How every report is built.

Every RadonZoneReport dossier is built from four data layers: the EPA Map of Radon Zones (county classification), EPA Table A-1 lifetime-risk data (the cancer-risk math), state-level radon screening surveys (the prevalence number), and the AARST mitigation standards (the cost and effectiveness ranges). No layer is invented. No layer is interpolated where a source has a published number.

Primary data sources

The report draws on six public documents and two certification directories. Each is named below with its publication number and a description of what it contributes to the dossier.

How each derived metric is computed

1. Your EPA Radon Zone (1, 2, or 3)

Source: EPA-402-R-93-071, the EPA's county-level radon zone classification table covering all 3,143 US counties. This is a direct county lookup: your ZIP is matched to its county, and the county's EPA zone is reported exactly as the EPA classifies it. We do not derive, interpolate, or override the zone.

The EPA zone definitions, taken directly from EPA-402-R-93-071:

• Zone 1: Predicted average indoor radon screening level greater than 4 pCi/L.
• Zone 2: Predicted average indoor radon screening level between 2 and 4 pCi/L.
• Zone 3: Predicted average indoor radon screening level less than 2 pCi/L.

2. Your county's average indoor concentration (pCi/L)

Source: state radon program publications, where available. For Colorado we use the Colorado Department of Public Health & Environment Radon Program data; for Iowa, the Iowa Department of Public Health; for Pennsylvania, PA DEP; for Maine, Maine CDC; and so on. Where a state has not published a county-level number, we report the EPA Zone classification midpoint as the indoor average estimate and explicitly mark the number as "EPA-zone-derived estimate" rather than a measured average.

3. Your home's estimated likelihood of testing above 4 pCi/L

This is a derived statistic, not a direct lookup. Method:

1. Start with the EPA-published national distribution of indoor radon levels (EPA-402-R-03-003).
2. Fit a log-normal distribution per zone using the EPA's published geometric means and geometric standard deviations.
3. Compute the probability mass above 4 pCi/L for the county's zone.
4. Apply two adjustments published in the underlying EPA technical support documents: +8 percentage points for homes with a basement (basements draw soil gas) and a small modifier for home type (single-family vs. condo).

The number we report is therefore a population-level statistical estimate for a hypothetical home of your type in your county. It is not a measurement of your home. The only way to know your home's radon is to test the house. See our DIY testing guide.

4. Mitigation cost range

Source: EPA-402-K-10-005 Consumer's Guide to Radon Reduction and American Lung Association published guidance. The EPA quotes typical sub-slab depressurization costs in the $800–$2,500 range for residential single-family homes; we apply a modest regional cost-of-living adjustment based on county-level construction-cost indices. We do not invent quotes and we do not vary the cost by mitigator; the range is what the EPA publishes, scaled for region.

5. Sub-slab depressurization effectiveness

Source: EPA-402-K-10-005. The EPA publishes effectiveness ranges of 50–99% reduction in indoor radon concentration for properly designed and installed sub-slab depressurization systems. We report this range as-is; we do not claim a specific reduction percentage for your home.

6. Lifetime lung-cancer risk

Source: EPA-402-R-03-003, Table A-1. The EPA publishes lifetime lung-cancer mortality per 1,000 persons at various indoor radon concentrations and smoking statuses, derived from BEIR VI risk models (National Research Council, 1999) and calibrated against the Iowa Radon Lung Cancer Study (Field et al., American Journal of Epidemiology, 2000). We report the EPA's published values verbatim. We do not interpolate, we do not adjust, and we do not extrapolate beyond the table.

Calibration

The "likelihood at or above 4 pCi/L" is a log-normal estimate calibrated against the EPA's National Residential Radon Survey, conditioned on your county's EPA zone and adjusted for basement presence and home type. It is a population-level statistic for the zone, not a prediction for your individual home; only a test can tell you that.

Known limits and uncertainty bounds

This report has real limits. We say them out loud:

• The 1993 vintage of the EPA map. EPA-402-R-93-071 has not been re-issued in a new edition. Zone classifications are stable (the underlying geology hasn't changed), but local construction patterns, foundation types, and HVAC practices have. Where a state has more recent data, we use it.
• House-to-house variance is enormous. Two homes on the same street can have radon readings that differ by an order of magnitude due to differences in foundation cracks, sub-slab drainage, soil moisture, and HVAC pressure balance. Our county number is a starting point for thinking about your risk, not a substitute for a $20 short-term test kit.
• Seasonal variance. Indoor radon is typically highest in winter (when houses are closed up and the stack effect pulls soil gas inward). EPA screening data is normalized for season; in-home tests should be performed in closed-house conditions per the EPA's protocol.
• Well water radon is separate. Our zone classification reflects soil-gas radon. Homes on private wells in granitic terrain may have an additional radon-in-water source that is not captured by the EPA Map of Radon Zones. See our radon in well water guide.
• Above-grade units. The zone classification assumes ground-contact construction. Apartments on the 3rd floor or higher of a multi-story building have substantially lower soil-gas radon exposure. See our radon in apartments and condos guide.

Data refresh cadence

We refresh the underlying data layers on the following schedule:

What this report is not

Corrections and updates

If you spot a factual error in the methodology or any individual report, email [email protected]. Every correction we make is logged publicly at /corrections.html with the date, the previous claim, the corrected claim, and the source that drove the change.