Acceptable Radon Levels: Fix at 4.0 pCi/L, Consider 2.0

Is any radon level safe?

No level has been shown to be safe. EPA says there is no known safe level of radon exposure, and its Citizen’s Guide states that any exposure carries some risk. The 4.0 pCi/L action level is a point for action, not a border between safe and unsafe. Because no level is known to be safe, EPA also recommends considering a fix between 2.0 and 4.0 pCi/L.

Congress set a long-term goal that indoor radon be no higher than outdoor air, which averages about 0.4 pCi/L. EPA says that goal is not yet technologically achievable in every case, but most homes today can be brought to 2 pCi/L or below. For comparison, the average American home has about 1.3 pCi/L.

Radon levels chart: what EPA says at each level

The steps below follow EPA’s testing guidance. They assume a short-term test placed in the lowest lived-in level of the home with windows and doors closed. Long-term tests stay in place for more than 90 days and are more likely to show the year-round average.

• •Below 2.0 pCi/L: some risk remains; EPA notes that going lower is difficult
• •2.0 to 3.9 pCi/L: EPA says consider fixing
• •4.0 or more on a first short-term test: run a follow-up test to be sure
• •More than 8.0 on a first short-term test: take a second short-term test right away
• •4.0 or more on a long-term follow-up: fix the home
• •Two short-term tests averaging 4.0 or more: EPA says consider fixing; the higher the results, the surer the case

What pCi/L means, and the metric version

A picocurie is one trillionth of a curie, a unit of radioactivity, so a reading in pCi/L measures the rate of radioactive decay in a liter of air. The international unit is the becquerel, and for air it is given per cubic meter. EPA’s unit table sets one curie at about 37 billion becquerels, which makes 1 pCi/L equal to 37 Bq/m³. EPA rounds the 4 pCi/L action level to 150 Bq/m³ and the 2 pCi/L line to 75.

Outdoor air makes a useful yardstick. Washington’s Department of Health points out that EPA’s 4.0 pCi/L is ten times the 0.4 pCi/L found in outdoor air. In metric units, EPA puts outdoor air at about 15 Bq/m³ and the average American home at about 50 Bq/m³.

Where other countries draw the line

WHO recommends a national reference level of 100 Bq/m³, about 2.7 pCi/L, and no more than 300 Bq/m³ where 100 cannot be reached. Washington’s Department of Health notes that other parts of the world recommend action at or above 2.7 pCi/L. Whichever point you choose, the department says, the goal is to reduce radon in the home as much as reasonably possible.

WHO assumes a linear relationship: lung cancer risk rises by about 16% for every 100 Bq/m³ increase in long-term average radon, in proportion to exposure. Across the state line, Idaho’s Department of Health and Welfare maps the share of home tests greater than 3.9 pCi/L, which it calls the recommended level for remediation.

Where Spokane County results fall on the chart

State lab records show how local results spread across these bands. Of 18,518 Spokane County tests reported from 1990 through 2020, 31.8% were below 2.0 pCi/L, 19.4% fell between 2.0 and 4.0, and 47.0% reached 4.0 or more; the rest were not assigned a band. Statewide, 28.3% of 46,161 tests reached 4.0. In 2020 alone the county share was 44%, against 22% for Washington.

The Washington Tracking Network warns that these results are a convenience sample from people who chose to test, and that some may have been taken after mitigation. They show how often measured homes crossed each line. They cannot tell any one household which band it belongs in; only a test in that home can.

Reading a result close to 4.0

Results near the line are the hardest to read. EPA’s guide gives an example: if two short-term tests average 4.1 pCi/L, there is about a 50% chance that the year-round average is somewhat below 4. The closer a result sits to 4.0, the less certain it is, and EPA points to a long-term test for a better picture of the year-round level. A reading in the 2s or 3s falls inside EPA’s consider-fixing band, so it is not a clean bill of health either.

Test conditions matter as much as the number. Washington’s Department of Health notes that a test run with windows open in mild weather mostly measures outdoor air blowing through the house. EPA advises testing again if a household starts spending time on a lower level, such as a finished basement.

Common questions