Get Informed | Radon

What is Radon?

Radon is a gas produced by the radioactive decay of the element radium whose remote parent is either uranium or thorium. Radioactive decay is a natural, spontaneous process in which an atom of an unstable or radioactive isotope decays or breaks down to form another isotope by losing atomic particles (protons, neutrons, or electrons). When solid radium decays to form radon gas, it emits a bundle of  two protons and two neutrons, an alpha particle, with great velocity (kinetic energy). Radon isotopes are also radioactive because they also decay, emitting another alpha particle and forming an unstable isotope of the element polonium, the first of four short-lived (radioactive) radon daughters, some of which emit yet more alpha radiation and some emitting beta and gamma radiation.

Elements that have important naturally-radioactive isotopes include uranium, thorium, and potassium, as well as radon and radium; potassium is the only element in this list that has stable, nonradioactive isotopes. Uranium and thorium are the parent elements in a long series of radioactive decays (see Uranium and Thorium Decay Chains in our Water Science Basics section) that produces intermediate radium and radon daughter isotopes on the way to a stable isotope of lead. The four isotopes that follow radon in the decay chains all have relatively short half-lives and are collectively known as radon daughters.

The decay of each radioactive element occurs at a very specific rate. How fast an element decays is measured in terms of the element "half-life", or the amount of time for one half of a given amount of the element to decay. Uranium has a half-life of 4.4 billion years, so a 4.4-billion-year-old rock has only half of the uranium with which it started. The half-life of radon is only 3.8 days. If a jar were filled with radon, in 3.8 days only half of the radon would be left. But the newly made daughter products of radon would also be in the jar, including isotopes of polonium, bismuth, and lead . Polonium is also radioactive - it is this element, which is produced by radon in the air and in people's lungs, that can hurt lung tissue and cause lung cancer.

Radioactivity is commonly measured in picocuries (pCi). This unit of measure is named for the French physicist Marie Curie, who was a pioneer in the research on radioactive elements and their decay. One pCi is equal to the decay of about two radioactive atoms per minute. Radon is measured in picocuries per liter and written as (pCi/L). One picocurie is one-trillionth of 37 billion disintegrations per second. One curie is the amount of radiation given off by one gram of radium.

How Does Radon become a problem?

One of the problems with understanding the amount of risk due to a specific radon level measurement is that the risk statistics are based on an average lifetime (70 years) spent in an exposed area, even though the average American moves every 7 years, and is thus exposed to many different radon levels. The American Society of Heating, Refrigeration, and Air Conditioning Engineers has set the lowest level, suggesting a radon action level of 2 picocuries per liter or less for commercial buildings and residences. For perspective, the outdoor radon level is typically on the order of 0.2 pCi/L. The EPA has adopted a 4 picocuries per liter of air action level. The U.S. Mine Safety and Health Administration, on the other hand, suggests an action level of 16 picocuries per liter (while miners are in underground mines). It is ironic that in the early 20th century, people would visit uranium mines to deliberately inhale this ‘energetic’ radon gas, a practice that continues today.

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 8 or 9 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 in it every minute.

When radon is inhaled or consumed, it will decay in the body. As Ra-220 (from the Th-232 decay chain) decays (half-life 55.6 seconds), it releases alpha particles and produces Polonium-216 (half-life 0.2 seconds) that then decays releasing alpha particles and producing Lead-212 (half-life 10.6 hours) then beta particles are released producing Bismuth-212 (half-life 10.6 hours). As Ra-222 (from the U-238 decay chain) decays (half-life of 3.8 days), it releases alpha particles and produces Polonium-218 (3 minutes) that then decays releasing alpha particle producing Lead-214 (half-life 26.8 minutes) then beta particles to yield Bismuth-214 (half life 26.8 minutes). Therefore, the intake of radon via inhalation or consumption and the associated decay of radon and its daughters can expose the body to both alpha and beta particles along with some gamma radiation associated with beta decay.

What are the Health Risks for Radon?

