Radon in basements vs. upper floors
Every county and ZIP page on this site shows two predicted numbers: a basement mean and an upstairs (above-ground) mean. The basement figure is almost always higher, and we grade areas on it. This guide explains where the split comes from, how big it really is, and when a basement reading is the one you should act on.
Why the basement reads higher
Radon is a soil gas. It enters through the surfaces in contact with the ground - the slab, floor drains, sump pits, the joint where the slab meets the wall, and the pores of block walls - and it is pulled in because a heated, ventilated house sits at slightly lower pressure than the soil beneath it. The basement is where the gas arrives and where air exchange with outdoors is lowest, so it accumulates there. Upper floors receive whatever drifts up through stairwells, ducts and floor penetrations, diluted by the much larger volume of air and by open windows and doors.
How big the gap is, in numbers
The National Radon Exposure Model predicts both figures for every ZIP. Across 29,215 ZIPs, the upstairs prediction is typically 57% of the basement prediction (median ratio); the middle 80% of ZIPs fall between 47% and 72%. Nationally the population-weighted basement mean is 1.8 pCi/L and the upstairs mean is 1.1.
The split matters most at the thresholds. 1,220 ZIPs are predicted at or above the 4 pCi/L action level in the basement, but only 2 upstairs. 14,231 ZIPs are at or above 2 in the basement versus 3,774 upstairs. And 23 ZIPs are predicted to exceed 4 in the basement while staying under 2 upstairs - the case where which floor you test decides whether you mitigate. At county level, 196 counties are predicted at or above 4 in the basement against 0 upstairs.
Predicted (model estimate). These are long-term area averages from a model trained on 2001-2021 test results, not readings from specific homes.
What measured tests show
Two state programs publish enough detail to check the model's direction against real tests. New York State Department of Health reports basement and first-floor averages separately for 58 counties (1987-2025): the simple average of county basement results is 5.7 pCi/L versus 2.5 on the first floor, and the first floor is lower in 58 of 58 counties. The widest gap is in Cortland County: 14.2 in basements against 6.5 on the first floor. In Pennsylvania, where DEP records the floor of every test, the basement-only mean is higher than the all-floors mean in 67 of 67 counties. Measured results run above the model overall because state tests are voluntary and basement-heavy, but the floor split points the same way.
When the basement reading matters
- Anyone spends time there. A finished basement, a bedroom, a home office, a gym or a playroom makes the basement reading the exposure that counts. Treat 4 pCi/L or above as a fix.
- You plan to finish it. Test before the drywall goes up; a sub-slab system is far cheaper to install in an open basement.
- You are buying or selling. Most transaction protocols test the lowest livable level, so expect the basement number to be the one negotiated over. See radon in a home sale.
- The basement result is very high. A basement at 10+ pCi/L usually means a strong source; even if upstairs is under 2 today, tightening the house or changing HVAC can move gas upward. A long-term first-floor test is the prudent follow-up.
When the upstairs reading is the one to use
If the basement is unfinished and rarely entered, the EPA's guidance is to test the lowest level you actually live in. A first-floor result below 2 pCi/L in that situation is genuinely reassuring - it is the air you breathe. Keep the basement result on file, retest every few years or after renovations, and read how to test for placement rules that keep either result valid.
Basement vs. upstairs: quick answers
- How much lower is radon upstairs than in the basement?
- Across 29,215 modeled ZIP codes, the predicted above-ground mean is typically 57% of the basement mean (a 43% drop), with the middle 80% of ZIPs between 47% and 72%. Measured New York State data agrees in direction: the average county basement result is 5.7 pCi/L against 2.5 on the first floor, and the first floor is lower in 58 of 58 counties.
- Should I test the basement or the first floor?
- Test the lowest level you use regularly. If the basement is finished, a bedroom, an office or a playroom, test there - that is where the exposure happens. If it is an unfinished space you enter a few times a month, the EPA says to test the lowest lived-in level instead, which is usually the first floor. For a real-estate transaction most protocols test the lowest level that could be lived in, so a basement reading is normal even if the sellers never use it.
- My basement is 5 pCi/L but upstairs is 1.5 - do I need to mitigate?
- It depends on how the basement is used. If people spend meaningful time there, or you plan to finish it, the EPA action level applies to that reading and mitigation is indicated. If nobody lives or sleeps there and the first-floor result is below 2, many state programs consider the upstairs number the relevant one - but note that a basement at 5 usually means the soil under the house is a strong source, and changes such as a new furnace, tighter windows or finishing the space can pull more of that gas upstairs. A long-term test on the first floor is the cheapest way to decide.
- Can radon be higher upstairs than in the basement?
- Rarely, and it usually signals a specific cause: a slab-on-grade wing with no basement under it, an attached garage or crawlspace venting into upper rooms, or well water releasing radon in showers and laundry. In the model data, 2 ZIPs are predicted at or above 4 pCi/L upstairs versus 1,220 in the basement, and only 154 of 29,215 ZIPs (0.5%) have an upstairs prediction above the basement one - so an upstairs-higher result in a home is a reason to look for a source, not something to expect.
Radon levels by state · Mitigation guide · Download the data