Skip to main content

Air Quality

Contact Information


Frequently Asked Questions on Radon and Health

Question: What is radon and where does it come from?

Radon is a radioactive gas found in nature. It has no color, odor or taste and is chemically inert. It's source is uranium. As the uranium molecule decays to form stable lead, a process taking many, many years, it changes from one radioactive element to another in a sequence known as the Uranium Decay Cycle. Partway through this cycle, the element radium becomes radon which as a gas moves up through the soil to atmosphere. Uranium is found in most soils and in granite. [Source: Colorado Department of Public Health and Environment]

Question: Is radon a problem in Colorado?

Excessive radon levels have been found in all of the 50 states. In Colorado between one-third and one -half of the homes have radon levels in excess of the EPA recommended action level of 4 picoCuries of radon per liter of air. [Source: Colorado Department of Public Health and Environment]

Question: What are the health effects from exposure to radon?

There are no immediate symptoms. Radon in indoor air is estimated to cause between 15,000 and 22,000 lung cancer deaths each year in the United States. Smokers are at higher risk of developing Radon-induced lung cancer. The only health effect which has been definitively linked with radon exposure is lung cancer. Lung cancer would usually occur years (5-25) after exposure. There is no evidence that other respiratory diseases, such as asthma, are caused by radon exposure and there is no evidence that children are at any greater risk of radon induced lung cancer than adults. [Source: US Environmental Protection Agency]

Thousands of preventable lung cancer deaths annually in the United States are attributable to indoor residential exposure to radon. Either smoking or radon exposure can independently increase the risk of lung cancer. However, exposure to both greatly enhances that risk. (At exposures to 4 pCi of radon per liter of air, the lifetime lung cancer risk attributable to radon rises from 2 cases per thousand in non-smokers to 29 cases per thousand in smokers). [Source: Colorado Department of Public Health and Environment]

Question: How much reliance can I put on these risk factors?

The risk factors were developed from epidemiological studies of underground miners exposed to radon. Because the studies collected data from human adult males rather than from animal subjects, they have a higher confidence level than is applied to toxicological studies.

The Surgeon General, the Environmental Protection Agency, the National Academy of Sciences, the American Medical Association, the American Lung Association and the World Health Organization have all identified indoor radon pollution as a national health problem. [Source: Colorado Department of Public Health and Environment]

Question: People are exposed to radiation from several sources –– how much of the overall radiation dosage comes from radon?

The average person in the U.S. gets more radiation dose from exposure to indoor radon (radon decay products) than from any other source of natural or man-made radiation according to the National Council on Radiation Protection and Measurements (NCRP 1987). At the average indoor radon concentration in the U.S. of about 1.3pCi/L, the dose from radon decay products is about 100 to 200 mrem/year*. In comparison, radiation from the ground contributes about 28 mrem/yr and radiation from outer space contributes another 27 mrem/yr. Radioactive materials in our bodies give another 40 mrem/yr. The use of radiation in x-rays and cancer therapy, consumer products, and nuclear power adds another 65 mrem/yr to our average dose. The U.S. Nuclear Regulatory Commission limits public exposures to man-made radionuclides to 100 mrem/yr. People in homes with radon at the EPA action level of 4 pCi/L could be receiving 300 to 600 mrem/yr. State and Federal programs for radiation protection should be putting more emphasis on the largest source of radiation exposures, namely radon. The radiation dose from naturally occurring radon decay products should not be addressed differently than man-made sources of radiation exposure. [Source: American Association of Radon Scientists and Technologists]

Question: What's the Debate on Radon?

All major national and international organizations that have examined the health risks of radon agree that it is a lung carcinogen. The scientific community continues to conduct research to refine our understanding of the precise number of deaths attributable to radon. EPA and the National Cancer Institute (NCI) have independently placed that number at about 15,000 lung cancer deaths each year in the United States.

A few scientists have questioned whether low radon levels, such as those found in residences, increase the risk of lung cancer because some small studies of radon and lung cancer in residences have produced varied results. Some have shown a relationship between radon and lung cancer, some have not. However, the national and international scientific communities are in agreement that all of these residential studies have been too small to provide conclusive information about radon health risks. All major scientific organizations continue to believe that approximately 10% of lung cancers in the United States -- or about 15,000 lung cancer deaths annually -- are attributable to radon. [Source: US Environmental Protection Agency]

Question: Are scientists sure that radon really is a problem?