Radon is a naturally-occurring radioactive gas that may cause lung cancer, and may be found in drinking water and indoor air. Some people who are exposed to radon in drinking water may have increased risk of getting lung cancer over the course of their lifetime primarily because radon gas can move from the water into the indoor air, especially by agitating the water in showers, sprays, and washing machines. Nevertheless, radon in soil under homes is the biggest source of radon in indoor air, and presents a greater risk of lung cancer than radon in drinking water. Radon will dissolve into groundwater and can be transported some way from the source. When the water is exposed to air the radon is released. If a well or bore hole is supplied from such water, it can become a significant source of radon in indoor air.

If you are a smoker and the indoor level of radon is 4 pCi/L, the risk of getting cancer is 29 out of 1000. If you are a non-smoker, the risk of getting cancer for the same exposure is 2 out of 1000. Another good reason to STOP SMOKING and protect your health! (Source) Statistically speaking, radon exposure is responsible for about 15% of all lung cancers in the U.S. – the other 85% is from smoking.

Radon is only associated with lung cancer because it is an alpha emitter; the alpha radiation cannot penetrate the outermost layer of dead skin cells.  When inhaled, however, if a radon atom happens to decay inside the lungs (if it doesn’t, it is just harmlessly exhaled), the emitted alpha particle can hit living lung tissue, hence, the possibility of developing lung cancer.  Some people wonder if radon in water could pose a stomach cancer risk. So far, that does not seem to be the case.  It might be that because water is much denser than air, the alpha particles are far more likely to harmlessly expend their energy on water molecules rather than on living tissue.  

The story and the risk do not end with the decay of the radon.  Although solid, the four short-lived radon daughters can remain suspended in air for some time (they are, after all, only single atoms) and be inhaled, generating more radiation as they decay.  The daughters do have an electric charge and tend to cling to surfaces which is good if the surface is outside the body, not so good if it is in the lungs.  This tendency of the daughters to cling to surfaces creates a synergistic effect between cigarette smoking and indoor radon exposure relative to lung cancer risk.  The lung cancer risk for both cigarette smoking and indoor radon can be estimated.  You would think that to compute the combined risk, you would just add the two separate risks but this is not the case.  The combined risk is considerably more than the sum of the individual risks.  How is this possible?  Smoking puts smoke particulates in the air.  Some of the radon daughters will cling to the smoke particulates and remain in the air longer than they otherwise would.  So, when you inhale the smoke particulates, you also get a load of radon daughters, multiplying the lung cancer risk.  

It wasn’t until the early 80s that people came to appreciate that radon and its daughters could be a significant indoor problem and how that came about is an interesting story.  There was a nuclear power plant (the Limerick power plant) under construction in Pennsylvania at that time.  As is normal for such plants, an entrance/exit gate was constructed that could measure any radiation emitted by people entering and leaving the plant.  One day, one of the construction workers, Stanley Watras, entered the plant.  The radiation monitors went bananas which was very puzzling because no radioactive material had yet been brought on-site.  It eventually turned out that Stanley and his clothing were heavily coated with radon daughters (not radon itself),  and it was the beta and gamma radiation from the radon daughters which set off the alarms.  The investigators tracked the source of these daughters to his house which was found to have a radon level of on the order of 4000 pCi/L.  That is when Pennsylvania and, eventually, the rest of the world began to wonder if indoor radon was a problem.  

What are the Standards for Radon?

Indoor Air Quality - The EPA has adopted a 4 picocuries per liter of air action level. Drinking Water - The EPA has only set a health advisory that ranges from 300 to 4000 pCi/L and a MCLG of Zero, but they also set a 10-4 Cancer Risk at 150 pCi/L.

Note: 10-4 Cancer Risk -The concentration of a chemical in drinking water corresponding to an excess estimated lifetime cancer risk of 1 in 10,000.

Radon levels in outdoor air, indoor air, soil air, and groundwater can be very different. Outdoor air ranges from less than 0.1 pCi/L to about 30 pCi/L, but it probably averages about 0.2 pCi/L. Radon in indoor air ranges from less that 1 pCi/l to about 3,000 pCi/L, but it probably averages between 1 and 2 pCi/L. Radon in soil air (the air that occupies the pores in soil) ranges from 20 or 30 pCi/L to more than 100,000 pCi/L; most soils in the United States contain between 200 and 2,000 pCi of radon per liter of soil air. The amount of radon dissolved in ground water ranges from about 100 to nearly 3 million pCi/L.

