Testing for Radon

Radon Measurement Key Points

  • There is no "totally safe" level of radon exposure
  • 4 picoCuries per liter is the "action level" recommended by the EPA
  • All radon problems can be fixed
  • Radon levels in homes can typically be reduced to between 2-4 picoCuries per liter
  • EPA's Testing Checklist


How to Test for Radon

There are reliable ways to measure indoor radon for short-term test periods that last only a few days, and for long-term test periods that may range from a few months to more than a year. Using recommended measurement devices, you can determine the radon potential of the home under "worst case conditions" (short-term test), or measure actual radon exposure under normal living conditions, (long-term test).

Homebuyers and sellers often prefer to have a radon test performed by a trained professional tester. In that case, EPA strongly recommends the use of a qualified radon measurement professional who has been trained in the proper placement of radon measurement devices and the interpretation of their results.

Short Term Tests – Potential Radon Exposure

Often, at the time of sale, it is desirable to know a building's potential for radon exposure, independent of how the building is currently used. Short-term tests are typically conducted over a two or three day period. Results of short-term tests represent the radon potential of the home, rather than the actual exposure encountered under normal living conditions, unless residents keep the home's windows and doors closed year-round. That's because EPA guidelines for short-term radon tests require "Closed-House Conditions," to promote maximum radon concentration during the brief test period.

The placement of the device within the home must follow the manufacturer's instructions and is dependent on whether or not the test is being conducted for a real estate transaction.

Short-term tests are usually 2 to 7 days long and are done under "closed-house" conditions that begin at least 12 hours before the test and remain in effect for the duration of the test.

Closed House Conditions

house a

Placing the test device in a closed room or leaving doors and windows open during the test is improper and can invalidate a test.

Short-term, "closed house" radon tests should not be conducted during rainy weather, especially when the rain is accompanied by persistent winds. Changes in barometric pressure and other forces can cause indoor radon levels to rise during a rain storm and skew the test results higher than found under otherwise normal conditions.

The following protocols should be followed to comply with the requirement for "closed-house" conditions:


  • Exterior windows and doors are kept shut, except for normal entrance and exit.
  • Fans and blowers which move air from the outside of the house to the inside, or exhaust inside air to the outside, are turned off, including swamp coolers
  • Whole house fans should be off
  • Air conditioners should be put on "recycle" or "max.-cool," but not on the "fresh air" setting
  • Combustion or makeup air to gas fired appliances are NOT to be closed
  • The house can be occupied during testing, provided the "closed house" conditions described above are maintained.
  • If testing during warm weather, keep the air-conditioning on and set the fan to recycle indoor air. Don't use the "fresh air" setting. Evaporative coolers, sometimes called "swamp coolers", should be turned off during the test, to avoid bringing outdoor air into the house

Radon Test Devices

test devices



There are several devices on the market. EPA recommends that you begin with a short-term test device placed in the home for a minimum of two days. The devices in most home test kits are actually small containers of activated charcoal. Look for kits approved by one of the national certification programs (NRPP or NRSB).

You can purchase a kit from hardware stores or other retail outlets, as well as through non-profit organizations such as the National Safety Council (800-767-7236). Most of the kits range in price from $10 to $20. The cost usually includes the test device, the price of postage to mail the detector back to the laboratory, and the written report you receive by return mail.

There are two types of short-term test devices:

  • Integrating Devices, which measure radon and provide an average reading over the time of the test; and
  • Continuous Radon Monitors, which measure radon and provide hourly readings as well as an overall average.

It is not appropriate to use "Grab Samples," which test for less than 48 hours, as this is more of a diagnostic device and will not provide a representative radon potential measurement for the home.

If a short-term radon test is conducted correctly for a minimum of two days, under closed-house conditions, one can reasonably say:

  • If the result is less than 4.0 pCi/L, the annual average of the home under normal lived-in conditions is also likely to be less than 4.0 pCi/L.
  • If the level is at or above 4.0 pCi/L, the house has the potential to average more than 4.0 pCi/L, and you should consider follow-up testing or taking action to reduce (mitigate) the radon in the home.

Test Device Location

The location of a radon test depends upon whether the test is being done for a real estate transaction or not. Since the purpose of the first, short-term test is to be able to identify homes that are clearly below 4.0 pCi/L, it is necessary to place the test device in a part of the home that would be expected to have the highest radon level.

test locationThe device should be placed in a room that is frequently occupied, but where high humidity in the air would not be expected. Examples of good locations would be bedrooms, dining rooms, and family rooms. Never place the device in a closet, crawl-space, storage area, kitchen, garage or bathroom.

