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Radon FAQ

The 64 questions people ask most about radon, answered in two sentences each, with the full answer and its sources one link away.

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What radon is10 questions

Where does radon come from?

Radon comes from uranium, which is present naturally in nearly all rock, soil and water and decays into radium and then into radon gas. Because radon is a gas, it escapes the ground into the air, where it dilutes outdoors but can build up inside buildings that sit on the soil.

Where is radon found?

Radon is found in rock, soil and groundwater across the country, because it forms wherever uranium in the ground breaks down. It is present at low levels outdoors and can build up to much higher levels inside buildings, where it collects instead of dispersing.

What causes radon in a house?

Radon in a house comes from uranium breaking down in the soil and rock beneath it. The gas moves up through the ground, and because air pressure inside a home is usually lower than in the soil around the foundation, the house draws it in through cracks and gaps, where it can build up.

How does radon get into a house?

Radon gets into a house from the soil beneath it, through openings in contact with the ground: cracks in floors and walls, construction joints, gaps around pipes, sump pits and crawlspaces. The house pulls it in, because the air pressure inside is usually lower than in the soil around the foundation.

Does radon have a smell?

No. Radon has no smell, color or taste, so you cannot detect it with your senses, and a musty, damp or rotten-egg smell in a basement is not a sign of radon; a radon test is the only way to know whether a home has a radon problem.

Does radon gas rise?

Radon rises out of the ground into a building's lowest levels, drawn in by lower indoor air pressure, so levels are usually highest in basements and rooms in contact with the ground. It does not stay there: considerable radon can be found above the ground floor, so homes below the third floor should be tested.

Where is radon found in homes?

Radon is usually highest in basements, cellars and other living spaces in contact with the ground, where it enters from the soil through cracks and openings in the foundation. Upper floors can have considerable radon too, so test the lowest lived-in level of any home below the third floor.

Where is radon most common?

Radon is most common where the soil and rock carry more uranium, the areas EPA's Map of Radon Zones marks as highest potential, and indoors it is usually highest in basements and ground-contact rooms. Elevated levels have been found in every state and in all three zones, so any home can have a problem.

What state is highest in radon?

There is no official ranking of states by measured radon, and the CDC testing figures the site holds are county figures. By EPA's Map of Radon Zones, a prediction, Iowa and North Dakota are the only states with every county in Zone 1, the highest predicted potential, followed by Wyoming, Montana and Colorado.

Is radon radioactive?

Yes. Radon is a radioactive gas: radon-222, its most common form, gives off an alpha particle and turns into polonium-218, losing half its atoms in about 3.8 days, and the solid decay products that follow are radioactive too, which is why breathing radon is linked to lung cancer.

Cutaway drawing of a two-storey house with a basement and a crawl space, numbered at EPA's seven radon entry routes: cracks in solid floors, construction joints, cracks in walls, gaps in suspended floors, gaps around service pipes, cavities inside walls, the water supply. — The seven ways EPA lists for radon to get into a house.

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Cracks in solid floors
Construction joints
Cracks in walls
Gaps in suspended floors
Gaps around service pipes
Cavities inside walls
The water supply

Radon from soil gas is the main cause of radon problems. It typically moves up through the ground to the air above and into your home through cracks and other holes in the foundation. Your home traps radon inside, where it can build up. Sometimes radon enters the home through well water.

Health and risk8 questions

Is radon gas a myth?

No: radon is a real, measurable radioactive gas, and the Surgeon General has warned that it is the second leading cause of lung cancer in the United States; DHHS, the International Agency for Research on Cancer and EPA consider it a human carcinogen. A simple home test shows whether your own air has a problem.

Is radon dangerous?

Yes. Radon is a known human carcinogen and, after smoking, the second leading cause of lung cancer in the United States, where EPA estimates it causes about 21,000 lung cancer deaths a year. The danger builds over years of breathing it, rises with the level, and is far greater for people who smoke.

What are the symptoms of radon in your home?

Radon in a home gives no warning signs: it has no smell, color or taste, and nothing about how a house looks or feels reveals it. Its health effect is lung cancer, which can appear many years after exposure, so a test is the only way to find radon.