Note: Radon in water appears to be a problem only to the extent that the radon can get out of the water and into the indoor air which is reflected in the considerably higher limit for radon in water as opposed to radon in air.  Another consideration is that you inhale much, much more air in a day than you drink water. 

Where did the action level of 4 pCi/L for indoor radon come from?  The EPA commonly sets exposure limits for many things based on one extra death in a population of one million because of exposure to something, in this instance, because of a long-time exposure to that level of indoor radon with the development of a lethal case of lung cancer.  The first step in this process, however, was to establish what the long-term exposure to indoor radon was for a given house.  

The PA Department of Environmental Resources, DER (now the Department of Environmental Protection, DEP) placed thousands of alpha-track-etch  detectors in thousands of homes in Pennsylvania for an entire year; the idea was to determine the annual average of the radon levels in the homes.  The detectors are simple pieces of plastic.  Should an alpha particle hit the plastic, it would bore an atomic-sized hole in it.  The hole would be too small to see but after a year’s exposure, the plastic would be etched with an acid which would enlarge the holes to microscopic size.  Some poor grunt would then look through a microscope and laboriously count the number of holes in a standard area.  The average annual radon concentration in the air to which the plastic had been exposed could then be computed from the concentration of holes.  The number of lung cancers associated with various levels of radon could then be compared to generate the 4 pCi/L standard.

Of course, nothing is simple.  Who wants to wait an entire year to find out what the radon levels are, especially if buying or selling a house?  So, devices were developed to determine radon levels in much shorter periods, typically from a few days to a week.  There was a problem.  The short-term test results could vary enormously depending on many things, especially in what season the test was conducted.  In the summer, houses usually have better ventilation.  Attic fans and open windows bring in fresh (low radon) air.  In winter, the house is close up tight and indoor radon levels tend to be higher.  Therefore, DER developed a testing protocol for short-term testing.  All short-term tests are to be conducted under closed-house conditions (close up the house as if it were winter).  This protocol ensured that you should get similar results regardless of what the season happened to be.  

Unfortunately, the 4 pCi/L standard is based on an annual average under normal house conditions (open in the summer, closed in the winter).  The DER eventually decided to go back to some 17,000 whose annual average radon level had been determined with alpha-track-etch detectors and retest them with various short-term monitors under closed-house conditions to see how they would compare.  What they found was that the short-term test results were from four to six times higher than the true annual average as determined by the alpha-track-etch detectors.  The DEP then decided to say that if a short-term test result came in between 4 and 20 pCi/L, then the homeowner should not worry, maybe undertake a few simple, inexpensive control measures, and test for a year with an alpha-track-etch detector, expecting that the long-term result would come in near 4 pCi/L.  Of course, no-body bothers with year-long tests anymore and most people have forgotten the origin of the 4 pCi/L limit.  Now, everybody thinks that the 4 pCi/L is the standard for short-term tests under closed-house conditions.  

Radon is an invisible, radioactive gas. The description includes three scare words which magnify the apparent risk for many people. Radon and its daughters are only associated with lung cancer and most of that is caused by cigarette smoking. It is important to put into perspective all risks to your life and health and not to become overly concerned with just radon. One of us spent many hours in the mid 80s talking with a man who had a short-term radon result of 4.1 pCi/L. He was overweight and chain-smoking. With which risks should he be most concerned?

Get Tested | Radon

Radon causes an estimated 14,000 lung cancer deaths each year. It is the earth's only naturally-produced radioactive gas and comes from the breakdown of uranium/thorium in soil, rock, and water. You cannot see or smell radon, but it can become a health hazard when it accumulates indoors. It can enter your home through cracks and openings in the foundation floor and walls. When radon decays and is inhaled into the lungs, it releases energy that can damage the DNA in sensitive lung tissue and cause cancer. Even though radon can not be seen or smelled, there has been a lot of testing that provides information on the relative amount of radon in indoor air.