In order to have confidence in the radon reading, the device should be placed in the lowest occupied space, for non-real estate transactions. A finished basement is normally chosen in those parts of the country that typically have basements.

In the case of a real estate transaction, the device should be placed in the lowest portion of the house that could be finished and occupied by future occupants.

If the radon measured is below 4.0 pCi/L, there is good reason to believe that the rest of the home is also below 4.0 pCi/L. Furthermore, if the closed-house test protocols are followed, there is good reason to believe that a low short-term test result (below 4.0 pCi/L) means that the average radon throughout the year will probably also be below 4.0 pCi/L, during normal use of the house.

Remember, short-term tests determine the radon potential of a home, independent of how future homeowners may operate or occupy the house.



Test Device Placement

test tableA proper location must be selected to obtain an accurate measurement of radon in air that represents the breathing space of the home. The test device should be at least 20 inches from the floor, 4 inches from another object, 12 inches from an exterior wall, and 3 feet from an outside window. The device can be placed near an interior wall, perhaps on a book shelf, but should be at least four inches from the wall or the back of the shelf, to allow good air circulation.

Devices that are designed to be hung by a string should be approximately 12 inches from the ceiling.

Test devices should also be located away from drafts and should not be placed in rooms with excess humidity.


  • 3 feet from windows or exterior doors
  • At least 20 inches above floor
  • 4 inches from other objects
  • Where it won't be disturbed
  • Away from drafts
  • Out of direct sunlight
  • Not on hot or warm surfaces

Long-Term Tests – Actual Radon Exposure

For the occupants of a home, actual radon exposure depends on how they use the home, where in the home the occupants spend their time, and how much freshair is brought into the living area. Since these factors may vary over time, the only reliable way of measuring the actual radon exposure is to conduct a long-term test for at least three months, under normal living conditions.

In the past, prospective homeowners have often been reluctant to purchase a home before performing a long-term test, for fear of not being able to correct a radon problem afterward. However, improved technology and the proven durability of radon mitigation systems have served to reduce much of that concern.

This does not mean that a short-term test is less valuable as part of a home inspection process; but rather, if the results of that test show a potential radon concern, a long-term test can more accurately show actual average radon levels. By conducting a long-term test after moving into a home, the homeowner can control test conditions and, if needed, make decisions on how a mitigation system will be installed.

The placement of the test device within the home must be in accordance with the manufacturer's instructions.

Test Results

test flowchart



If you perform the short-term test and the results are higher than 4.0 pCi/L, the EPA recommends you take further action. The next step should be to retest the home on a long term basis, ideally for a year, then decide if mitigation is necessary.

However, if the initial short-term test finds radon levels are significantly elevated, such as 10 pCi/L or more, you may want to repeat the short term test using a different test device to confirm the radon is still elevated. You can then average the two results and base your mitigation on the average.

The health risk from radon is cumulative, increasing over time if the radon level is elevated and not corrected. The health risks from radon occur over a long period of time and radon concentrations vary from season to season. An average radon level, measured over all four seasons, is a better indicator of actual health risk over time.

On the other hand, if your initial readings are significantly elevated, you should take action to quickly confirm these readings and then proceed to mitigate the home. Again, you may want to repeat the short-term test, using a different test device to confirm the radon is still elevated, then average the two results and base your mitigation on the average.

Testing for Radon Decay Products

Radon decay products can also be measured using a special monitor that reports in Working Levels (WL). This can be done as an initial measurement or, more typically, after initial measurements have identified a potential concern in commercial buildings or homes with relatively low initial radon readings. The EPA guidance for radon decay products (comparable to 4.0 pCi/L of radon) recommends people should avoid long-term exposures in excess of 0.02 WL of radon decay products.

Testing Water for Radon

Radon in the ground can dissolve into water that finds its way to a well. When well water that contains radon is brought directly into a building, the dissolved gas is released indoors as the water leaves the faucet, showerhead or other outlet.

The amount of radon brought into the building will depend upon the amount of water used and the amountof radon in the water. Given typical water usage rates and radon concentrations found in wells, this entry mechanism only accounts for about 1-2% of the radon that enters homes in the U.S.

Most indoor radon comes from the soil. Thus, most remediation efforts concentrate on reducing the entry of radon from soil rather than water. As a rule of thumb, it takes 10,000 pCi/L in the water to add 1pCi/L to the air in a home, after the radon dilutes and dissipates within the large volume of air indoors. This is above and beyond that which comes from the soil.

The US EPA has recommended maximum contaminant levels (MCL) ranging from a low of 300 pCi/L to as high as 4,000 pCi/L of water for community water supplies. The regulation, as proposed, would not impact private wells, and no firm implementation date has been set.