Last updated 2026-07-17
New Jersey publishes one of the more useful state radon maps in the country: every municipality is assigned a radon potential tier based on real indoor test results, not just geology. Here’s how the tiers are defined, where the high-potential areas are, and what the map does — and doesn’t — tell you about your own basement.
The tiers come from measured data. A municipality gets a tier once at least 25 homes there have been tested, based on the share of those homes at or above EPA’s 4 pCi/L action level:
| Tier | Radon potential | Definition (share of tested homes ≥ 4 pCi/L) |
|---|---|---|
| Tier 1 | High | 25% or more |
| Tier 2 | Moderate | 5% to 24% |
| Tier 3 | Low | Less than 5% |
Put plainly: in a Tier 1 town, at least one in four tested homes has come back at or above the mitigation action level. In a Tier 2 town it’s somewhere between one in twenty and one in four. Even in Tier 3, elevated homes exist — the definition allows up to 1 in 20.
NJDEP’s current tier assignments are published as a statewide list and map (last comprehensively updated from 2015 testing data), plus an online municipal lookup.
High radon potential in New Jersey tracks geology. The Reading Prong — a uranium-bearing rock formation running from Pennsylvania through northwestern New Jersey into southern New York — underlies much of the state’s Tier 1 territory. All or part of seven northern and western counties fall in Tier 1: Sussex, Warren, Hunterdon, Morris, Passaic, Somerset, and Mercer. Homes in Hunterdon, Somerset, Morris, Warren, and Sussex counties produce some of the most elevated indoor readings in the country, with individual houses not uncommonly testing above 20 pCi/L.
Central Jersey is a patchwork of Tier 1 and Tier 2, while much of the coastal plain in South Jersey — sandier soil, different bedrock — sits in Tier 2 and Tier 3. But the pattern is a tendency, not a rule: NJDEP is explicit that radon concentration can vary widely even within a tier area, depending on local geology and the amount of uranium in the soil beneath a specific house.
For existing homes: nothing mandatory. Testing is recommended for every NJ home in every tier, but no law requires an owner to test or mitigate an existing house, in any tier.
For new construction: Tier 1 triggers the radon hazard subcode. New Jersey’s Uniform Construction Code includes a radon hazard subcode (N.J.A.C. 5:23-10) that requires radon-resistant construction in Tier 1 municipalities (listed in the subcode’s Appendix 10-A) for new residential and school construction. Required features include:
These “passive” features make a house cheaper to fix later — often just a fan added to the existing vent stack.
For child care centers: testing is required statewide. Under N.J.S.A. 30:5B-5.2, licensed child care centers must be tested for radon at least once every five years, with results posted in the building. This applies regardless of tier.
EPA’s national Map of Radon Zones puts counties into Zones 1–3 using geology, aerial radioactivity, and soil data — and much of northern New Jersey is EPA Zone 1 (predicted average above 4 pCi/L). NJDEP’s tier map is the better tool for New Jersey specifically because it’s municipality-level and built from actual indoor measurements. Use the EPA map for national context; use the NJDEP lookup for your address.
NJDEP publishes a municipal radon potential lookup and a statewide tier list on its Radon Program site (dep.nj.gov/rpp/radon). You can look up your municipality directly or view the color-coded statewide map.
Yes. NJDEP recommends every home be tested regardless of tier. Tiers describe the odds across a whole municipality, not your house — a Tier 3 designation means fewer than 5 percent of tested homes were elevated, not zero, and radon varies house to house with local geology.
Northern and western New Jersey. All or part of seven counties — Sussex, Warren, Hunterdon, Morris, Passaic, Somerset, and Mercer — fall in Tier 1 territory, largely because of the uranium-bearing Reading Prong geology that crosses northwestern NJ.
For existing homes, no — testing is recommended but voluntary. For new construction, New Jersey's radon hazard subcode (N.J.A.C. 5:23-10) requires radon-resistant construction features in Tier 1 municipalities, such as a sub-slab vapor barrier and a soil-gas collection pipe.
No. EPA's national map assigns whole counties to Zones 1–3 using geology and other predictors, while NJDEP's tier map classifies each municipality using actual indoor test results from at least 25 homes. The NJ map is the more granular tool for NJ addresses.
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