How does radon affect human health?

Breathing radon at home over a long period raises the risk of lung cancer, the one health effect that is established. The risk grows with the radon level and the length of exposure, and it is much higher for people who smoke.

How long is long-term exposure to radon?

There is no set number of years: radon's risk builds with both the level and the time spent breathing it, and EPA describes risk as exposure over a lifetime. A long-term radon test is something different, a measurement that stays in the home for more than 90 days.

How does radon cause lung cancer?

Radon decays into radioactive particles that, when breathed in, are deposited on the cells lining the airways, where their radiation can damage DNA and lead to lung cancer over many years. Radon is the second leading cause of lung cancer in the United States, and smoking greatly raises the risk.

How can you protect yourself from radon?

Test your home, because radon cannot be seen, smelled or tasted, and if the level is high, have a trained contractor install a radon reduction system and test again to confirm it worked. Then retest every two years, and if you smoke, stop, since smoking and radon together raise lung cancer risk.

Is it safe to live in a house with radon?

Every house has some radon, so safety depends on the level: the lower it is, the lower the risk, and even levels below 4 pCi/L pose some risk. A high result is a reason to confirm it and fix the home, not to move out overnight, because the risk builds over years.

Bar chart of EPA's radon risk estimates for people who smoked and people who never smoked: out of 1,000 people exposed over a lifetime, about how many could get lung cancer at 20, 10, 8, 4, 2, 1.3 and 0.4 pCi/L, from about 260 smokers at 20 pCi/L to about 3 at 0.4 pCi/L, with EPA's advice at each level and its comparison to other risks. — EPA's risk chart: out of 1,000 people exposed over a lifetime, about how many could get lung cancer at each radon level.

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EPA estimate, lifetime exposure

Out of 1,000 people who smoked
Out of 1,000 people who never smoked

About how many people out of 1,000 exposed over a lifetime could get lung cancer, by radon level
Radon level	Could get lung cancer, per 1,000	EPA's comparison	What to do
20 pCi/L740 Bq/m³	Smoked: About 260Never smoked: About 36	Smoked: 250 times the risk of drowningNever smoked: 35 times the risk of drowning	Fix your home
10 pCi/L370 Bq/m³	Smoked: About 150Never smoked: About 18	Smoked: 200 times the risk of dying in a home fireNever smoked: 20 times the risk of dying in a home fire	Fix your home
8 pCi/L300 Bq/m³	Smoked: About 120Never smoked: About 15	Smoked: 30 times the risk of dying in a fallNever smoked: 4 times the risk of dying in a fall	Fix your home
4 pCi/L150 Bq/m³	Smoked: About 62Never smoked: About 7	Smoked: 5 times the risk of dying in a car crashNever smoked: The risk of dying in a car crash	Fix your home
2 pCi/L75 Bq/m³	Smoked: About 32Never smoked: About 4	Smoked: 6 times the risk of dying from poisonNever smoked: The risk of dying from poison	Consider fixing between 2 and 4 pCi/L
1.3 pCi/L50 Bq/m³	Smoked: About 20Never smoked: About 2		Average indoor radon level
0.4 pCi/L15 Bq/m³	Smoked: About 3Never smoked: No figure given		Average outdoor radon level

Smoked: If you are a former smoker, your risk may be lower.
Never smoked: If you are a former smoker, your risk may be higher.
Reducing radon levels below 2 pCi/L (75 Bq/m3) is difficult.
Lifetime risk of lung cancer deaths, as EPA estimates it.
The risk comparisons are EPA's.

Levels7 questions

What is a safe level of radon?

No level of radon is known to be safe: any exposure carries some risk, and lowering the level lowers the risk. EPA recommends fixing a home at 4 pCi/L, its action level, and considering a fix between 2 and 4 pCi/L, while WHO's reference level is 100 Bq/m3, about 2.7 pCi/L.

What radon level is dangerous?

EPA treats 4 pCi/L as the level at which to fix a home, and recommends considering a fix from 2 to 4 pCi/L, but there is no line where radon turns from safe to dangerous. The risk rises steadily with the level, with the years of exposure, and sharply with smoking.

