon was. Studies of radon-exposed miners during the 1950s and 1960s confirmed the link between radon exposure and lung cancer.
Higher levels of radon exposure are also more likely for people who work in uranium processing factories or who come in contact with phosphate fertilizers, which may have high levels of radium (an element that can break down into radon).
Does radon cause cancer?
Long-term exposure to radon can lead to
lung cancer. Radon gas in the air breaks down to other radioactive elements (radon progeny). Radon progeny are tiny radioactive particles that can lodge in the lining of the lungs, where they continue to break down into other radioactive elements by releasing radiation. The radiation released in this “radioactive decay” process can damage lung cells and eventually lead to lung cancer.
Cigarette smoking is by far the most common cause of lung cancer in the United States, but radon is the second leading cause. Scientists estimate that about 20,000 lung cancer deaths per year are related to radon.
Exposure to the combination of radon gas and cigarette smoke creates a greater risk for lung cancer than either factor alone. Most radon-related lung cancers occur among smokers. However, radon is also thought to cause a significant number of lung cancer deaths among non-smokers in the United States each year.
Some studies have suggested that radon exposure may be linked to other types of cancer as well. But the evidence for such links has been inconsistent and not nearly as strong as it is for lung cancer. Because radon and its progeny are absorbed mainly by inhaling, and because the alpha particles they give off travel only a short distance, it is unlikely they would affect other tissues in the body.
The evidence that radon causes lung cancer comes from studies in people and studies done in the lab.
Studies in people
Several types of studies in people have found that exposure to radon increases lung cancer risk:
- Studies of people working in underground mines with high levels of radon exposure. Many of these studies looked at people working in uranium mines, but working in other types of mines also showed a link to lung cancer risk.
- Studies comparing radon levels in homes of people with lung cancer with the levels in homes of similar people without lung cancer
- Studies comparing lung cancer cases or deaths in areas with differing levels of radon exposure
These studies also show that the risk of lung cancer from radon is even higher in smokers and former smokers.
Some long term studies of uranium miners have found that they had higher risks of certain other cancers. But since the people with the higher risk weren’t exposed to higher amounts of radon and radon progeny, it isn’t clear that radon is the cause of those cancers. They may instead be linked to uranium dust or other exposures in the mines. .
Studies done in the lab
Studies in lab animals have also shown an increased risk of lung cancer with exposure to radon. These studies revealed that breathing in radon and its progeny significantly increases the risk of lung tumors. The risk is higher if the animal breathes in both cigarette smoke and radon. In some animals, the risk of certain other cancers was increased, such as cancer of the
lip,
nasal cavity, and
bladder.
In lab studies using human cells, radon and its progeny have also been shown to cause damage to chromosomes (packets of DNA) and other types cellular damage. These types of changes are often seen in cancer cells.
What expert agencies say
Several agencies (national and international) study different substances in the environment to determine if they can cause cancer. (A substance that causes cancer or helps cancer grow is called a
carcinogen.) The American Cancer Society looks to these organizations to evaluate the risks based on evidence from laboratory, animal, and human research studies.
Based on animal and human evidence, several expert agencies have evaluated the cancer-causing potential of radon.
The
International Agency for Research on Cancer (IARC) is part of the World Health Organization (WHO). Its major goal is to identify causes of cancer. Based on the data available, IARC classifies radon and its progeny as "carcinogenic to humans."
The
National Toxicology Program (NTP) is formed from parts of several different US government agencies, including the National Institutes of Health (NIH), the Centers for Disease Control and Prevention (CDC), and the Food and Drug Administration (FDA). The NTP has classified radon as "known to be a human carcinogen."
The US
Environmental Protection Agency (EPA) monitors the human health effects from exposure to various substances in the environment. The EPA lists radon as the second leading cause of lung cancer and the number one cause of lung cancer among non-smokers, estimating it is responsible for about 20,000 lung cancer deaths every year.
(For more information on the classification systems used by these agencies, see our document,
Known and Probable Human Carcinogens.)
Does radon cause any other health problems?
Some studies of miners have suggested a link between radon exposure and some non-cancerous respiratory diseases, especially pulmonary fibrosis (scar tissue forming in the lungs that leads to shortness of breath). These effects appear mainly in miners with high levels of exposure.
It's not clear that the lower levels commonly seen in homes can cause the same types of problems. Still, a study by the American Cancer Society did find that people living in areas with higher radon levels did have a higher risk of dying from chronic obstructive pulmonary disease (this includes emphysema and chronic bronchitis).
氡气还会引起其它肺部疾病
