Is radon exposure dangerous during pregnancy?

What does the evidence say about radon exposure during pregnancy?

1. Radon is the leading cause of lung cancer among people who have never smoked and the second leading cause overall, linked to an estimated 21,000 lung cancer deaths a year in the US; federal guidance recommends fixing a home at a radon level of 4 pCi/L or higher. 1
2. The health risk from radon comes from inhaling its decay products, which lodge in lung tissue and irradiate nearby cells directly. That lung-specific mechanism is part of why toxicology reviewers haven't attributed fetal or reproductive effects to radon exposure in humans. 2
3. The same federal toxicological profile states plainly that no developmental effects in humans following radon inhalation have been studied at all, an evidence gap rather than a clean bill of safety, and recommends home testing as the only way to know your actual exposure level. 3
4. A 2024 study of more than 68,000 Connecticut births found that higher basement radon during the first trimester, even at levels below the EPA's 4 pCi/L guideline, was associated with somewhat higher odds of a baby being born small at a full-term birth. 4
5. That same study relied on modeled rather than directly measured household radon levels and didn't have data on individual mitigation systems or precise smoking status, limitations the authors themselves flag as reasons the finding needs confirmation before it changes guidance. 4

Is radon exposure safe in each trimester?

• First trimester. The Connecticut study that flagged a possible link to lower birth weight specifically measured first-trimester basement radon exposure, so if you're going to test and mitigate, doing it as early as possible in pregnancy lines up with the window that study looked at.
• Second trimester. No trimester-specific findings exist for the second trimester beyond the first-trimester signal in that one study. If a test comes back at or above 4 pCi/L, this is a reasonable time to schedule mitigation, since installing a sub-slab suction system typically takes only a day or two.
• Third trimester. Radon's established risk is about years of cumulative exposure, not a late-pregnancy spike, so there's nothing specific to this trimester beyond keeping up with whatever testing or mitigation plan you started earlier.