When are radon levels highest?

Radon levels are usually highest in the colder months, when homes are closed up and heated and radon is most likely to be trapped and build up, and on the lowest floors, closest to the ground. Levels also rise and fall from day to day and hour to hour with weather and ventilation.

According to the EPA, at what level does radon become a health concern requiring mitigation?

EPA's action level is 4 pCi/L: at a confirmed result of 4 pCi/L or more, EPA recommends fixing the home, usually with a vent pipe and fan system. Between 2 and 4 pCi/L, EPA recommends considering a fix, and it says any radon exposure carries some risk, so the line is a decision point rather than a safe limit.

What causes radon spikes?

Indoor radon varies from hour to hour and day to day with air pressure, ventilation and weather, and a continuous monitor makes those swings visible as spikes. Base decisions on the average over a full test, and if you have a radon system, check that its fan is running.

Is a radon level of 5 pCi/L bad?

A result of 5 pCi/L is above EPA's 4 pCi/L action level, so plan to fix the home once the number is confirmed, with a followup test if it came from one short-term test. It calls for a planned fix rather than alarm: most homes can be brought to 2 pCi/L or below.

How do you read a radon test result?

Read a radon result in picocuries per liter (pCi/L) against EPA's marks: 4 pCi/L is the action level for fixing a home, and 2 to 4 pCi/L is the range to consider a fix. First check the test was valid, and confirm a high short-term result with a followup test.

Key radon numbers as EPA states them: the 4 pCi/L action level, consider fixing between 2 and 4 pCi/L, average indoor 1.3 pCi/L, outdoor 0.4 pCi/L, nearly 1 in 15 U.S. homes with elevated levels, about 21,000 lung cancer deaths a year, reduction up to 99% and a 48-hour minimum test. — The radon numbers that matter most, each in the agency's own words.

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As the agencies state them

EPA's action level
4pCi/L
“Fix your home if your radon level is 4 picocuries per liter (pCi/L) or higher.”
EPA
Consider fixing
2 to 4pCi/L
“Consider fixing between 2 and 4 pCi/L.”
EPA
Average indoor level
1.3pCi/L
“The average indoor radon level is estimated to be about 1.3 pCi/L.”
EPA
Outdoor air
0.4pCi/L
“The average indoor radon level is estimated to be about 1.3 pCi/L, and about 0.4 pCi/L of radon is normally found in the outside air.”
EPA
Homes with elevated levels
1 in 15U.S. homes
“Nearly 1 out of every 15 homes in the U.S. is estimated to have elevated radon levels.”
EPA
Lung cancer deaths from radon
21,000a year
“Radon is responsible for about 21,000 lung cancer deaths every year.”
EPA
What a system can do
Up to 99%lower
“Some radon reduction systems can reduce radon levels in your home by up to 99%.”
EPA
Shortest valid test
48hours
“All radon tests should be taken for a minimum of 48 hours.”
EPA

Testing11 questions

What is radon testing?

Radon testing is measuring the radon concentration in a building's air over a set period, with a detector that is either analyzed by a laboratory afterward or reports the result itself. It is the only way to know the level in a specific building, because a map or a neighbor's result describes somewhere else.

What is a radon test?

A radon test is a small device that samples a building's air for a set time, at least 48 hours, to measure its radon concentration in picocuries per liter. Passive tests, such as charcoal canisters, go to a lab for analysis afterward; active tests, continuous monitors, record radon themselves and report hourly readings and an average.

How do I test for radon?

Use a short-term test kit in the lowest lived-in level of your home, or hire a qualified tester: close windows and doors 12 hours before, set the device at least 20 inches off the floor away from drafts, leave it at least 48 hours, then mail it to the lab. A high result gets a follow-up test.

How often should I test for radon?

Test every home at least once, then again when something changes: after a radon fix and at least every two years after that, after major renovations, and when you start living on a lower level. After a low result, you may want to test again sometime in the future.

Do I need a radon test if I don't have a basement?

Yes: any home can have a radon problem, with or without a basement, because radon enters through any part of the building in contact with the ground, such as a slab or the floor over a crawlspace. Test in the lowest lived-in level, which without a basement is the first floor.

