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Radon Testing Frequently Asked Questions

  • At what level is Radon considered an emergency?

    The EPA risk chart is based on a lifetime of exposure. Dangers from Radon gas are based on the amount of time exposed and the average radon level.  If a Radon mitigation system cannot be installed in the next few weeks and the radon concentration is greater than 50 pCi/L, you may consider ventilation by leaving a basement window continuously open and/or a bedroom window open.

    • 0.4 - 0.7 pCi/L  - Average exterior Radon concentration.
    • 2.0 pCi/L -  Most homes will fall below 2 pCi/L following mitigation. The US EPA recommends you consider mitigation between 2 and 4 pCi/L. Not all homes are capable of reaching levels below 2 pCi/L without major reconstruction.
    • 2.7 pCi/L - The World Health Organization's (WHO) action level
    • 4.0 pCi/L  - The US EPA action level at which point mitigation is recommended. 

    The EPA guideline of 4.0 pCi/l is not a safe level. There is no safe level of radon, only reduced risk.

  • Do I need a Radon measurement professional to test my home?

    For non-real estate transactions, consider conducting the test yourself. By following the directions you can get an accurate understanding of your home's radon levels.

    You can test for radon using a do-it-yourself test kit. They are available for purchase on this website, from the Missouri Department of Health and Senior Services or from a local hardware store. You may wish to confirm these tests by hiring a professional to deploy a Continuous Radon Monitor.

    You can run a non-professional passive radon test, as the home seller in advance of selling your home. It may help you with insight to a potential hidden cost before negotiations on the price of your home. It is unlikely a homeowner's at home test will satisfy the buyers need to test for Radon.

  • Are Continuous Radon Monitors more accurate than passive tests?

    The radon test kits from local hardware stores and the state are accurate. Continuous Radon monitors that give hour by hour radon results can be more accurate than passive test kits, however correct use of the passive devices will provide an accurate understanding of the radon concentration. CRM's offer the advantage of measuring the radon levels every hour, which can indicate unusual radon patterns due to weather, possible radon test tampering, or other home system interference.

  • Is short term or long term testing the best?

    For a real estate transaction a short-term test is necessary. 

    For a non-real estate transaction a short recommend short term testing be performed initially. If the test results are marginal, a homeowner may choose to deploy a long term test.

    Most of the radon tests being done today are short term. Short-term measurements are great to quickly find out if your home has a high radon concentration. The EPA recommends deployment of at least two tests if you are not involved in a real estate transaction.

    If the initial test results are above 10 pCi/L, it is best to forego long term testing and consider mitigation and/or perform another short term test.

    EPA Recommendations for Radon Testing: Radon Testing for Real Estate Transactions

    "If you are testing in a real estate transaction, any of the following three ways to conduct Short-Term Tests are acceptable"

    Short Term Testing Options What to Do Next
    Active: (Continuous Radon Monitors)
    Test the home with a continuous monitor for at least 48 hours.
    Fix the home if the average radon level is 4 pCI/L or more.

    Passive: (Charcoal Canisters, Electrets, etc)
    Take an initial short-term test for at least 48 hours. After the first test has been completed, take a follow-up short-term test for at least 48 hours.


    Take two short-term tests at the same time in the same location for at least 48 hours.

    Fix the home if the average of the two tests is 4 pCI/L or more.

    Active Testing: Continuous Monitors

    • Helpful for time sensitive decisions and when very accurate hourly information is needed
    • Graphs hourly data regarding radon fluctuations which may assist with analyzing borderline results
    • Our CRM machines report device movement, (test tampering)

    Passive Testing with Charcoal Canisters

    • Cost effective for home owner testing when there is time to double check readings
    • Produces a single number which represents the US EPA radon level average
    • More than one test or testing device is recommended when using passive testing devices
    • Test deployment should be simultaneous

    Please note that there is no safe level of radon, and that all levels pose some risk.

  • Can Radon test results be manipulated to show below average readings?

    Tampering with a Radon testing device, is a serious moral offense, creating artificial security and potentially a life threatening health hazard. Test tampering could threaten the lives of an innocent family by providing an artificial sense of security.

    Most CRM machines have tamper resistant features. If so moving of the machine would be marked on the results. Data cannot be manipulated from the CRM machine, it prints directly into a "read only" /unchangeable document.

    Radon levels will change when closed house conditions are not meet. Especially if the open window or door is in the same area as the test kit. Opening windows or doors and leaving them open could either raise or lower Radon concentration depending on the air flow within the home.

    Continuous Radon Monitors are more likely to detect changes in the radon testing environment. Passive systems could be moved to an outside location and returned. Most Continuous Radon Monitors report Radon concentration hourly, and detect movement.

  • Does a vacant house have higher radon levels?

    The lowest level / basement level of a vacant house, is not likely to be significantly different than if the home was occupied. Radon does not build up to higher levels during vacancy because of natural ventilation and the 3.8 day half life of Radon Gas.

    It is recommended that average normal living condition temperatures be maintained during Radon testing, regardless of occupancy. Heating and cooling systems should be ran as if the home was occupied.

  • Is the radon level higher in the basement?

    The highest levels are almost always in the basement or lowest level of the home. If you have a basement and there is a door between the basement and the upper floors then the upper floors will likely have significantly lower levels. Often the levels above the basement are as low as 30%-50% of what is reported in the lowest level. This will vary depending on the season, heating system and separation of basement and first floor. This information should be confirmed if used as part of the decision to mitigate.

  • How to increase the accuracy of your at home radon test kits?


