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Testing

Radon testing: how to test your home and read the result

By Radon411 Editorial Team

Updated October 1, 2026

Methodology

Short answer

Radon testing means leaving a small detector in the lowest lived-in level of a home for at least 48 hours, with windows and doors kept closed, then having a lab or a monitor report the average radon level in pCi/L. You can do the radon testing yourself with a kit from a store or a state program, or hire a qualified tester, which EPA recommends when you are buying or selling.

Next step: pick the right test for your home. Which test to use

Key takeaways

  • Testing is the only way to know a home's radon level: you cannot see, smell or taste radon, and neighbors' results do not predict yours.
  • EPA and the Surgeon General recommend testing all homes below the third floor.
  • Short-term tests run 2 to 90 days and long-term tests more than 90 days. Every test should run at least 48 hours.
  • Test in the lowest lived-in level, at least 20 inches off the floor, away from drafts, heat, humidity and outside walls, with windows and doors closed.
  • A high first result is checked with a second test before fixing. The action level is 4 pCi/L.

Why test

What radon testing is, and why every home needs it

Radon testing measures how much radon is in the air of a building, reported in (pCi/L). Testing is the only way to know the radon level in a specific building. Put plainly: you cannot see radon, and you cannot smell it or taste it, yet it may be a problem in your home, and testing is the only way to know if you and your family are at risk.

Nearly 1 in 15
U.S. homes estimated to have an elevated radon level (4 pCi/L or more), according to EPA
48 hours
The least any radon test should run. Some devices need longer.
2 to 90 days
How long a short-term test stays in the home. Long-term tests run more than 90 days.
Below the third floor
EPA and the Surgeon General recommend testing all homes below the third floor.
Read moreTwo paragraphs, 1 min

Location does not answer the question. Radon problems vary from area to area, but high levels have been found in every state, homes next to each other can have different levels, and a neighbor's result cannot stand in for yours. That is why EPA and the Surgeon General recommend testing all homes below the third floor, and testing in schools as well.

Testing is inexpensive and easy and should only take a few minutes of your time. This page covers every part of it, from choosing a test to reading the lab report. If you just want the short version, see how do I test for radon.

How to test

How to test for radon in your home, step by step

Steps in a radon test

  1. Get a test

    Buy a kit at a hardware or home improvement store or online, order one from the National Radon Program Services at Kansas State University, or ask your state radon office about free or discounted kits. See free radon test kits.

  2. Close up the house

    For a short-term test, keep windows and outside doors closed except for normal entry and exit. For a test of 2 or 3 days, close them at least 12 hours before you start.

  3. Place the device

    In the lowest lived-in level, in a room used regularly but not a kitchen or bathroom, at least 20 inches above the floor and away from drafts, high heat, high humidity and exterior walls.

  4. Record the start

    Write down the start date and time, as the instructions ask.

  5. Leave it alone

    Leave the kit in place for as long as the package says, without moving it.

  6. Stop and send

    Record the stop time, reseal the package and send it to the lab named on it right away.

  7. Read the result

    Results usually come back within a few weeks. Then follow the steps for your number, below.
Read moreA figure and a paragraph, 1 min
Flowchart of EPA's radon testing steps: take a short-term test; at or above 4 pCi/L follow up with a long-term or a second short-term test; then fix the home or consider fixing based on the result or the average of the two tests.
What to do after the first test.
Read this figure as text

  1. 1Step 1: Take a short-term test.

    • Below 4 pCi/LEven if your test result is below 4 pCi/L, you may want to test again sometime in the future.
    • At or above 4 pCi/LIf your result is at or above 4 pCi/L, take a followup test to be sure.

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

    • Long-term testFor a better understanding of your year-round average radon level, take a long-term test.
    • Second short-term testIf you need results quickly, take a second short-term test. If your first short-term test result is more than twice EPA's 4 pCi/L action level, you should take a second short-term test immediately.

  3. 3Step 3: Decide whether to fix your home.