Where should a radon test be placed?

Place a radon test in the lowest lived-in level of the home, in a room used regularly such as a living room or bedroom, not the kitchen or bathroom. Set it at least 20 inches above the floor where it won't be disturbed, away from drafts, high heat, high humidity and exterior walls.

How long does a radon test take?

A radon test takes at least 48 hours. Short-term tests stay in the home from 2 to 90 days, depending on the device, and long-term tests more than 90 days; a lab result then comes back within a few days or weeks of mailing the device.

How much does a radon test cost?

A do-it-yourself radon test kit costs about $10 to $30 in Wisconsin's health department figure and $20 to $30 in Pennsylvania's, and many states give kits away or sell them for less. A professional test costs more: Virginia's health department says it usually costs $100 to $200, depending on the device.

How does humidity affect radon testing?

Humidity is a placement rule for radon tests: keep the device away from drafts, high heat, high humidity and exterior walls, and do not test in a bathroom, kitchen, laundry room or other enclosed humid area. Use a regularly used room on the lowest lived-in level instead.

What is a radon test kit?

A radon test kit is a small, low-cost detector you set out in the lowest lived-in level of your home for a set time, then reseal and mail to a lab, which reports the radon level. Short-term kits stay out two days to 90 days, and long-term kits more than 90 days.

Should I test for radon?

Yes. EPA and the Surgeon General recommend testing every home below the third floor, whatever its age, foundation or location, because radon cannot be seen, smelled or tasted, and a test, which is inexpensive and takes only a few minutes of your time, is the only way to know a home's level.

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. — EPA's testing steps: a short-term test, a follow-up test at or above 4 pCi/L, then the rule for fixing.

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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.
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.
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.

Mitigation systems13 questions

How do I reduce radon in my home?

To reduce or get rid of radon, the main method is soil suction: a vent pipe and fan that pull radon from beneath the house and vent it outside before it can enter. Confirm a high result first, hire a qualified contractor, and test again after the system is installed.

How much does radon mitigation cost?

Public health agencies put a radon mitigation system in an existing home at $500 to $3,500. EPA's range for retrofitting an existing home is $800 to $2,500, and state health departments give ranges such as Wisconsin's $1,000 to $2,000 and Minnesota's $1,500 to $3,000.

How do you reduce radon gas in a basement?

The standard way to reduce radon in a basement is soil suction: a vent pipe and fan pull radon from beneath the floor and release it outdoors before it can get in, typically cutting levels by 50 to 99 percent. Sealing cracks helps that system work, but sealing alone is not recommended.

Would an exhaust fan in a basement reduce radon?

Probably not, and it can make things worse. An exhaust fan pushes basement air outdoors, and the make-up air that replaces it can pull more soil gas, and radon, in through the foundation. The fan that does work is a radon mitigation fan, which draws radon from under the slab and vents it above the roof.

Does radon mitigation work?

Yes: some radon mitigation systems can cut the radon level in a home by up to 99 percent, and most homes can be brought to 2 pCi/L or below. The proof for your home is a follow-up test within 30 days of installation, with the fan running continuously after that.

What is a radon mitigation system?

A radon mitigation system is a permanent installation, usually a vent pipe and a fan that runs all the time, that pulls radon-laden soil gas from beneath a house and releases it outdoors above the roof before it can get in. It is designed for the building's foundation, and a follow-up test shows whether it lowered the level.

How do you install a radon mitigation system in a basement?

Have a qualified mitigation contractor install it. The contractor puts one or more suction pipes through the basement slab, connects a radon vent fan outside the living space that runs continuously and vents above the roof, seals cracks, fits a warning device, and then a follow-up test within 30 days confirms the level came down.

Can you do radon mitigation yourself?

You can install a radon system yourself, but a trained contractor is recommended, because lowering high radon levels takes technical knowledge and special skills, and done wrong it can even raise the level. If you go ahead, first contact your state radon office, and test after the system is running.

How long after radon mitigation can you retest?

Wait at least 24 hours after the system starts running with its fan on, and test within 30 days of installation. A test of two to seven days is recommended, with windows and doors closed from 12 hours before. After that, retest at least every two years and after any remodel.

