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People are exposed to radiation from a variety of sources, including cosmic rays, medical and dental x-rays, and trace amounts of radioactive elements in the materials around us. Even our own bodies contain small quantities of radiactive material (carbon-14, which is used to date artifacts, is an example). However, by far the largest source of radiation in our lives is due to radon. By some estimates, radon accounts for more exposure than all other sources combined. Furthermore, the exposure due to radon occurs primarily inside our throats and lungs. Thus, radon may be the second most common cause of lung cancer (if you had any doubt, cigarette smoking is by far the most common cause). Estimates range from 7,000 to 30,000 cases of cancer due to radon each year in the US.

In this article, we will use some of the physics we have learned to understand what radon is, how it affects us, and what can be done about it.

Radon is an inert gas (chemical symbol Rn) with atomic number 86; you can find it in the periodic table just below xenon (Xe). It is colorless, odorless and tasteless. The most common isotope of radon, the one which people worry about, is ²²²Rn. This isotope is produced when ²²⁶Rd (radium) decays by emitting an alpha particle. The radium itself comes from decay of heavier radioactive elements. The whole chain starts with ²³⁸U, and thus is called the "uranium series." The series ends with ²⁰⁶Pb, which is a stable isotope. Uranium, and hence radon, is a component of soil in many parts of the world. Thus radon can be found in many places, including the entire United States.

Radon is dangerous to people primarily through the threat of lung cancer. Radon exposure outdoors is fairly small, however, radon can build up in homes and businesses, causing high radiation exposure, particularly to the lining of the throat and lungs (the tracheobronchial epithelium, often abbreviated TBE).

In actuality, it is not radon itself that causes most of the harm. Radon 222 has a half-life of about 3.8 days. Thus, chances are that you will exhale any radon that you inhale. The damage is actually caused by the short half-life "progeny" of radon: ²¹⁸Po, ²¹⁴Pb, ²¹⁴Bi, and ²¹⁴Po. These isotopes have much shorter half-lives (measured in minutes) and they are not inert. An airborn radon atom decays into an alpha particle and a ²¹⁸Po atom which is still airborn. If you inhale the polonium, chances are that it will stick inside your lungs and emit an alpha particle of its own while it is there. The resulting atom (usually ²¹⁴Pb) will also decay inside your lungs, etc.

It is worth noting that this whole chain of events is especially serious for cigarette smokers. Not only do cigarettes cause cancer on their own, but they also increase the damage do to radon. This may be because smoking reduces the lungs ability to clear themselves of dust, aerosols, etc., which may carry ²¹⁸Po, etc., into the lungs.

What can be done about this? The most effective solution is to make sure that homes, schools, etc., do not have high concentrations of radon inside.

Image from "The Geology of Radon" by James K. Otton, Linda C.S. Gundersen, and R. Randall Schumann of the United States Geological Survey

Radon builds up in buildings because the radium decaying in the ground creates a resevoir of radon in the tiny spaces between grains of soil. This radon gets sucked into the lower levels of buildings through cracks in the foundation, porous building materials (e.g., concrete blocks), sumps, joints, etc. In fact, all houses get some fraction of their inside air from the soil. Usually the amount is small, but in some houses the fraction can be as high as 20%. In some cases, radon can also enter homes through well water.

The process of reducing the amount of radon that gets into buildings is called radon mitigation. Calculating the risks of low radon exposure is difficult, so it is up to individuals to decide how much radon they can live with. The EPA recommends that radon in homes be reduced to below 4 picocuries per liter of air (recall that a Curie is 3.7 x 10¹⁰ disintigrations per second). For most homes, this can be accomplished for $500-$2500. Other recommendations are somewhat less cautious, but most are within a factor of two. The National Council on Radiation Protection and Measurement (NCRP) recommends 8 pCi/l as a maximum acceptable level. As a reference point, typical outside air has an activity of about 0.2-0.3 pCi/l.

There are several ways to determine the level of radon in a building. There are test kits that you can buy, usually for about $25. In these tests, you open the test kit and let it "collect" for a specified period of time, anywhere from a few days to several months. You then return the kit to the manufacturer to be read. The findings are returned to you by mail. This type of test is fairly accurate (use kits that are labeled "meets EPA requirements") and relatively inexpensive. Furthermore, they measure the average radon level over the test period. Since the radon level in a house often varies with weather, season, etc., it is important to measure the average radon level to get a clear idea of your exposure. If you need to know the radon level quickly, it is also possible to hire a professional who will bring more sensitive test equipment that reads the radon level accurately in a short period of time. In some cases, this equipment monitors the radon level at hourly intervals, and produces a graph of radon concentration vs. time over a day or two. Tests of this kind are becoming increasingly common in connection with real estate transactions.

Image coutesy of Infiltec

There are many contractors all over the US who handle this work. The Environmental Protection certifies professionals who provide radon measurement and mitigation services through its radon proficiency program (RPP). Lists of certified contractors are available on the EPA web site, or through state radon offices.

You can get a lot more information about this subject on the internet. Here are a few search engines

1. Alta Vista

2. Google

3. Northern Light

4. Ask Jeeves

5. Infoseek

And here are a few good links to get you started.

1.

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3.

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This site is made possible by funding from the National Science Foundation (DUE-9981111).

©2001 Andy Gavrin