An invisible gas. A measurable risk. A problem you can do something about.
Radon is a naturally occurring radioactive gas released from the ground. It can collect indoors without a smell, a color, or a warning sign. Understanding how it moves, how it affects the lungs, and how to measure it is the first step toward healthier indoor air.
A practical guide for homeowners, buyers, and curious minds.
01 / THE ATOM LABRn · 222
RADON-222 · 86 protons + 136 neutrons. Drag to rotate; arrow keys also work.
Conceptual nucleus and electron cloud. Sizes and particle motion are illustrative; the rings are not literal electron paths.
InvisibleNo color, taste, or smell
3.8 daysRadon-222 half-life
4 pCi/LU.S. EPA action level
Test firstYour home needs its own measurement
01 / FROM ROCK TO RADIOACTIVE GAS
It starts beneath our feet.
Trace amounts of uranium occur naturally in rock and soil. As uranium-238 decays through a sequence of elements, it eventually produces radium-226. Radium releases an alpha particle and becomes radon-222. Unlike its solid parent, radon is a gas: it can move through pores and fractures and dissolve in groundwater.
Radon is a noble gas, with the chemical symbol Rn and atomic number 86. Chemically unreactive does not mean radiologically harmless. Its unstable nucleus continues to transform, releasing ionizing radiation.
DECAY CHAIN / SIMPLIFIED
²³⁸UUranium in rock
… →
²²⁶RaRadium solid parent
α →
²²²RnRadon mobile gas
α →
²¹⁸PoPolonium solid progeny
Intermediate stages before radium are omitted. Further alpha and beta decays ultimately lead to stable lead-206.
An alpha particle is two protons and two neutrons. Its energy is deposited over a short distance, which matters when the emitting material is inside the lungs.
What “half-life” actually means
After about 3.8 days, half the radon-222 atoms in an isolated sample have transformed. After another 3.8 days, half the remainder has transformed. This does not mean a house clears itself in four days: the ground can continually supply more gas.
50.0% of original radon atoms remain.
N/N₀ = 2−t/3.8. This illustrates radioactive decay only, not ventilation, new radon entry, or a home's measured concentration.
One name, different isotopes
Residential radon discussions usually focus on radon-222. Radon-220, also called thoron, comes from the thorium decay series and has a much shorter half-life. Different isotopes share the same number of protons but have different numbers of neutrons.
Why outdoors is different
Outdoor air usually disperses the gas. Enclosed rooms can allow it to accumulate when entry outpaces removal. Radon is natural; elevated indoor exposure is something we can reduce.
Drag to orbit · Arrow keys to rotate · Illustrative model, not installation instructions or a measurement.
Small openings. A continuous route.
Cracks in floors and foundation walls, construction joints, and the floor-wall joint can connect indoor air to radon-bearing soil gas. Visible damage is not required: small openings can be enough.
In entry mode, orange particles travel from the soil through openings into the occupied space. Particle numbers and speeds do not represent concentration.
A
Pressure draws gas in
Warm air rising through a building, wind, and mechanical exhaust can make the lower part of a house less pressurized than surrounding soil. The pressure difference pulls soil gas through available openings.
B
Geology sets the supply
Uranium content, soil permeability, moisture, and fractures influence how much gas reaches a foundation. A regional map describes potential; it cannot determine the reading inside one house.
C
The building changes the result
Foundation design, ventilation, weather, season, and occupant habits all matter. A new house, an older house, a slab home, and a drafty home can each have elevated radon.
Radon decays into solid radioactive progeny. Some attach to airborne particles; when inhaled, they can deposit along the airways. Their alpha emissions can damage DNA in nearby cells. Over years, that damage can contribute to lung cancer.
Radon is the second-leading cause of lung cancer in the United States after smoking. EPA estimates about 21,000 radon-related lung cancer deaths annually. People who have never smoked are also at risk; smoking and radon together greatly increase risk.
Risk depends on concentration, time exposed, and smoking history. A test measures the building's air during the test period; it does not diagnose disease or predict an individual's outcome.
Mechanism schematic. Particle sizes, anatomy, and radiation tracks are not to scale.
You cannot feel your radon level.
Radon exposure has no reliable immediate warning symptoms. Do not wait for a cough or other symptoms before testing. Discuss persistent respiratory symptoms or a substantial exposure concern with a healthcare professional; routine medical tests cannot reconstruct your past radon dose.
A neighbor's result, a radon map, and an ordinary home inspection cannot substitute for a radon measurement in your home.
