Maryland Department of Natural Resources

Radon and Your Home

by James R. Brooks
March, 1988

A Radon-Free Home?

Radon, first discovered in 1910, is a colorless, odorless, radioactive gas formed from the radioactive decay of radium. Radium in turn is formed from uranium which is present to some extent in all rocks but is most common in those of granitic composition. It is not unusual for granites to contain as much as 3.9 parts per million uranium and .0013 parts per billion radium. When rocks weather, these radioactive elements find their way into the soil. Radon has a half-life of only 3.8 days; consequently, most of the gas decays harmlessly in the ground or atmosphere.

Under certain circumstances, radon can migrate through the soil and enter homes in quantities sufficient to become a health hazard. In order for this to happen, there must be a source of radon, the soil must be permeable, and there must be a conduit into the home. The most common points of entry are porous basement walls, cracks in concrete floors or slabs, and openings around utility accesses. Certain home sites exhibit reasonably predictable radon problems. These are :

  1. Homes built in areas where the rocks contain greater than normal amounts of uranium.
  2. Homes built over uranium-bearing veins and granite pegmatites.
  3. Home sites containing radium contamination.
  4. Sites in areas containing high radon concentrations in the ground water.
  5. Homes built on or constructed with radioactive materials such as uranium mill tailings.

In most areas, however, the soils and construction materials exhibit normal concentrations of radioactive material, and the problems of indoor radon are far less predictable. Nevertheless, some generalities can be made. The number of radon-problem houses in an area is usually in a direct proportion to the amount of uranium in the underlying soils and rocks. Granites and rocks derived from quartz-rich igneous rocks normally exhibit higher concentrations of radioactive material than quartz-deficient rocks, so areas of quartz-rich rocks can be expected to present more problems than normal. Houses built over serpentinite will have few if any problems. Houses built in other areas of the Piedmont Province, particularly those built on the zone of phyllites running through central Montgomery, western Howard, eastern Frederick and central Carroll Counties can have indoor radon values far in excess of those considered safe by the Environmental Protection Agency. The sands and gravels of the Maryland Coastal Plain, in general, have been well oxidized and most of the uranium has been leached out and carried away. Certain Coastal Plain sediments, however, contain uranium-rich phosphate beds which are resistant to oxidation, and homes built on these formations may show elevated radon values. Studies indicate that often extremely localized geologic environments, coupled with the idiosyncrasies of building construction, play a major role in those Maryland homes containing abnormal concentrations of radon.

Uranium 238
4,500,000,000 years
Thorium 234
24.1 days
Protactinium 234
1.17 minutes
Uranium 234
247,000 years
Thorium 230
80,000 years
Radium 226
1602 years
Radon 222
3.82 days
Polonium 218
3.05 minutes
Lead 214
27 minutes
Bismuth 214
19.7 minutes
Polonium 214
0.00001 seconds
Lead 210
19.4 years
Bismuth 210
5.01 days
Polonium 210
138.4 days
Lead 206
*alpha radiation = helium nucleus
beta radition = electron

Radon decays into a series of short-lived elements which are inhaled and trapped in the lungs. These elements, along with radon, emit sub-atomic sized particles known as alpha radiation which have high energy but little penetrating power. However, when this energy is expended in the thin lung tissue, it is possible to create cell damage that can eventually lead to lung cancer.

Concentrations of radon are usually expressed either as working levels or as picocuries per liter. A working level month is defined as a potential energy release of 130,000 Mev (Million electron volts) over a period of 170 hours. One picocurie per liter represents the decay of 2.2 atoms of radon per minute in slightly more than a quart of air. The two units do not measure exactly the same thing, but for approximations it can be said that 1.0 working level is equivalent to 200 picocuries per liter.

A variety of methods can be used for detecting radon. The two most popular for the home owner are the carbon canister and the track etch. These devices are placed in either the basement or living area for a specific length of time and then returned for analysis to wherever they were purchased. It should be remembered that measurements can vary with location in the home as well as with the season. Ideally several readings should be taken at different times. Readings taken in the basement during the winter are usually the highest.

The risk of radon in homes has presumably been with us since man first built permanent structures, but did not become a concern until the advent of energy efficient homes and the concern about indoor air pollution. Scientists do not know with certainty how great the radon exposure in homes must be before it becomes an unacceptable health risk. Studies in this respect are difficult to make because the exposure must be over long periods of time, people do not generally live a lifetime in any one place and living habits such as smoking can have great influence on the development of lung cancer. The Environmental Protection Agency, based on studies of uranium miners, suggests that homes ideally should not exceed concentrations of 4 picocuries per liter. Current studies suggest this may be overly conservative. It is generally agreed, however, that values under 20 picocuries per liter or 0.1 working level do not pose an immediate health hazard.

Persons who have encountered excessive radon levels in their homes and wish to take remedial action are advised to read the following pamphlets published by the Environmental Protection Agency :

More information in radon can be found on the EPA web site at:

The Uranium-238 decay series, showing the half-lives of elements and their modes of decay. For example, Radon-222 is produced from the radioactive decay of radium-226, which is, in turn, a product of the decay of uranium-238.


Hanson, M.C., 1986, Radon; in Ohio geology news letter, fall, 1986, Ohio Dept. of Natural Resources, p. 1-6.

National Council on Radiation Protection and Measurements, 1984, Exposures from the uranium series with emphasis on radon and its daughters, NCRP Report No. 77, 131 pp.

Spencer, J.E., 1986, Radon gas: a geologic hazard; in Field notes, winter, 1986, Arizona Bureau of Geology and Mineral Technology, p. 1-6.

Tanner, A.B., 1964, Radon migration in ground water: a review; in The natural radiation environment, Adams and Wayne, eds, University of Chicago Press, p. 161-189.

Tanner, A.B., 1986, Indoor radon and its source in the ground, USGS Open File Report 86-222, 5 pp.

US Environmental Protection Agency, 1986, A citizen's guide to radon, 16 pp.

US Environmental Protection Agency, 1986, Radon reduction techniques for detached houses, 50 pp.

This pamphlet was prepared by James R. Brooks, 1988.
Compiled by the Maryland Geological Survey, 2300 St. Paul Street, Baltimore, MD 21218
This electronic version of "Radon and Your Home " was prepared by Bob Conkwright, Division of Coastal and Estuarine Geology, Maryland Geological Survey.