    • After a long-term testFix your home if your long-term test result is 4 pCi/L or more.
    • After a second short-term testConsider fixing your home if the average of your first and second test is at or above 4 pCi/L.
Show the whole figure

Always follow the instructions that come with your kit as well, because some devices need longer than others. Answers to the most common follow-up questions are on how long does a radon test take, can I test for radon myself and can you be in the house during a radon test.

Choosing a test

Short-term or long-term radon test

There are two general ways to test. remain in the home for two days to 90 days, depending on the device, and are the quickest way to test. remain for more than 90 days. Because radon levels tend to vary from day to day and season to season, a short-term test is less likely than a long-term test to tell you your year-round average, but if you need results quickly a short-term test followed by a second short-term test can be used to decide whether to fix your home.

Table comparing a short-term radon test, a long-term radon test and a continuous radon monitor: how long each runs and what each is for.
Short-term tests, long-term tests and continuous monitors: how long each runs and what it tells you.
Read this figure as text
TestHow long it runsWhat it is for
Short-term testStays in the home from 2 to 90 days, depending on the device, and for a minimum of 48 hours.A faster screening result. Because radon levels vary from day to day and season to season, it is less likely than a long-term test to tell you the year-round average.
Long-term testStays in the home for more than 90 days.It is more likely than a short-term test to tell you the home's year-round average radon level.
Continuous monitorLogs readings for the whole test, run as a short-term or a long-term test under the same durations in EPA guidance.Logs a graph over time, so you can see trends and spot anomalies rather than a single number.
Show the whole figure
Read moreA table, a paragraph and a note, 1 min
Choosing a test for the situation
SituationWhat to use
A first test of the home you live inA short-term test is the quickest start; a long-term test gives the better year-round picture
A first short-term result of 4 pCi/L or moreA follow-up test: long-term for a better year-round average, or a second short-term test if you need results quickly
A first result more than twice the action levelA second short-term test immediately
Buying or selling a homeOne of the three short-term options for real estate, with steps to prevent or detect interference
Checking a new mitigation systemA two- to seven-day test, no sooner than 24 hours and within 30 days of start-up
Show all 5 rows

Whichever you pick, all radon tests should be taken for a minimum of 48 hours, and some devices need longer, such as a 7-day charcoal canister. The longer the test, the better the result reflects your home's radon level and your lifestyle. More on each: short-term radon test kits and long-term radon test kits.

Devices

Radon testing equipment: passive and active devices

Since radon cannot be seen or smelled, special equipment is needed to detect it, and that equipment falls into two groups. Passive devices need no power: they are exposed to the air for a set time and then sent to a laboratory for analysis, and both short- and long-term passive devices are generally inexpensive. Active devices need power, continuously measure and record radon or its decay products, and usually give hourly readings plus an average for the test period; they may cost more.

The common radon testing devices

  • Passive

    Charcoal canister

    Short-term tests; some need 7 days

  • Passive

    Charcoal liquid scintillation detector

    Short-term tests

  • Passive

    Alpha track detector

    Short-term and long-term tests

  • Passive

    Electret ion chamber

    Short-term and long-term tests

  • Active

    Continuous radon monitor

    Short-term tests, with an hour-by-hour record

  • Active

    Continuous working level monitor

    Continuously measures and records radon or its decay products
Show all 6 cards
Read moreA paragraph, 1 min

Use a device from a qualified laboratory. A state or local radon official can explain the differences between devices and recommend the ones that suit your needs and testing conditions. The active devices' reports can reveal unusual or abnormal swings in the level during the test, and some are built to deter and detect interference. Which device to buy is covered on radon test kits, radon detectors and continuous radon monitors.

Test kits and monitors we cover42

All products

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Placement

Where to place a radon test: basement or first floor?

Test the lowest level that is lived in. That is the basement if it is used often and otherwise the first floor, in a room used regularly such as a living room, playroom, den or bedroom. A home sale goes lower still: the lowest level that could be used regularly, finished or not, because a buyer may live in a level the seller does not use.

