Radon definition

The word radon is used in three senses, and a lot of confusion comes from mixing them up. It names a chemical element, it names the gas that element forms, and in everyday use it names a measured level in a building.

Radon the element

EPA's Radionuclide Basics page gives radon's chemical symbol as Rn, the type of radiation it emits as alpha particles, and its half-life as 3.8 days. It forms from the natural decay of uranium and radium found in nearly all rocks and soils.

Radon gas definition

As a gas, radon moves. EPA explains that it typically moves up through the ground to the air above and into a home through cracks and other holes in the foundation, and that the home traps it inside, where it can build up. Outdoors, WHO says, it quickly dilutes to very low concentrations. The radon gas guide covers where it comes from and how it moves.

Radon as a level

When a home inspector, a test report or a neighbor says a house "has radon", they mean a level: the concentration of radon in the air of the tested room. All air has some. CDC says all outdoor and indoor air has some radon in it, and EPA estimates the average indoor level at about 1.3 pCi/L. So the useful question is never whether a house has radon, but how much.

Radon progeny and decay products

Radon decay products (radon progeny)

The radioactive particles radon produces as it decays. WHO explains that when they are breathed in they are deposited on the cells lining the airways, where they can damage DNA and potentially cause lung cancer. EPA's Home Buyer's and Seller's Guide notes that active test devices continuously measure and record the amount of radon or its decay products in the air.

Radon does nothing you can notice. It has no immediate effect and no smell or taste to warn you. What it does is long term: EPA's Citizen's Guide explains that radon gas decays into radioactive particles that can get trapped in your lungs when you breathe, and that as they break down further they release small bursts of energy that can damage lung tissue and lead to lung cancer over the course of a lifetime.

EPA identifies radon as the second leading cause of lung cancer and estimates it is responsible for about 21,000 lung cancer deaths every year in the U.S. It also says not everyone exposed to elevated levels will develop lung cancer, and that the time between exposure and the onset of the disease may be many years. The radon and lung cancer guide sets out the evidence.

What does radon look like or smell like?

Nothing. EPA says you can't see radon, and you can't smell it or taste it. CDC calls it odorless and invisible. There is no sign in a room, a wall or a basement that shows radon is present, which is why EPA says testing is the only way to know whether you and your family are at risk.

Science and health writing about radon uses a handful of terms again and again. ATSDR's ToxFAQs, one in a series of summaries about hazardous substances and their health effects, defines most of them. EPA's own definition starts from the source: radon is a radioactive gas that forms naturally when uranium, thorium, or radium, which are radioactive metals, break down in rocks, soil and groundwater.

Is radon a metal?

No. PubChem's periodic table lists radon as a gas in the noble gas group, and ATSDR calls it a noble gas. The metals in the story are its parents: EPA calls uranium, thorium and radium radioactive metals, and radon forms when they break down. The radon element guide covers the periodic table in detail.

What is a picocurie?

CDC's radon feature says a picocurie is a common unit for measuring the amount of radioactivity, and EPA's Citizen's Guide says the amount of radon in the air is measured in picocuries per liter of air, or pCi/L. WHO's unit, the becquerel, is defined in the next section.

pCi/L (picocuries per liter)

The unit EPA uses for radon in air. EPA's Citizen's Guide says the amount of radon in the air is measured in picocuries per liter of air, or pCi/L.

Bq/m3 (becquerels per cubic metre)

The unit WHO and many other countries use. WHO explains that one becquerel corresponds to the transformation of one atomic nucleus per second, and radon in air is measured by the number of transformations per second in a cubic metre of air.

The two units cannot be read against each other without converting. EPA's Health Risk of Radon page prints both side by side, and those pairs are the table below. They are EPA's own rounded pairs, so read them as EPA prints them rather than as an exact formula.

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2 pCi/L

4 pCi/L

00 Bq/m³274 Bq/m³4148 Bq/m³6222 Bq/m³8296 Bq/m³10+370 Bq/m³

• Below 2 pCi/L (below 74 Bq/m³)Below both of EPA's guidance levels.
• 2 to 4 pCi/L (74 Bq/m³ to 148 Bq/m³)EPA recommends considering a fix when the level is between 2 and 4 pCi/L.
• At or above 4 pCi/L (148 Bq/m³ and up)EPA recommends fixing a home when the radon level is 4 pCi/L or higher.

