Radon is not limited to basements. While basements typically have the highest concentrations because they sit closest to the soil where radon originates, the gas regularly reaches upper floors of homes and apartment buildings. In some structures, radon levels on upper floors are just as high as on lower ones. Radon causes about 21,000 lung cancer deaths in the United States each year, and assuming you’re safe because you live above ground level is one of the more common mistakes people make about this invisible, odorless gas.
Why Basements Have the Highest Levels
Radon forms naturally in soil and rock as uranium breaks down. The gas seeps into buildings through cracks in foundations, gaps around pipes, and openings in concrete slabs. Basements sit in direct contact with the ground, so they’re the first point of entry and usually show the highest readings. In a study of a 19-story building in Mumbai, ground floor radon concentrations reached 41 Bq/m³ compared to 15 Bq/m³ on the 19th floor, with the peak winter reading hitting 59 Bq/m³ at ground level.
This pattern is what creates the common assumption that radon is a “basement problem.” But the gas doesn’t stay put.
How Radon Travels to Upper Floors
The main force pushing radon upward through a building is called the stack effect. When indoor air is warmer than outdoor air, the warm air rises and exits through the upper parts of the building. This creates negative pressure in the lower levels, pulling soil gas (including radon) in through the foundation. But it also draws that gas upward through the building’s interior, floor by floor.
The greater the temperature difference between inside and outside, the stronger this effect becomes. That’s why radon levels in many homes peak during winter, when heating systems keep indoor air much warmer than the cold air outside. Wind pressure on the exterior of a building adds to the effect, further driving air movement through the structure.
Several building features act as vertical highways for radon. Elevator shafts, stairwells, plumbing chases, and gaps between floors all give the gas a path upward. A comprehensive study of an apartment building in Rome’s historic center found something striking: while the basement had the highest radon concentration, levels on the upper floors were comparable to each other, with no clear decrease as floors got higher. The researchers specifically noted that “elevated floors are not safer than lower floors” and that the lift shaft likely served as a preferential pathway for upward gas movement.
Homes Without Basements Are Still at Risk
Slab-on-grade homes, which sit directly on a concrete pad with no basement underneath, can have significant radon levels on the main living floor. Radon enters through cracks in the slab, construction joints, and gaps around utility penetrations. Crawl spaces present similar risks, as radon accumulates in the enclosed space beneath the home and migrates upward through the floor system.
Even mobile homes and manufactured housing can have elevated radon if they sit over soil with high uranium content, particularly if the underside isn’t well ventilated.
Building Materials Can Release Radon Too
Soil gas is the dominant source of indoor radon, but it’s not the only one. Building materials like brick, concrete, marble, and granite contain trace amounts of naturally occurring radioactive elements. As these materials slowly decay, they release small quantities of radon directly into indoor air. The EPA notes that while this source is a far smaller health risk than soil gas entry, it can still contribute to elevated indoor levels, and it means radon can appear on any floor regardless of soil contact.
Well Water as a Radon Source
If your home uses a private well, the water itself can carry dissolved radon into your living space. When you run a shower, dishwasher, or washing machine, the agitation releases radon gas from the water into the air. The general ratio is that every 10,000 pCi/L of radon in your water adds about 1 pCi/L to your indoor air. Since the EPA recommends action at 4 pCi/L in air, water with very high radon levels can meaningfully contribute to the problem, and this happens wherever the water is used, not just in the basement.
Why Every Floor Should Be Tested
The assumption that testing only the basement matters can leave real exposure undetected. If you spend most of your time on the first or second floor, the radon level on those floors is what determines your actual health risk. A basement reading of 6 pCi/L might drop to 3 pCi/L on the main floor, but in some buildings, the difference is far smaller than expected.
Short-term test kits are inexpensive and available at most hardware stores. Place them on the lowest lived-in level of your home for accurate results that reflect your actual exposure. If you live in an upper-floor apartment, testing still makes sense, particularly in older buildings with elevator shafts, internal stairwells, or stone and concrete construction. The Rome study challenged the widespread belief that upper-floor apartments are inherently low-risk, finding indoor radon levels that remained “almost uniform” across floors above the basement.
Radon mitigation systems, which typically use a fan to pull gas from beneath the foundation and vent it outside, are effective at reducing levels throughout the entire building. If testing reveals elevated radon on any floor, the problem is solvable regardless of building type.