Although some scientists dispute the precise number of deaths due to radon, all the major health organizations (like the Centers for Disease Control and Prevention, the American Lung Association and the American Medical Association) agree with estimates that radon causes thousands of preventable lung cancer deaths every year. This is especially true among smokers, since the risk to smokers is much greater than to non-smokers. [Source: US Environmental Protection Agency]

Question: How do we know radon causes lung cancer in humans?

Radon is classified as a Class A known human carcinogen according to the World Health Organization's International Agency for Research on Cancer (IARC 1988). This classification is based on the strong evidence of lung cancers in underground miners. Data from 11 studies of radon-exposed underground miners show that about 40% of the 2,700 lung cancer deaths which occurred among 65,000 miners are due to radon according to the National Cancer Institute (NCI 1995). Among never-smokers, 70% of the lung cancer deaths are believed to be due to radon and 39% of the lung cancer deaths in smokers. While there are many differences in radon exposure conditions between mines and homes, it is notable that persons living in homes with radon above the EPA action level of 4pCi/L could accumulate radon exposures similar to underground miners. Miner studies show a consistent linear no-threshold relationship between lung cancer and radon exposure down to radon levels commonly found in homes. Epidemiology studies on residential radon exposures are consistent with expectations based on the linear model. Therefore, the National Cancer Institute indicates that 10% of all lung cancer deaths in the U.S. could be due to indoor radon exposures, 11% of lung cancer deaths in smokers and 30% of lung cancer deaths in never- smokers. When differences between mine and home exposures are taken into account, we find that exposures above 4pCi/L represent substantially greater risks than allowed for any other source of radiation exposure. [Source: American Association of Radon Scientists and Technologists]

Question: How do we know radon is a carcinogen?

The World Health Organization (WHO) and the US Department of Health and Human Services, as well as EPA, have classified radon as a "Class A" known human carcinogen, because of the wealth of biological and epidemiological evidence and data showing the connection between exposure to radon and lung cancer in humans.

The World Health Organization, the National Academy of Sciences and other scientific organizations agree that studies of the miners are the best scientific information for estimating radon health risks in homes. The Lubin/Boice meta-analysis paper also concludes that the miners studies are the best data source for analyzing residential radon risk. Based on the miner data, NCI has previously estimated that 15,000 people die of lung cancer from residential radon each year in the U.S.

There have been many studies conducted by many different organizations in many nations around the world to examine the relationship of radon exposure and human lung cancer. The largest and most recent of these was an international study, led by the National Cancer Institute (NCI), which examined the data on 68,000 underground miners who were exposed to a wide range of radon levels. The studies of miners are very useful because the subjects are humans, not rats, as in many cancer research studies. These miners are dying of lung cancer at 5 times the rate expected for the general population. Over many years scientists around the world have conducted exhaustive research to verify the cause-effect relationship between radon exposure and the observed increased lung cancer deaths in these miners and to eliminate other possible causes.

In addition, there is an overlap between radon exposures received by miners who got lung cancer and the exposures people would receive over their lifetime in a home at EPA's action level of 4 pCi/L, i.e., there are no large extrapolations involved in estimating radon risks in homes. [Source: US Environmental Protection Agency]

Question: Is radon's health risk currently being investigated?

We already have a wealth of scientific data on the relationship between radon exposure and the development of lung cancer. The scientific experts agree that the occupational miner data is a very solid base from which to estimate a risk range of 7,000-30,000 lung cancer deaths annually. While residential radon epidemiology studies will improve what we know about radon, they will not supersede the occupational data. EPA is funding efforts by the NAS. Health authorities like the Centers for Disease Control (CDC), the Surgeon General, the American Lung Association, the American Medical Association, and others agree that we know enough now to recommend radon testing and to encourage public action when levels are above 4 pCi/L. The most comprehensive of these efforts is the National Academy of Science's Biological Effects of Ionizing Radiation (BEIR VI) Report. As in the case of cigarette smoking, it would probably take many years and rigorous scientific research to produce the composite data needed to make an even more definitive conclusion. [Source: US Environmental Protection Agency]

Question: What is the BIER VI report by the National Academy of Sciences?

