What does 2, 4, 5, 10 pCi/L mean?

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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.

Any single number is also a snapshot. Radon varies from day to day and season to season, so a short-term result is less likely than a long-term test to show your year-round average. A long-term test, more than 90 days, is the better reading of where your home really sits on this scale.

EPA recommends considering a fix when the level is between 2 and 4 pCi/L. A result of 2 is above the national indoor average of about 1.3 pCi/L. Radon below 4 pCi/L still poses some risk, and any exposure carries some, so a fix at this level is a reasonable choice rather than a requirement.

Even below 4, you may want to test again sometime in the future. More on this level is on the 2 pCi/L page.

EPA recommends fixing a home when the radon level is 4 pCi/L or higher. In metric units, 4 pCi/L is about 150 Bq/m3. By EPA's estimate, nearly 1 in 15 U.S. homes has an elevated level.

A result right at 4 is where short-term tests are least certain. If two short-term tests average 4.1 pCi/L, there is about a 50 percent chance that the year-round average is somewhat below, or above, 4. That is why a high first result gets a follow-up test, long-term or a second short-term, before the decision. See the 4 pCi/L page.

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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.

Both call for the same step as 4: fix the home. The risk grows with the level. EPA's risk chart estimates how many of 1,000 people exposed over a lifetime could get lung cancer at each level, and the gap between smokers and people who never smoked is large.

The chart has no row for 5; it falls between the 4 and 8 rows. Even very high levels can be reduced to acceptable levels, and some systems cut radon by up to 99 percent. More on each level: 5 pCi/L and 10 pCi/L.

Outside the United States, radon is usually reported in becquerels per cubic meter (Bq/m3). WHO recommends a national reference level of 100 Bq/m3, and no more than 300 Bq/m3 where 100 cannot be reached. By WHO's estimate, lung cancer risk rises by about 16 percent per 100 Bq/m3 of long-term average radon, in a straight line.

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• U.S. EPA
• World Health Organization

0100200300 Bq/m³

• EPA: average indoor air48.1 Bq/m³ (1.3 pCi/L)The average for indoor air is 48.1 Bq/m3 (1.3 pCi/L).
• EPA: consider fixing2 pCi/L (74 Bq/m³, calculated)Consider fixing between 2 and 4 pCi/L.
• WHO: reference level100 Bq/m³ (2.7 pCi/L, calculated)A national annual average residential radon concentration reference level of 100 Bq/m3.
• EPA: fix your home148 Bq/m³ (4 pCi/L)EPA recommends fixing your home when the radon level is at or above 148 Bq/m3 (4 pCi/L).
• WHO: upper limit for a reference level300 Bq/m³ (8.1 pCi/L, calculated)If 100 Bq/m3 cannot be reached under the prevailing country-specific conditions, the reference level should not exceed 300 Bq/m3.

Calculated by Radon411: figures marked calculated convert the publisher's unit at 37 Bq/m³ per pCi/L, EPA's factor: “1 pCi/L corresponds to 37 Bq/m3”. A becquerel is one atomic nucleus transforming each second. WHO sets reference levels for countries; EPA's level is a recommendation to fix a home.

For any other number, the radon level interpreter reads a result for you.