Radon: The Invisible Risk Worth Testing For

Last updated September 24, 2026 · Monitor My Air

Radon is a colorless, odorless radioactive gas and a leading cause of lung cancer. The test is cheap, the fix works, and no monitor for other pollutants detects it.

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What it is

Radon is a naturally occurring radioactive gas that seeps up from soil and rock into buildings through cracks in foundations, floor slabs, and sump pits. You cannot see, smell, or taste it, and it is the second leading cause of lung cancer after smoking. Because it comes from the ground, it tends to concentrate in basements and lower floors. The critical point most people miss: your air quality monitor almost certainly does not measure radon. It needs a dedicated test. The good news is testing is cheap and mitigation reliably works.

The numbers that matter

4 pCi/L: the EPA action level. At or above this, you should fix the home.
2 to 4 pCi/L: the EPA suggests considering mitigation, since no level is truly risk-free.
Below 2 pCi/L: low risk for most homes, though periodic re-testing is still wise.

Radon is measured in picocuries per liter (pCi/L) in the US, or becquerels per cubic meter elsewhere (the WHO reference is roughly 100 Bq/m³, about 2.7 pCi/L). Levels vary by geology, season, and even day to day, which is why a proper test runs over time rather than for a single moment.

What NOT to waste money on

Do not assume your multi-pollutant air monitor covers radon. Almost none do, and radon needs its own test. Continuous digital radon monitors exist and are useful for tracking, but you do not need one to start. A cheap kit answers the only urgent question, which is whether you are above the action level. Do not skip testing because your neighbor tested fine; radon varies house to house even next door. And do not attempt DIY mitigation piping for a high reading. This is one area where a certified professional is worth the cost, because a poorly designed system can fail silently.

What professionals know

Radon risk is driven by cumulative exposure to its short-lived decay products (radon progeny), which are the actual carcinogenic agents; they attach to airborne particles and deposit in lung tissue where their alpha emissions damage cells. This is why radon and smoking are strongly synergistic. Measurement reflects this indirectly through the parent gas concentration.

Testing method matters. Short-term charcoal kits (2 to 7 days) adsorb radon and are read by a lab, but they capture a snapshot and are sensitive to weather and ventilation during the test window. Long-term alpha-track detectors (90 days to a year) integrate exposure and give a far better estimate of true annual average, which is what health risk depends on. Radon levels swing with barometric pressure, soil moisture, frozen ground, and heating season stack effect, so a single short test can mislead in either direction; the EPA protocol accounts for this by recommending confirmatory testing before mitigating.

Continuous radon monitors using ionization chambers or scintillation cells provide hourly trend data and are genuinely useful for verifying mitigation performance and understanding seasonal swings, but they require periodic calibration and cost more than kits. Note the units: 1 pCi/L equals 37 Bq/m³. Mitigation via active sub-slab depressurization routinely reduces levels by 80 percent or more when properly installed. Sources: US EPA "A Citizen's Guide to Radon," WHO Handbook on Indoor Radon, national radon proficiency programs (NRPP/NRSB).

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