Radon is a naturally occurring radioactive gas produced by the decay of uranium in soil and bedrock. It's colorless, odorless, tasteless, and chemically inert — meaning you cannot detect it without specialized equipment. It seeps up from the ground and accumulates in indoor air, particularly in basements, crawl spaces, and lower floors of homes.
In the United States, radon is the second leading cause of lung cancer after smoking and the leading cause among non-smokers, responsible for approximately 21,000 deaths every year, according to the EPA.
Where radon comes from
Radon is part of a natural radioactive decay chain. Uranium-238, found in most rocks and soils worldwide, slowly decays through a series of intermediate isotopes, eventually producing radium-226. Radium decays to radon-222 — the form of radon we worry about — which has a half-life of about 3.8 days. As radon decays further, it produces radioactive "daughter products" (polonium, lead, bismuth) that lodge in lung tissue and emit alpha particles, damaging DNA and causing cancer over years of exposure.
Some types of bedrock contain much higher uranium concentrations than others. Granite, shale, limestone, and certain phosphate deposits are notably uranium-rich. The Cincinnati region sits on Ordovician limestone and shale formations that are some of the most uranium-rich rocks in the eastern United States — which is why our metro is classified as EPA Radon Zone 1.
How radon enters your home
Your home operates at slightly lower air pressure than the soil beneath it, especially in winter when warm indoor air rises and creates a "stack effect" that pulls air up through the building. This pressure difference draws soil gas — including any radon present — through every opening in the foundation:
- Cracks in the concrete slab
- Gaps around plumbing penetrations
- Floor-to-wall joints (cove joints)
- Sump pits and floor drains
- Crawl space dirt floors
- Porous concrete (cinder block walls especially)
- Construction joints in the foundation
Why radon levels vary so much
Even neighboring homes can have wildly different radon levels due to:
- Foundation type — basements typically test higher than slab-on-grade
- Foundation age and condition — older foundations with more cracks have higher entry rates
- Soil permeability — sandy/gravelly soil moves more gas than clay
- Heating season vs. cooling season — winter levels are typically 2-7× higher than summer
- HVAC patterns — homes with negative-pressure ventilation pull more radon in
- Sealing and air sealing — counter-intuitively, tightly sealed energy-efficient homes can trap radon more effectively
What level of radon is dangerous?
The EPA action level is 4.0 pCi/L. Above this level, mitigation is strongly recommended. The EPA also advises considering mitigation between 2.0 and 4.0 pCi/L, especially for homes with children, smokers, or long-term occupants.
No level of radon exposure is risk-free. Even at 2.0 pCi/L, your lifetime lung cancer risk is comparable to the risk from a non-trivial number of chest X-rays per year.
Risk in plain English
The EPA publishes risk comparisons that help contextualize radon exposure:
| Radon Level (pCi/L) | Lifetime Cancer Risk (non-smoker) | Comparable to |
|---|---|---|
| 20 | ~36 in 1,000 | 35× the risk of dying in a car crash |
| 10 | ~18 in 1,000 | Smoking 1 pack/day exposure |
| 8 | ~15 in 1,000 | Smoking 8-10 cigarettes/day |
| 4 (action level) | ~7 in 1,000 | 200 chest X-rays/year |
| 2 | ~4 in 1,000 | 100 chest X-rays/year |
Risk multiplies dramatically for smokers — smoking and radon exposure together are far worse than either alone. EPA Citizen's Guide to Radon estimates.
How to know if you have a problem
Test your home. There is no other way. Radon levels vary so much by home and even by room that you cannot predict what your levels are based on neighbors, geology, or how new your house is. Read about professional radon testing or request a free testing quote.