Can I turn off my radon system?

No. A radon fan should never be turned off: it must run continuously for the system to work. The fan draws radon from under the floor and vents it above the roof, and when a system stops venting, radon levels in the building rise. Running it only slightly raises the electric bill.

Does opening windows reduce radon?

Opening windows can lower radon for a while, because outdoor air dilutes the radon indoors, but it is only temporary: once the windows are closed, levels most often return to where they were within about 12 hours. It costs heated or cooled air and is not considered radon mitigation.

How do you seal cracks in a basement floor for radon?

Seal cracks in the floor and walls, floor-wall joints and gaps around pipes with plaster, caulk or other materials made for the purpose, then test again. Sealing is a basic part of most radon fixes, but on its own it has not been shown to lower radon significantly or consistently.

What filters radon?

For radon in indoor air, the recommended fix is not a filter: it keeps radon out, usually with a vent pipe and fan that pull it from under the house. For radon in well water, point-of-entry treatment works: a granular activated carbon filter typically removes 85 to 95 percent and aeration 95 to 99 percent.

Cutaway drawing of a house with a sub-slab radon mitigation system: a suction point through the basement slab into crushed rock, a vent pipe with a manometer warning device, a fan in the attic and the exhaust above the roof, with the slab edges sealed. — The parts of a sub-slab depressurization system, as EPA's Consumer's Guide describes them.

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Suction point. One or more suction pipes are inserted through the floor slab into the crushed rock or soil underneath. Often, only a single suction point is needed.
Negative pressure under the slab. A radon vent fan connected to the suction pipes draws the radon gas from below the home and releases it into the outdoor air while simultaneously creating a negative pressure or vacuum beneath the slab.
Sealed cracks and openings. Sealing foundation cracks and other openings makes this kind of system more effective and cost-efficient.
Vent pipe, clearly labeled. Radon reduction systems must be clearly labeled. This will avoid accidental changes to the system that could disrupt its function.
Radon vent fan. Common fan locations include unconditioned home and garage spaces, including attics and the exterior of the home. The exhaust fan must not be located in or below a livable area.
Exhaust above the roof. The exhaust pipes of soil suction systems must vent above the surface of the roof and 10 feet or more above the ground, and must be at least 10 feet away from windows, doors or other openings that could allow radon to reenter the home, if the exhaust pipes do not vent at least 2 feet above these openings.
Warning device. A warning device must be installed to alert you if an active system stops working properly. Examples of system failure warning devices are: a liquid gauge, a sound alarm, a light indicator, and a dial, or needle display, gauge.

There are several proven methods to reduce radon in your home, but the one primarily used is a vent pipe system and fan, which pulls radon from beneath the house and vents it to the outside. Keep the fan running at all times: it must run continuously for the system to work correctly.

Buying and selling5 questions

Should I buy a home with a radon mitigation system already installed?

Yes, it can be a good sign: a system means a radon problem was found and fixed, and where radon problems have been fixed, home sales have not been blocked. Before you buy, ask the seller for the system's paperwork and test results, check the warning device, and test with the fan running.

Should I buy a house with radon?

You can buy a house with radon: a high level is a problem that can be fixed, usually with a vent pipe and fan system that costs about the same as other common home repairs. Get a valid test before you buy, and agree with the seller on when any fix happens and who pays.

Should I get a radon test with my home inspection?

Yes, unless the house has a recent test you can rely on: if the home has not been tested, have it tested, and a qualified radon tester is very often a home inspector. Ask for the test by name, and schedule it early, since every radon test runs at least 48 hours.

Does a radon mitigation system hurt resale value?

We know of no published figure showing that a radon mitigation system lowers resale value. Where radon problems have been fixed, home sales have not been blocked or frustrated, and a working system with test records showing low levels is sometimes a good selling point.

Is it hard to sell a house with radon?

Radon need not block a sale: where radon problems have been fixed, home sales have not been blocked or frustrated. The hard part is time, so test before you list, fix a high result while you can choose the schedule, and keep the records a buyer will ask for.