Where to put a radon test, and where not to

Put it

  • The lowest lived-in level, or for a sale the lowest level that could be used regularly
  • A room used regularly: living room, playroom, den, bedroom, family room, exercise room or workshop
  • At least 20 inches above the floor
  • Where it will not be disturbed
  • In a multi-unit building, the lowest level within your own unit

Keep it out of

  • Kitchens and bathrooms
  • Closets, stairways, hallways and crawl spaces
  • Laundry rooms, furnace rooms and other enclosed areas of high humidity or high air velocity
  • Drafts, high heat, high humidity and spots next to exterior walls
  • Places where it may be moved or knocked over
Read moreA paragraph, 1 min

A simple rule is to raise the device three feet off the ground in the middle of the room, which also clears the 20-inch minimum. If you start using a lower floor later, such as a basement, retest on that level. More detail is in where should a radon test be placed and, for slab homes, do I need a radon test if I don't have a basement.

Closed house

Closed-house conditions: windows, doors, fans and HVAC

A short-term test measures a house closed up, not a house airing out. mean keeping all windows closed, keeping doors closed except for normal entry and exit, and not running fans or other machines that bring in air from outside. For a short-term test of 2 to 4 days, keep those conditions for at least 12 hours before the test starts and for the whole test; for 4 to 7 days, keep them throughout.

During a short-term radon test
AllowedNot allowed
Normal entry and exit through outside doorsLeaving windows or outside doors open
Heating and air conditioning that recirculate indoor air, run normallyFans or other machines that bring in air from outside
Fans that are part of a radon reduction systemFor tests under a week, air conditioners that are not recirculating indoor air
Small exhaust fans run only for short periodsShort tests of 2 or 3 days during unusually severe storms or unusually high winds
Show all 4 rows

What affects it

What affects a radon test result

Radon moves with the house and the weather, so a short test is a snapshot. Levels tend to vary from day to day and season to season, which is why long-term tests better reflect the year-round average. Short tests of 2 or 3 days should not be run during unusually severe storms or periods of unusually high winds. If you can, test in winter, when the heating system runs day and night, which is more likely to give an accurate result.

Things that can change or spoil a radon test, and what to do

  • Open windows or outside doors

    Break closed-house conditions; interference may invalidate the result

  • Fans that pull in outdoor air

    Not to be run during a short-term test

  • Storms and high winds

    Avoid short tests of 2 or 3 days during unusually severe storms or high winds

  • Season

    Levels vary season to season; testing in winter may be more accurate

  • Renovations and HVAC changes

    Can change radon levels; a buyer may ask for a new test after them

  • A moved or disturbed device

    Do not disturb the device at any time during the test

  • Late return to the lab

    Promptly return the device, with start and stop times and location filled in
Show all 7 cards
Read moreA paragraph, 1 min

Improper testing may yield inaccurate results and require another test. Why readings jump within a test is covered on what causes radon spikes, humidity on how does humidity affect radon testing, and the full list on what can affect a radon test.

Your result

Reading your radon test result

A lab report or monitor gives the average radon concentration over the test, in pCi/L. For scale, EPA estimates the average indoor level at about 1.3 pCi/L, and about 0.4 pCi/L is normally found in outdoor air. EPA recommends fixing a home when the radon level is 4 pCi/L or higher. EPA recommends considering a fix when the level is between 2 and 4 pCi/L.

4 pCi/L
EPA's action level: the point at which EPA recommends fixing a home.
Between 2 and 4 pCi/L
Where EPA recommends considering a fix.
About 1.3 pCi/L
EPA's estimate of the average indoor level in U.S. homes.
Read moreA paragraph, 1 min

Results close to 4 pCi/L are the least certain. An average of 4.1 pCi/L from two short-term tests gives about a 50 percent chance that the year-round average is somewhat below 4. Any radon exposure also carries some risk, and no level of radon is safe, so a result under 4 is not an all-clear. What each number means is on radon levels, and the result interpreter reads a specific result for you.

After the result

After the result: EPA's follow-up steps

The steps are set. If a first short-term result is 4 pCi/L or more, take a follow-up test to be sure: a long-term test for a better sense of the year-round average, or a second short-term test if you need results quickly. The higher the first result, the more certain you can be that a short-term follow-up is the right choice, and a first result more than twice the calls for a second short-term test immediately.