01
Short-term testing
2–90 DAYS
Useful for an initial assessment or a time-sensitive decision. Charcoal kits, electret detectors, and qualified continuous radon monitors are examples. Use the device's prescribed test period, with at least 48 hours for standard residential measurements.
02
Long-term testing
MORE THAN 90 DAYS
Better reflects the average exposure across everyday living and seasonal changes. Alpha-track and some electret devices can remain in place for months. A full-year measurement captures more seasonal variation than a brief snapshot.
03
Continuous monitoring
TRENDS OVER TIME
Electronic monitors reveal changes and averages. A qualified professional's calibrated instrument and quality controls differ from a consumer display. Treat consumer readings as useful signals; confirm elevated results with an appropriate test before making decisions.
A practical test-day checklist
Choose the location. Follow the kit or professional's placement instructions. Test the lowest regularly occupied level; for a sale, include the lowest level suitable for occupancy.
Control the conditions. For short tests, keep exterior doors and windows closed except normal entry and exit. For tests under four days, begin closed-house conditions at least 12 hours beforehand.
Protect the device. Use a regularly occupied room away from drafts, heat, and humidity. Do not move, cover, or tamper with it. Avoid a kitchen or bathroom.
Document and return. Record location, start and stop times, and any unusual conditions. Return laboratory devices promptly as instructed.
Run normal heating and cooling as instructed. Keep an existing radon system operating. Follow device-specific guidance about ventilation and weather; severe storms or unusually high winds can make a brief test unrepresentative.
Buying or selling?
Use a qualified tester and the applicable transaction protocol. EPA describes simultaneous or sequential passive tests, or a qualified continuous monitor test of at least 48 hours. A single consumer monitor snapshot is not an equivalent substitute.
Keep the full report, not only its headline number: test dates, room, device, duration, conditions, and quality-control notes help explain what was measured.
U.S. reports usually use pCi/L (picocuries per liter). International reports often use Bq/m³ (becquerels per cubic meter). Both describe radioactive activity per volume of air, not a count of visible particles.
1 pCi/L = 37 Bq/m³
The EPA action level is a practical decision point, not a boundary between safe and dangerous. Reducing exposure below 4 pCi/L can still reduce risk.
WHO recommends a national residential reference level of 100 Bq/m³ where feasible (about 2.7 pCi/L), and no higher than 300 Bq/m³ if that cannot be achieved. These are different policy reference levels, not interchangeable “safe limits.” This tool uses U.S. EPA homeowner guidance.
148 Bq/m³
<22–<4≥4 pCi/L
Follow up your initial test
A first short-term result at or above 4 pCi/L calls for follow-up. Use a long-term test for a better annual estimate, or a second short-term test if you need an answer sooner.
Educational guidance for ordinary residential testing. Real estate protocols and local requirements may differ. This tool does not assess individual medical risk.
Below 2
Lower exposure
Keep your result and retest when the home or its use changes. A low result is useful information, not a lifetime guarantee.
2 to below 4
Consider reduction
Discuss practical ways to lower exposure. A longer measurement can help clarify an uncertain short-term result.
4 or higher
Act on confirmed results
EPA recommends fixing when a long-term result or the average of two short-term tests is at least 4 pCi/L.
If the initial result is above 8 pCi/L, arrange a second short-term test promptly instead of waiting months for a long-term test. The higher the reading, the sooner to follow up.
Mitigation changes how gas moves around the building. A qualified professional chooses a system for the foundation, soil conditions, and measured radon level.
01 / CAPTURE
Below the foundation
Active sub-slab depressurization draws soil gas from beneath a concrete slab. For crawl spaces, a sealed membrane with suction below it can serve a similar purpose. Mixed foundations may need more than one collection area.
02 / ROUTE
Through a dedicated system
A continuously operating fan moves the gas through sealed piping to an appropriate outdoor discharge above the roof. System design must address exhaust placement, fan location, electrical safety, and combustion-appliance backdrafting.
03 / VERIFY
Measure the outcome
Do a post-mitigation test within 30 days of installation, after the system has operated for at least 24 hours. Agree on testing responsibility and the performance target before work begins.
Sealing helps. It is rarely enough alone.
Sealing cracks and sump covers supports pressure control. Sealing alone is not a reliable radon solution. Open windows may temporarily dilute indoor air, but weather and pressure changes make that an inconsistent long-term strategy.