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. — EPA's three short-term testing options for a home sale, and what to do after each.

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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.

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.
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.
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.

The element10 questions

Is radon heavier than air?

Yes: radon is about 7.8 times as dense as nitrogen and 6.8 times as dense as oxygen, and it was the heaviest known gas when first measured in 1908. Even so, it does not stay in the basement: a home's lower air pressure draws it in, and it reaches upper floors too.

Is radon a noble gas?

Yes: radon is in group 18, the noble gases, with helium, neon, argon, krypton and xenon, and it is the heaviest and densest of them. It is a naturally occurring, radioactive noble gas that is odorless, colorless and tasteless, and it can build up in homes.

What is radon used for?

Radon has two kinds of use. It was once sealed in tiny tubes, called seeds, to treat cancer, a practice now largely discontinued, and scientists use it as a natural tracer to follow groundwater into streams, date groundwater and study how gases move underground and in the air.

Is radon a metal, nonmetal or metalloid?

Radon is a nonmetal: it is classified as a non-metal and belongs to the noble gases, group 18, with element number 86. It is not a metal or a metalloid, and it is a colorless gas at ordinary temperatures, unlike its neighbors polonium, a metalloid, and radium, a metal.

How many protons and electrons does radon have?

Radon has 86 protons, because its atomic number is 86, and every radon isotope shares that count. A neutral radon atom also has 86 electrons, one for each proton, arranged as [Xe] 6s2 4f14 5d10 6p6 with eight in the outer shell, while the neutron count varies by isotope: radon-222 has 136.

How many neutrons does radon have?

Radon-222, the most common isotope, has 136 neutrons: its mass number, 222, minus its 86 protons. Other isotopes have different counts: radon-220, for example, has 134, and thirty-nine radon isotopes are known in all, each with the same 86 protons but its own neutron count.

How many valence electrons does radon have?

Radon has 8 valence electrons, the 6s2 and 6p6 electrons in its outermost shell, out of 86 electrons in all. Radon is a noble gas with that full outer shell, and it is essentially inert, though fluorine has been reported to react with it to form radon fluoride.

What is the atomic mass of radon?

Radon's atomic mass is about 222: it is listed as 222.01758 in the periodic table and written [222] as its atomic weight. Those figures follow radon-222, the most common and most stable of radon's 39 known isotopes, whose own isotopic mass is 222.017576017.

What group is radon in on the periodic table?

Radon is in group 18, the noble gases, also called the zero group of gases, and in period 6. It is element 86, symbol Rn, a non-metal that sits directly below xenon in the column, with oganesson, element 118, listed after it.

When was radon discovered?

Radon was discovered in 1900 by the German chemist Friedrich Ernst Dorn, who found it while studying radium's decay chain and called it radium emanation. Ramsay and Gray isolated it in 1908 and named it niton, and it has been called radon since 1923.

Decay chain from uranium-238 through uranium-234, thorium-230 and radium-226 to radon-222, radon's decay products and stable lead-206, with radium-226's half-life of 1600 years and radon-222's of 3.823 days. — Radon is one step in the decay of uranium-238, which ends in stable lead-206.

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Uranium-238
Solid
Uranium-234
Solid
Thorium-230
Solid
Radium-226
Solid
Half-life 1600 years
Radon-222
Gas
Half-life 3.823 days
Radon's decay products
Lead-206
Solid

Uranium-238. More than 99 percent of the uranium found in the environment is in the form of U-238. Uranium decays by alpha particles.
Uranium-234. Less than one percent of natural uranium, but much more radioactive.
Thorium-230. An intermediate EPA names in the uranium-238 chain.
Radium-226. Radium decays to produce radon gas.
Radon-222. The most common radon isotope, and an alpha emitter. As a gas it can move up through the ground and into a building.
Radon's decay products. Radon gas decays into radioactive particles that can get trapped in your lungs when you breathe.
Lead-206. Where the chain ends. Only the final, stable atom in the chain is not radioactive.

EPA names uranium-234, thorium-230, radium-226 and radon-222 as intermediates "such as" these, so the chain has more steps than are drawn. Half-lives are shown only where a cited source states one.

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