Guide
What is radon, and why does it matter?
Radon is a naturally occurring radioactive gas from the breakdown of uranium in soil and rock. You cannot see, smell, or taste it — and EPA links it to about 21,000 U.S. lung cancer deaths every year.
Guidance reviewed against EPA and PA DEP source material: July 22, 2026
Health risk
What does radon do to people?
Breathing radon over years damages lung tissue. EPA calls radon the number one cause of lung cancer among non-smokers and the second leading cause of lung cancer overall, attributing about 21,000 U.S. deaths per year to it — roughly 2,900 of them among people who never smoked. The U.S. Surgeon General's 2005 health advisory put it plainly: indoor radon is the second-leading cause of lung cancer in the United States, the threat is completely preventable, and it "can be detected with a simple test and fixed through well-established venting techniques."
How it enters
How does radon get into a house?
Radon moves up from soil and bedrock and gets pulled indoors through slab cracks, foundation joints, sump pits, crawlspaces, and utility penetrations — the lower pressure inside most homes literally draws it in. That is why the lowest livable level is where testing belongs, and why finishing a basement changes the exposure question. A secondary path exists for homes on private wells: PA DEP's rule of thumb is that every 10,000 pCi/L of radon in well water adds about 1 pCi/L to indoor air, so DEP suggests testing the water when the air result is elevated and the home uses a well. Pennsylvania does not regulate private well water quality — that check is on the homeowner.
Action levels
What does a radon test number mean?
| Radon result (pCi/L) | What EPA guidance supports | Practical next step |
|---|---|---|
| Below 2.0 | No action level triggered. | Keep the report; retest after renovations, foundation changes, or before a sale. |
| 2.0 to 4.0 | EPA recommends considering mitigation in this range. | Weigh how the lowest level is used; retest or plan mitigation if the space is occupied often. |
| 4.0 or higher | EPA recommends mitigation at or above this action level. | Move into mitigation planning with a PA DEP-certified provider, then retest to confirm. |
Pennsylvania
Why is Pennsylvania a radon hotspot?
Geology. Uranium-bearing rock underlies much of the state, and PA DEP reports that approximately 40% of homes tested in Pennsylvania come back above EPA's 4.0 pCi/L action guideline. Chester County sits in EPA Zone 1, the highest radon-potential category — a reason to test, not a prediction for any single address. Two neighboring homes can differ several-fold; only a measured result settles it.
Next step
What should a Chester County homeowner do with this?
Test the lowest livable level — see the radon testing guide for how. If the result is 4.0 pCi/L or higher, move to mitigation planning; the how-mitigation-works guide explains what a system actually does. Every claim on this page traces to the EPA and PA DEP documents on the sources page.
Clear next step
Request Chester County radon testing or mitigation routing.
Use this for first tests, real-estate deadlines, 4.0+ pCi/L results, mitigation planning, and post-mitigation retests.
Requests are routed only where an appropriate PA DEP-certified provider is available.
FAQ
Common homeowner questions
How many deaths does radon cause each year?
EPA attributes about 21,000 U.S. lung cancer deaths per year to radon, including about 2,900 among people who never smoked.
Is radon really the top cause of lung cancer for non-smokers?
Yes. EPA states radon is the number one cause of lung cancer among non-smokers and the second leading cause of lung cancer overall.
Is there a safe level of radon?
No known safe level exists. EPA recommends mitigation at 4.0 pCi/L or higher and says to consider mitigation between 2.0 and 4.0 pCi/L; lower is better.
How bad is radon in Pennsylvania?
PA DEP reports that approximately 40% of homes tested in Pennsylvania are above EPA’s 4.0 pCi/L action guideline, and calls Pennsylvania one of the most serious radon problem areas in the country.
Can well water add radon to a home?
Yes. PA DEP’s rule of thumb is that every 10,000 pCi/L of radon in water contributes about 1 pCi/L to indoor air. If the air result is elevated and the home uses a private well, DEP suggests testing the water too.