A standard particle filter does not remove radon gas. Treat an air cleaner as a separate indoor-air tool, not proof that radon is controlled.
A manometer or warning device indicates system pressure or operation. It does not measure the radon concentration in your living space. Check it regularly, keep the fan running, and contact your installer if the reading or alarm changes.
Retest at least every two years after mitigation, and after significant changes or repairs. Keep system documentation, baseline gauge readings, test results, and maintenance details together.
Ask about current credentials, applicable state requirements, diagnostic testing, the proposed pipe and fan route, post-installation testing, operating costs, warranties, and what happens if the target is not reached. Request a written, property-specific scope. A model on a website cannot determine the correct installation for a particular building.
Radon-resistant construction is easier to plan before the slab is poured. Typical features include a gas-permeable layer, a soil-gas barrier, sealed openings, a vent pipe to the roof, and electrical provision for a fan.
A passive system relies on natural airflow. An active system adds a fan. Every new home still needs a test after occupancy; “radon resistant” does not mean “radon free.” If testing shows a problem, a qualified professional can assess activation or adjustment.
Finishing a basement or moving a bedroom or home office downstairs.
Major foundation, ventilation, heating, or cooling changes.
Weatherization or additions that alter airflow.
Buying a home with missing, old, or poorly documented results.
Repairing a mitigation system or noticing a warning signal.
Keep records so future testing can be compared in context. Changes in season and conditions can explain some differences; a new elevated result deserves attention.
Can I smell radon, or detect it with a carbon monoxide alarm?
No. Radon has no smell, color, or taste. Smoke and carbon monoxide alarms detect different hazards. Use equipment designed specifically to measure radon.
My neighbor tested low. Do I still need a test?
Yes. Neighboring homes can differ because of foundation openings, soil pathways, pressure, and ventilation. Your neighbor's result does not measure your air.
Is radon only a basement problem?
No. Basements and ground-contact rooms often have higher levels, but slab and crawl-space homes can also accumulate radon. Gas can move to upper floors. Choose testing locations based on actual and potential occupancy and the testing protocol.
Does a result below 4 pCi/L mean zero risk?
No. There is no established risk-free radon concentration. EPA recommends considering reduction between 2 and 4 pCi/L. Lower exposure is the goal, and future changes may warrant another test.
Could radon come from my well water?
Yes. Groundwater can carry dissolved radon, which is released into indoor air during showering and other water use. Soil gas is usually the larger source. If indoor air is elevated and you use a well, discuss a water test with a qualified professional. Water treatment needs a separate assessment; an under-sink drinking-water filter does not address gas released throughout the house.
Start by testing the indoor air. Natural stone can emit radon, but EPA identifies soil beneath the home as the main source in most cases. Granite is not typically a major contributor. A qualified professional can investigate unusual sources if needed.
Yes, if conducted correctly. It measures conditions during that period and can guide follow-up or a time-sensitive decision. A longer test is generally better for estimating annual average exposure. Interpret the result with its duration, conditions, and purpose.
Is a high reading an immediate poisoning emergency?
Radon is primarily a long-term lung-cancer risk, unlike an acute carbon monoxide emergency. Do not ignore a high result: arrange prompt follow-up and a mitigation assessment. An unusually high reading calls for faster professional advice, not months of delay. If someone is acutely ill, seek medical help rather than assuming radon is the cause.
Will mitigation make my home radon-free?
The goal is to reduce exposure, not promise zero radon. Performance depends on the building and system. A test after installation verifies the actual result; continued operation, maintenance, and retesting help keep it low.
I rent. What should I do?
Ask the property owner or manager about testing and share documented results. Contact your state radon program for location-specific guidance on testing and available resources. Radon can affect rented homes and multifamily buildings as well as owner-occupied properties.
Should I use a DIY kit or hire a professional?
A suitable kit used exactly as directed can be a practical way to test your own home. A qualified professional helps with placement, equipment, quality control, and interpretation, especially during a sale or when a result is uncertain. Mitigation requires specialized assessment and skills.
What if I have lived in the house for years?
Testing and reducing ongoing exposure can still be worthwhile. A current air test cannot reconstruct every past year's exposure, and no routine medical test can predict whether radon will cause cancer. Discuss personal health concerns with your clinician.
THE SCIENCE BEHIND THE GUIDE
Go deeper.
Primary public-health resources for understanding the science and planning your next step. Reviewed October 22, 2026. U.S. homeowner guidance is emphasized; follow applicable local requirements and your test provider's instructions.