The steps after a radon test

  • First short-term result below 4 pCi/L

    You may want to test again sometime in the future

  • First short-term result of 4 pCi/L or more

    Take a follow-up test: long-term, or a second short-term test

  • First short-term result above 8 pCi/L

    Take a second short-term test immediately

  • Long-term follow-up of 4 pCi/L or more

    Fix the home

  • Average of two short-term tests of 4 pCi/L or more

    Consider fixing the home; the higher the results, the more certain the need
Show all 5 cards
Read moreA paragraph, 1 min

If the home needs fixing, radon mitigation covers the decision and the project, and radon mitigation systems the hardware.

Home sale

Radon testing for a home sale

Real estate deals move fast, so they have testing rules of their own. Three short-term options are accepted, each at least 48 hours: two passive tests side by side at the same time, two passive tests one after the other, or a continuous monitor. Each has a trade-off: simultaneous tests improve precision, and sequential tests are more likely to represent the seasonal average.

EPA's three short-term radon testing options for a real estate transaction: two tests at the same time, one test then a second, or a continuous monitor, each for at least 48 hours, with what to do after each.
The three short-term testing options for a home sale, and what to do after each.
Read this figure as text

If you are testing in a real estate transaction and you need results quickly, any of the following three options for short-term tests are acceptable in determining whether the home should be fixed.

  1. Option 1. Two tests at the same time

    Passive

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

    What to do nextFix the home if the average of the two tests is 4 pCi/L or more.

    Two tests taken at the same time (simultaneous) would improve the precision of this radon test.

  2. Option 2. One test, then a second

    Passive

    Take an initial short-term test for at least 48 hours. Immediately upon completing the first test, do a second test using an identical device in the same location as the first test.

    What to do nextFix the home if the average of the two tests is 4 pCi/L or more.

    One test followed by another test (sequential) would most likely give a better representation of the seasonal average.

  3. Option 3. A continuous monitor

    Active

    Test the home with a continuous monitor for at least 48 hours.

    What to do nextFix the home if the average radon level is 4 pCi/L or more.

    Both active and passive devices may have features which help to prevent test interference.

Any real estate test for radon should include steps to prevent or detect interference with the test device. Your state radon office can help you decide which option is best.

Show the whole figure
Read moreTwo paragraphs and a photo, 1 min

Any real estate test should include steps to prevent or detect interference: a device that records levels often enough to show unusual swings, motion and proximity detectors, and records of barometric pressure and temperature to show whether weather or open doors and windows affected the test. Disturbing the device or closed-house conditions may invalidate the result, and it is illegal in some states.

For buyers and sellers the details are in buying a house with radon, selling a house with radon, should I get a radon test with my home inspection and how to manipulate a radon test, which explains why tampering does not work.

A technician in a high-visibility vest holds a card reading "Do not disturb, radon test in progress" under a pair of radon detectors on a wall
A tester's card on a test in progress, asking that the detectors not be disturbed.

DIY or hire

Doing it yourself or hiring a qualified radon tester

Either works: you can test your home yourself or hire a qualified radon test company, and either approach takes only a small amount of time and effort. EPA recommends hiring a qualified tester when you are buying or selling, and buyers often want a test made by someone not involved in the sale.

How to check a radon tester's qualifications

  • State rules

    Ask your state radon office; many states require radon professionals to be licensed, certified or registered, and most can give you a list

  • National credential

    In states that do not regulate radon services, ask whether the tester holds a proficiency or certification credential, from NRPP or NRSB

  • Photo ID

    Credentialing programs usually issue a photo ID card showing the qualification and its expiry date; some states do too

  • The report

    The tester's ID number, if there is one, should be noted in the test report

  • Training

    Alternatively, ask whether they have completed formal training, such as a course in radon measurement
Show all 5 cards
Read moreA paragraph, 1 min

A qualified tester can also evaluate the home, recommend a testing approach, explain how to keep proper conditions, analyze the data and provide an independent result. Certified testers are listed by state in the measurement professionals directory.

