How Radon Works

Defocused periodic table of the elements with the element Radon magnified through a glass marble.
Radon and uranium have a lot in common. Adrienne Bresnahan / Getty Images

If you live in certain parts of the United States, you might be aware of the threat that radon gas poses. Radon gas is radioactive, and in tightly insulated houses it can accumulate to concentrations that pose a health threat. If you inhale the gas into your lungs, its decay can increase your chance of getting lung cancer. A study reported in 1990 by the National Safety Council estimated that about 14,000 deaths a year could be attributed to radon, and that the number could range from 7,000 to 30,000.

In this edition of How Stuff Works, we'll talk about what radon is, how you can find it in your home and what the health risks are for it.


What is Radon?

Radon gas, like carbon-14 gas, is completely natural. It forms during the decay of uranium-238, an element with a fairly interesting decay sequence (to learn more about decay sequences and radioactivity in general, see How Nuclear Radiation Works):

  1. Start with a uranium-238 atom. This atom has 92 protons and 146 neutrons. It has a half-life of 4.5 billion years. When it decays it emits an alpha particle, leaving behind a thorium-234 atom.
  2. A thorium-234 atom has 90 protons and 144 neutrons. It has a half-life of 24.5 days. When it decays it emits a beta particle and a gamma ray, leaving behind a protactinium-234 atom.
  3. A protactinium-234 atom has 91 protons and 143 neutrons. It has a half-life of 269,000 years. When it decays it emits a beta particle and a gamma ray, leaving behind a thorium-230 atom.
  4. A thorium-230 atom has 90 protons and 140 neutrons. It has a half-life of 83,000 years. When it decays it emits an alpha particle and a gamma ray, leaving behind a radium-226 atom.
  5. A radium-226 atom has 88 protons and 138 neutrons. It has a half-life of 1,590 years. When it decays it emits an alpha particle and a gamma ray, leaving behind a radon-222 atom.

That radon atom is a gas atom, and it has a half-life of only 3.825 days. Accumulations of radon atoms from the natural nuclear decay of uranium-238 is where radon gas comes from. That means that radon gas concentrations are higher where uranium is plentiful in the soil. For completeness, here is the rest of the sequence:


  1. radon-222, with a half-life of 3.825 days, emits an alpha particle to become polonium-218.
  2. polonium-218, with a half-life of 3.05 minutes, emits an alpha particle to become lead-214.
  3. lead-214, with a half-life of 26.8 minutes, emits a beta particle and a gamma ray to become bismuth-214.
  4. bismuth-214, with a half-life of 19.7 minutes, emits either an alpha particle or a beta particle and a gamma ray to become either thallium-210 or polonium-214.
  5. polonium-214, with a half-life of a 150 microseconds, emits an alpha particle to become thallium-210.
  6. thallium-210, with a half-life of 1.32 minutes, emits a beta particle to become lead-210.
  7. lead-210, with a half-life of 22 years, emits a beta particle and a gamma ray to become bismuth-210.
  8. bismuth-210, with a half-life of five days, emits a beta particle to become polonium-210.
  9. polonium-210, with a half-life of 138 days, emits an alpha particle and a gamma ray to become lead-206.
  10. lead-206 is a stable isotope of lead.

How Does Radon Get into a Home?

As you saw in the previous section, radon atoms are fairly short-lived. Over the course of several days a radon atom becomes a lead atom. While it is a radon atom, however, it is a gas. Because radon is a gas, it can seep from the ground into the air in a house. The primary way that radon enters a house is through the foundation (crawl space, basement) by a variety of paths:

  • cracks in basement floors
  • drains
  • sump pumps
  • exposed soil
  • construction joints (mortar, floor-wall)
  • loose fitting pipes

Radon may also enter the air of a house from well water, but this is a minor source compared to that coming in through the foundation. Some parts of the United States have higher risks of radon than others. The U.S. Environmental Protection Agency has a map that shows which counties and states have higher average radon levels. You can see the map here.


How Do You Test for Radon?

Radon gas has no smell or color so a test must be done to detect it. In fact, the U.S. Office of the Surgeon General and U.S. EPA recommend that all homes be tested for radon, especially if you are buying, selling or building a home. Testing can be done by a professional or homeowner using a "do-it-yourself" kit. It is important that either the professional service or home test kit be EPA-approved.

Because radon levels can vary from day to day and from season to season, testing can be done on the short-term (two to 90 days) or long-term (greater than three months). Short-term tests are best done if the results are needed quickly and should be followed by another short-term test. Long-term tests will yield better information on a home's average year-round radon levels. Radon test devices are placed in the lowest occupied level of the home.


Radon Test Devices

Radon tests detect either radon gas directly or the daughter products of radon's radioactive decay. There are two categories of radon test devices, passive and active. Passive devices require no electrical power and generally trap radon or its daughter products for later analysis by a laboratory. Passive devices include charcoal canisters, charcoal liquid scintillation detectors, alpha track detectors and electret ion detectors.

  • Charcoal canister and charcoal liquid scintillation devices absorb radon or its products on to the charcoal. In the laboratory, the radioactive particles emitted from the charcoal are counted directly by a sodium iodide counter or converted to light in a liquid scintillation medium and counted in a scintillation detector.
  • The alpha track detectors have a plastic film that gets etched by the alpha particles that strike it. In the laboratory, the plastic is chemically treated to make the tracks visible, then the tracks are counted.
  • Electret ion detectors have a Teflon disc, which is statically charged. When an ion generated from radon decay strikes the Teflon disc, the electrical charge is reduced. In the laboratory, the charge reduction is measured and the radon level is calculated.

Typically, all of the passive devices, except electret ion detectors, are available in hardware stores or by mail; electret ion detectors are usually only available through laboratories. The passive devices are generally less expensive than active devices and may require little or no special training for their use. Of the passive devices, the charcoal canisters and charcoal liquid scintillation detectors are typically used for short-term tests.

In contrast to passive devices, active devices need electrical power and include continuous monitoring devices (continuous radon monitors, continuous working level monitors). Active devices detect and record radon or its daughter products continuously. They are generally more expensive and require professionally trained testers for their operation.

Radon levels in the average home are about 1.25 picocuries/liter of air (pCi/L). If a radon test discloses levels of 4 pCi/L or greater, then some action should be taken to reduce the radon level. Radon can be reduced by preventing its entry into the home or by removing it once it has entered the home. The general solution involves active ventilation either in the basement or below the slab of the home.

What is the Health Risk of Radon?

There have been no reports of short-term effects or symptoms caused by radon exposure. The only reported long-term effect is lung cancer. If you inhale a radon atom, the atom can disintegrate while it is in your lungs. When it disintegrates, it becomes polonium-218, which is a metal. This metal atom can settle in your lungs, and over the next hour or so it will emit a number of alpha particles, beta particles and gamma rays. It eventually turns into lead-210 with a half-life of 22 years, which is fairly stable in this context. But now you have an atom of lead in your system, which causes its own problems. It is the quick, hourlong sequence of alpha, beta and gamma emissions that can lead to the mutations in the lung tissue, which can cause cancer. Smoking enormously increases the risk of lung cancer from radon exposure.

So you can see that a high concentration of radon gas, despite the fact that it is completely natural, is not something you want in your home.