The NAS has prepared its latest analysis of health research on radon, the Biological Effects of Ionizing Radiation (BEIR VI) Report. This is the most comprehensive review effort to date. The Committee was charged with:

  • reviewing all current miner and residential data, as well as all existing cellular-biological data
  • comparing the dose per unit exposure effects of radon in mines and homes
  • examining interactions between radon exposure and smoking
  • Examining any exposure-rate effect (alteration of effect by intensity of exposure).

The report confirms that radon is the second leading cause of lung cancer in the U.S. and that it is a serious public health problem. The study fully supports EPA estimates that radon causes about 15,000 lung cancer deaths per year. You can read a summary of the report at and read the full report online at

Why does Colorado have high radon levels and low lung cancer rates?

Relative to other states, Colorado has the third lowest lung cancer death rate in the nation. For the period 1993-1997, the Colorado cancer death rate per 100,000 population was 48.2 among males, 25.6 among females. These rates are well below the national averages of 69.4 for males, 34.0 for females.

Colorado radon levels are well above the national average. The average short term radon test done in Colorado comes in at 7.3 pCi/L. USEPA estimates the average indoor radon level nationwide is 1.3 pCi/L.

So why don't cancer rates match up better with radon levels? Mobility is a key factor. Colorado's population is growing rapidly- people move here from other parts of the nation where they might have been exposed to more or less radon. Also death statistics are from the county in which the person died. Many residents move away from Colorado (retire in another state for example) and are not residents when they die, so their deaths are not recorded in Colorado. This in addition to the fact that they have moved here after exposure to unknown amounts of radon elsewhere shows that this is not a reliable way to equate any state's radon levels with the state's lung cancer rates.

Second it must be remembered that smoking is the chief cause of most lung cancers. USEPA estimates that only about 10% of all lung cancers are attributable to radon. Most of these 10% are smokers, who have succumbed to the combined risks of smoking and radon. Therefore Colorado's low smoking rate might help explain our low lung cancer rate.

How many Fort Collins homes need radon mitigation? Since your cost-benefit analysis says the average radon level is 3 pCi/L, does that mean the average house does not need mitigation? Why, then, is the City pushing radon-resistant new construction?

The confusion about radon exposure levels in Fort Collins has arisen because of the way we have estimated lung cancer risk, and we hope the information below will help clear it up. USEPA and other health authorities recommend fixing a house if its radon level is above 4 pCi/L. Statistics on short-term tests in Fort Collins show that about 70% are at or above 4 pCi/L. Short-term tests are usually done on the lowest living level of the home under closed-house conditions - i.e., the worst-case scenario. These results are often used to determine whether to mitigate radon in an existing house. A study of Fort Collins homes using more-realistic, long-term tests showed a similar result: 75% of homes with basements had levels above 4 pCi/L in the basement. However, looking at first floor, long-term tests in the Fort Collins study, the number of tests above 4 pCi/L drops to about 25%.

For our cost-benefit analysis of radon-resistant new construction, we needed an estimate of people's day-to-day, long-term exposure to radon. Basement levels are too high for this purpose. Therefore, we used long-term, first floor measurements from the Fort Collins study, which are lower than the level found in basements, and are close the average of all floors measured in the study. The average ground-floor level in the Fort Collins study was 3.0 pCi/L.

However, the original question is still valid - if only 25% of the first floor, long-term tests are above 4 pCi/L, does that mean that radon resistant new construction is not needed in 75% of the homes? Arguably, yes. However, many people put in a radon mitigation system based on high readings found in the basement, and there is some logic to that approach. Perhaps family members spend a lot of time in the basement; or perhaps the homeowner prefers to have every level of his home below 4 pCi/L rather than depend on the averaging effect, which occurs when family members eat on one level, sleep on another, and play on a third. For that reason, the benefit of having a passive radon reduction system that reduces radon by about half would help many homeowners who would otherwise end up paying a lot more for an after-construction radon mitigation system. [Source: Fort Collins Natural Resources Department]

Question: Who compiled this Question and Answer Sheet? How can I learn more?

This document was compiled by Brian Woodruff and Zoë Shark, staff from the City of Fort Collins, Community Planning and Environmental Services, Natural Resources Department. To learn more about radon you can call Brian Woodruff at 221-6604 or Zoë Shark at 416-2616.