Is radon radioactive?

A half-life is the time it takes for half of the radioactive atoms present to decay. After 3.8 days, half of any given amount of radon-222 has become polonium-218; after another 3.8 days, half of what is left has gone. That is fast, but it does not mean the radon in a house runs out, because the ground beneath it keeps making more.

Radon is one stop on a long chain of radioactive decay. When a radioactive atom decays, it transforms into a different atom, and the atoms keep transforming until they reach a stable state. The chain that begins with uranium-238 forms uranium-234, thorium-230, radium-226 and radon-222 along the way and ends at lead-206.

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1. 238U

   Uranium-238

   Solid

2. 234U

   Uranium-234

   Solid

3. 230Th

   Thorium-230

   Solid

4. 226Ra

   Radium-226

   Solid

   Half-life 1600 years

5. 222Rn

   Radon-222

   Gas

   Half-life 3.823 days

6. ···

   Radon's decay products

7. 206Pb

   Lead-206

   Solid

• Uranium-238. More than 99 percent of the uranium found in the environment is in the form of U-238. Uranium decays by alpha particles.
• Uranium-234. Less than one percent of natural uranium, but much more radioactive.
• Thorium-230. An intermediate EPA names in the uranium-238 chain.
• Radium-226. Radium decays to produce radon gas.
• Radon-222. The most common radon isotope, and an alpha emitter. As a gas it can move up through the ground and into a building.
• Radon's decay products. Radon gas decays into radioactive particles that can get trapped in your lungs when you breathe.
• Lead-206. Where the chain ends. Only the final, stable atom in the chain is not radioactive.

EPA names uranium-234, thorium-230, radium-226 and radon-222 as intermediates "such as" these, so the chain has more steps than are drawn. Half-lives are shown only where a cited source states one.

Every decay product in such a chain is radioactive; only the final, stable atom is not. Radon forms in three of these chains, which begin with uranium or thorium. Each uranium or thorium atom decays about a dozen times, and radium and then radon form midway through. Radium-226, radon's parent, has a half-life of 1,600 years, so the ground under a house goes on producing radon for a very long time.

Radon gives off alpha particles, which are so heavy that they use up their energy over short distances. All three common kinds of radiation from radioactive decay are ionizing: they carry so much energy they can knock electrons out of atoms, which is how they damage tissue and DNA. Each kind is dangerous in a different way.

That is the whole story of radon in one row. Alpha decay usually isn't considered a great radiological hazard, because the particles are easily stopped, but radon is a gas, so it is easily inhaled and living tissue is directly exposed. Once inside, alpha damage is severe: the ionizations an alpha particle causes are very close together, so it can release all its energy in a few cells, which means more severe damage to cells and DNA.

Much of the concern about radon is about what it turns into. The short-lived radioactive atoms radon quickly decays into are called radon progeny. They are solid particles that can attach to dust and other particles and move with the air, and many scientists believe the alpha radiation dose from long-term exposure to elevated levels of radon progeny is what raises the chance of getting lung cancer.

1. 1

   Radon is breathed in

   It is a gas, so it goes wherever the air goes, including into the lungs.

2. 2

   It decays into solid particles

   Radon gas decays into radioactive particles that can get trapped in the lungs.

3. 3

   The particles keep decaying

   As they break down further, they release small bursts of energy.

4. 4

   Lung cells take the hit

   The particles are deposited on the cells lining the airways, where they can damage DNA and potentially cause lung cancer.

Radon levels are a measure of radioactivity, not of weight or volume. In the United States the unit is picocuries per liter of air, or pCi/L. The other common unit, used by WHO, is becquerels per cubic meter: one becquerel is the transformation of one atomic nucleus per second, so Bq/m3 counts the transformations each second in a cubic meter of air.

Those small numbers add up to a large share of the radiation people receive. For most people, radon is the single greatest environmental source of radiation exposure, and in many countries radon in homes accounts for half of the public's exposure to naturally occurring radiation. The Department of Health and Human Services, the International Agency for Research on Cancer and EPA all classify radon as a human carcinogen.