    • Passive test problems occur mostly from improper use, extreme weather patterns, miss-interpretation, or insufficient data.
    • Passive tests are otherwise to be considered an accurate method for measuring Radon.
    • Charcoal testing can be bias towards the latter part of the testing period. (Avoid testing in severe weather patterns)

    Simultaneous charcoal testing helps to make sure the test results are an accurate measurement of radon concentration.

    If the score is above 4 Pico curies, then the two units should be within 36% of each other to be considered accurate.

    If the score is below 4, then they need to be within 67% of each other to be considered accurate. Place the kits about one foot apart. Do not place them in separate locations. If you want to test different rooms or floors, then you need to buy additional kits.

    The closer the short-term testing result is to 4.0 pCi/L, the less certainty there is about the home's year round average being above or below the EPA's action level.

  • What if my first radon test is lower than the EPA's action level of 4pCi/L?

    The EPA recommends that more than one passive test be run to accurately determine radon concentration. Following those guidelines and determining that radon concentration is modest, a homeowner may choose to retest the home during the opposite season, or perform a long term test to be certain.

    It is beneficial to test for radon periodically throughout the years, even if initial tests are low as the soil composition and structure under your home may change form, especially if major ground moving events are reported, i.e. blasting, earthquakes, global earth shifts.

    The EPA recommends that homes with mitigation systems be retested at least every two years.

  • What is a long-term radon test?

    A long-term radon test is three months to one year in length. Long-term tests do not require closed house conditions. A long-term test may be preferred if the initial short term test averages are low to moderate and the homeowner is interested in a larger testing period that may include more than one season.

  • What is the seasonal effect on radon in my home?

    Home radon concentration is likely to be higher in the winter than in the other seasons, because of an increase in the stack effect. Stack effect is the natural transfer of heated air elevating through and out of the top of your home, which creates a greater air draw from under your home to enter the home. Upstairs measurements in particular can be significantly different in heating versus cooling seasons. Basement measurements tend to be more consistent throughout the changing seasons.

  • We had severe storms, should I retest?

    Weather patterns can play a role in both raising and lowering radon concentration in your home. When possible, Radon testing should be avoided during heavy storm conditions, periods of high winds, saturated or frozen soil conditions. Minor rains of less than ½ in a twelve hour period are not considered to have much of an impact on radon testing.

    Periods of high winds, heavy rains and saturated soil conditions can influence radon concentration in your home. In most homes, radon concentration marginally varies from day-to-day, week-to-week, season-to-season, and to a lesser extent year-to-year.

    If there is continuous hard rain (more than 1/2") it can cause radon levels to rise in most houses and to fall in other homes. Understanding the affect of the storm on your radon measurement report becomes more of an issue when the results are near the EPA's action level of 4pCi/L.

    If the radon test is done to determine if mitigation is necessary for a real estate transaction, then prolonged rain and a borderline test (4 pCi/L to 5 pCi/L) may be a reason to retest. The length of rain time as a percentage of the overall testing period can increase or decrease the concern.

    If you are not part of a real estate transaction and time is on your side, the EPA recommends a second measurement before deciding on mitigation. The EPA recommends that more than one passive testing device be used - even during real estate transaction testing.

    Continuous Radon Monitors can help you understand the effect severe storm patterns have on radon concentration in your home. The level of detail provided and help from a Radon Measurement Professional may assist in determining the next course of action.

    If the radon levels are greater than 10 pCi/l it is unlikely that the second test will be below the guideline. If the results from two tests have a large variance, consult with a Radon measurement professional.

  • What about Radon in the Water Supply?

    The EPA states in the Home Buyers and Sellers guide to radon, "If you have tested the air in your home and found a radon problem, and your water comes from a well, have the water tested."
    It takes approximately 10,000 pCi/L in water to produce 1 pCi/L in the air.

    Well water is more likely to have a problem with Radon gas than public water supplies.

  • What about Radon in my Granite Countertops?

    There has been concern about the possibility of granite counter tops emitting Radon Gas. Studies have found that very few granites contain enough radium (the parent of radon) and or other radioactive elements to be any concern.

    There are however granites that have measurable radiation release and some that will emit significant radon.

    The American Association of Radon Scientists and Technologists, (AARST), has recommended that if you are concerned about granite in your home you should purchase three radon in air test kits and place them in the following locations: one in the lowest livable area (basement), one in the main living area (bedroom or living room) and one in the kitchen, 20 inches or more from the granite. If any of the test kits are at or above the EPA guideline of 4.0 pCi/l consult a radon professional.

  • What about testing after the radon system is installed?

    Following mitigation, we retest each home with either a CRM machine, an ePerm (Electret Ion Chamber) or if preferred, we can supply a passive third party charcoal test kit with independent laboratory analysis. 

    The EPA recommends that homes with Radon systems be re-tested every two years. All mitigation systems should be retested following installation.



The first step to help ensure the safety of you and your family’s health and safety is to have your home tested for radon. At St. Louis Radon, our professional testers deploy the latest in computerized electronic real-time radon monitoring technology to provide you with a comprehensive report, including a complete visual graph of radon level data points as recorded over a 48-hour sampling period.

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If testing indicates levels of radon that require action, our team of engineers and technicians will design and install a custom radon mitigation system to safely and effectively redirect radon and other soil gasses out of your home. We also take additional measures to seal and close other possible breaches in the foundation that could let soil gasses into the through cracks or drain tiles systems.

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New Construction

With new home construction projects, it is crucial to determine the level of radon that may be present in the soil before the home is built. It’s also critical to have the new home retested for potential radon concentrations immediately following occupancy. We also highly recommend installing an Active Dampness Control System (ADC) with every new build.

Learn More About New Construction