How often

When and how often to test for radon

When to test or retest
SituationWhat to do
The home has never been tested, or its level is unknownTest now
Preparing to buy or sellTest before the sale
Renovations, especially repairs to reduce radonTest before and after
Someone will spend more time in the basement or lower levelTest before the move
A mitigation system was just installedTest no sooner than 24 hours and within 30 days
A home with a mitigation systemRetest at least every two years
A home without a mitigation systemRetest every 5 years, the Minnesota health department's advice
A result below 4 pCi/LTest again sometime in the future
Show all 8 rows
Read moreA paragraph, 1 min

A buyer may also ask for a new test if the last one is not recent, meaning within two years, if the home was renovated or altered since, or if the buyer plans to use a lower level than was tested. The full answer is on how often should I test for radon and when should you test for radon.

After a fix

Testing after radon mitigation

A system is checked with a test, not a gauge. Run a two- to seven-day test within 30 days of installation but no sooner than 24 hours after the system starts, with windows and doors closed 12 hours before and during the test. The contractor may test too, but an independent follow-up measurement, by an independent tester or by you, avoids any conflict of interest.

No sooner than 24 hours
After the system starts, before the test begins.
Within 30 days
Of installation, for the test that checks the system.
A two- to seven-day test
With windows and doors closed 12 hours before and during it.
Read moreA paragraph, 1 min

If a house already has a system when you test it for a sale, make sure the fan is working properly and runs for the whole test. The full timeline after a system goes in is on radon mitigation, and the waiting period in how long after radon mitigation can you retest.

Other buildings

Testing water, apartments, schools and new homes

Air comes first. Although radon gets into some homes through water, it is important to test the air first; if you find high levels and have a private well, have the water tested too, with devices and procedures that differ from air tests. How that is done is on radon water testing.

Testing other kinds of buildings

  • Apartments and other multi-unit buildings

    Place the device in the lowest level within your unit

  • Schools

    EPA recommends testing in schools; ask your state radon office about schools, child care and workplaces

  • New homes, even radon-resistant ones

    Test after occupancy

  • Homes on private wells

    Test the air first; if it is high, test the water as well
Show all 4 cards
Read moreA paragraph, 1 min

For new construction see new home radon testing.

Questions

Radon testing questions

How do radon tests work?

A passive test collects radon from the air for a set time and is then analyzed by a lab; an active monitor measures continuously and reports hourly readings and an average. Both report the average concentration in pCi/L.

Should I test in the basement or the first floor?

The lowest lived-in level: the basement if it is used often, otherwise the first floor. For a home sale, the lowest level that could be used regularly, finished or not.

Can you open windows during a radon test?

Not during a short-term test. Keep windows closed and doors closed except for normal entry and exit; for a 2- to 4-day test, start 12 hours before.

How long does a radon test take?

At least 48 hours. Short-term tests run 2 to 90 days depending on the device, long-term tests more than 90 days, and lab results usually come back within a few weeks.

How accurate are radon tests?

A test is only as good as how it was run: improper testing may give inaccurate results. Use a device from a qualified laboratory, follow the instructions and closed-house conditions, and confirm a high first result with a second test.

What can make a radon test read too high or too low?

Open windows or fans pulling in outdoor air, severe storms or high winds during a short test, a device placed in the wrong spot or disturbed, and normal day-to-day and seasonal swings. A long-term test smooths out the swings.

Can a radon test be cheated?

Real estate tests are designed to catch it, with motion and proximity detectors, frequent readings and temperature and pressure records. Interference may invalidate the test and is illegal in some states.

Do I need a certified radon tester?

Not to test your own home: you can do it yourself. Some states require testers to be licensed or certified, and EPA recommends a qualified tester for a home sale.

Where can I get a radon test kit?

Home improvement and hardware stores, online, the National Radon Program Services at Kansas State University (1-800-SOS-RADON), and some American Lung Association offices. Some state and local health departments give kits free.

How often should I test for radon?

After any change that could affect it, such as a renovation or moving into the basement, and at least every two years with a mitigation system. Even after a low result, test again sometime in the future.

Is one short-term test enough?

A decision to fix rests on a long-term result or the average of two short-term tests. One short-term result tells you whether a follow-up is needed.

Does a radon test detect radon in water?

No. Air tests measure radon in the air. Water is sampled separately with different devices and procedures; see radon water testing.