Radon isotopes form naturally through the radioactive decay of uranium or thorium. These two elements have been present since the earth was formed; thus radon will remain indefinitely at about the same levels as it is now.

Due to the many sources of indoor radon levels, local geology alone is not an adequate predictor of indoor concentration.

Transport of radon in indoor air is primarily a function of the enclosure’s outflow ventilation rate.

• Most residential heating and air conditioning systems operate in a total recirculation mode and thus do not contribute to a building’s ventilation rate.
• Air pressure inside a home is normally lower than that in the soil underneath. This vacuum pulls radon from the ground into the home where levels can increase. Poorly sealed homes tend to ventilate better and not build up radon.
• Under most conditions, the indoor radon concentration increase is in direct proportion to the ventilation rate decrease (WHO 1983).
• Some indoor radon studies showed variability in radon concentrations greater than those attributable to ventilation rates alone. The authors suggested that the strength of the radon source mainly caused the wide range in observed indoor radon levels (Nero 1987).
• Modeling has also extensively studied and predicted the behavior of radon in enclosed areas (Bowring 1992; Eichholz 1987; Kitto 2003).

Other factors found to predispose homes to increased levels of radon include

• Building ventilation rates,
• Entry points for soil gas,
• Foundation type,
• Location,
• Soil porosity,
• Source of water supply, and
• Type of building materials used.

If the home was built over mine or mill tailings, the probability of radon gas seeping into the building is higher, as the tailings are likely to have higher levels of radium and emit more radon than will the ambient ground.

In addition to pressure differences, the type of building foundation can affect radon entry.

• Basements allow more opportunity for soil gas entry, while slab-on-grade foundations (no basement) allow for less.
• Although slab-on-grade foundations allow for less soil gas entry than do basements, both types of foundations permit entry of radon, especially if the foundation or slab is cracked and an underlying diffusion barrier is absent.
• Building foundations may also produce radon and release it into buildings.

Home “tightening” for energy conservation retains more of the radon and its progeny that enter the home from soil and building materials, allowing levels to increase.

Further research is underway on how to predict those homes most likely to have significant radon levels.

Methods of Detection

Short-term and long-term tests (lasting a few days to several months) are available to identify whether you have increased levels of radon gas in your home.

• Testing should be conducted in the lowest-inhabited area of the home. “Closed house conditions” should be met, which involves keeping doors and windows shut; placing the test units away from windows, doors, and vents; and meeting other parameters that help ensure accuracy.
• “Do-it-yourself” short-term testing kits are available at hardware stores.

Short-term testing is the quickest way to determine the presence of a potential problem.

Short-term radon detection tests.

Short-term tests remain in your home for two days to 90 days, depending on the device. "Charcoal canisters," "alpha track," "electret ion chamber," "continuous monitors," and "charcoal liquid scintillation" detectors are most commonly used for short-term testing.

• Charcoal canister tests are inexpensive (approximately $25) and generally used for short-term testing (3 to 7 days).
• Alpha track detectors are suitable for measuring cumulative exposure over a short or long period (several weeks to a year) and cost roughly twice that of the charcoal canister.
• Electret ion chamber
• Continuous monitors
• Charcoal liquid scintillation

Exposed devices are mailed to a certified laboratory for analysis. Because these devices measure radon gas levels rather than radon progeny, the units reported are in pCi/L.

Long-term radon detection tests.

Long-term tests remain in your home for more than 90 days and will give a better reading of a home's year-round average radon level than will a short-term test. These testing methods are available only through a professional service.

The most common long-term testing devices are

• Alpha track detectors, and
• Electret ion detectors.

EPA Recommends the Following Testing Steps:

Step 1. Do a short-term test. If your result is 4 pCi/L or higher, do a follow-up test (Step 2) to be sure (See Standards and Regulations).

Step 2. Follow up with either a long-term test or a second short-term test.

For a better understanding of your year-round average radon level, do a long-term test.

If you need results quickly, do a second short-term test.

The higher your initial short-term test result, the more certain you can be that you should do a short-term rather than a long-term follow-up test. If your first short-term test result is more than twice EPA's 4 pCi/L action level, you should immediately do a second short-term test.

Step 3. If you followed up with a long-term test, fix your home if your long-term test result is 4 pCi/L or more. If you followed up with a second short-term test, the higher your short-term results, the more certain you can be that you should fix your home. Consider fixing your home if the average of your first and second test is 4 pCi/L or higher.

Radon Abatement and Remediation

Subslab depressurization with suction lowers the soil pressure below that inside of the home, preventing inward soil gas migration.

• Subslab depressurization is one of the most effective methods of lowering radon levels in many homes.
• Subslab depressurization can reduce indoor radon levels by as much as 99%.

Figure 3. Sub slab Depressurization
Pipes, attached to a suction fan, are inserted into the ground below the basement floor, creating a low-pressure region under the house. Adapted from Brenner (1989).

Subslab depressurization is not expected to reduce indoor radon levels below those found outdoors.

Real Estate Transactions

To determine potential increased exposures to radon gas, a standard practice in some states has been to measure radon levels in homes at the time a property is sold.

Today, many homes are built to prevent radon from entering them. Your state or local area may require such radon-resistant construction features.

If you are buying or renting a new home, ask the owner or builder if it has radon-resistant features. In high radon potential (Zone 1) areas, the EPA recommends building new homes with radon-resistant features. Even if every new home is built radon-resistant, after occupancy it still should be tested for radon.

The EPA Home Buyer’s and Seller’s Guide to Radon (also available in Spanish) includes valuable information on radon testing and mitigation for real estate transactions. http://www.epa.gov/radon/pubs/hmbyguid.html

EPA Map of Radon Zones

The EPA map of radon zones was developed to assist national, state, and local organizations to target their resources and to implement radon-resistant building codes.

http://www.epa.gov/radon/zonemap.html

http://www.epa.gov/radon/images/zonemapcolor_800.jpg

This map is not intended to determine whether a home in a given zone should be tested for radon.