Reference level

WHO's term for the level a country sets for homes. WHO recommends a national annual average residential reference level of 100 Bq/m3, and where that cannot be reached under a country's conditions, a reference level that does not exceed 300 Bq/m3.

A radon test measures the concentration of radon in the air of one room over a set period. Testing is the only way to know the radon level in a specific building. These are the terms a test kit or a report will use, defined as EPA uses them. The radon testing guide explains how to run each kind of test.

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Test	How long it runs	What it is for
Short-term test	Stays 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 test	Stays 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 monitor	Logs 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.

Closed-house conditions

EPA's Home Buyer's and Seller's Guide defines them as keeping all windows closed, keeping doors closed except for normal entry and exit, and not operating fans or other machines which bring in air from outside. For a short-term test of 2 to 4 days, it says to keep them for at least 12 hours before the test begins and for the whole test.

Passive and active devices

Passive devices do not need power; EPA lists charcoal canisters, alpha-track detectors, charcoal liquid scintillation devices and electret ion chamber detectors. Active devices need power; EPA lists continuous radon monitors and continuous working level monitors, which record radon or its decay products continuously.

Lowest lived-in level

Where EPA says a test belongs: the basement if it is frequently used, otherwise the first floor, in a room used regularly but not the kitchen or bathroom.

A result is a number in pCi/L. EPA's guidance says what to do with it. 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. EPA's Citizen's Guide adds that any radon exposure carries some risk, and that even levels below 4 pCi/L pose some risk, so a lower number is better but never described as risk-free.

1. 1

   First short-term test

   If the result is at or above 4 pCi/L, EPA recommends a follow-up test to be sure before deciding on a fix.

2. 2

   Follow-up test

   A long-term test gives a better picture of your year-round average. If you need results quickly, take a second short-term test. If the first result is more than twice EPA's 4 pCi/L action level, EPA says to take a second short-term test immediately.

3. 3

   Decide

   After a long-term follow-up, EPA says to fix the home if the long-term result is at or above 4 pCi/L. After a second short-term test, consider fixing if the average of the two tests is at or above 4 pCi/L.

For scale, EPA says the U.S. Congress has set a long-term goal that indoor radon levels be no more than outdoor levels, and that while this goal is not yet technologically achievable in all cases, most homes today can be reduced to 2 pCi/L or below. The radon level interpreter walks a result through EPA's bands.

Radon zone

An area's place on EPA's Map of Radon Zones, which EPA developed in 1993 to identify areas of the U.S. with the greatest potential for elevated indoor radon levels, based on indoor radon measurement data, geology, aerial radioactivity, soil parameters and foundation types. It is a prediction for an area. Look up any county on the radon zone map.

The same PDF says homes with elevated levels of radon have been found in all three zones, and all homes should be tested for radon. A zone describes an area's predicted potential, never the level in one house.

Action level

The level at which EPA recommends fixing a home: 4 pCi/L. EPA's zone map page adds that the radon action level should comply with the guidance of the country, state or other local jurisdiction where the test is being conducted.

Radon mitigation

Reducing the radon level in a building. EPA's Citizen's Guide says the method primarily used is a vent pipe system and fan that pulls radon from beneath the house and vents it outside, which it calls a soil suction radon reduction system. The radon mitigation guide explains the options.

Subslab depressurization

EPA's Consumer's Guide calls active subslab suction, also called subslab depressurization, the most common and usually the most reliable radon reduction method: one or more suction pipes inserted through the floor slab, with a fan drawing radon from below the home.

Radon-resistant construction

Features built into a new home to reduce radon entry. EPA's Home Buyer's and Seller's Guide lists a gas-permeable layer, plastic sheeting, sealing and caulking, a vent pipe and junction boxes for a future fan.

NRPP and NRSB

The National Radon Proficiency Program and the National Radon Safety Board, the two national radon proficiency programs EPA currently recognizes for credentialing radon service providers. The radon professionals directory lists measurement and mitigation providers.

Zone and state figures describe areas, not buildings. The state and county radon reports, such as the one for Minnesota, show an area's predicted potential and reported testing data as background for your own test.

Some radon phrases come from everyday speech rather than from the agencies. Here is what each one means when it is set against what EPA, CDC and WHO actually say, with the page on this site that goes further.

For what exposure does over time, see radon poisoning and radon symptoms; for what counts as elevated, see high radon levels.