Radon Levels by Floor: Basement vs. First Floor vs. Upper Floors

How Radon Distributes Through a Home

Radon enters your home from the ground up. It seeps through the foundation, accumulates in the lowest space, and then gradually mixes upward into the rest of the house — diluted by normal air exchange with each floor it rises through.

Typical distribution (same home):

These are rough averages — actual distribution depends on floor plan, HVAC configuration, staircase location, and how much air moves between floors. Open floor plans distribute radon more evenly; homes with closed stairwells concentrate it more in the basement.

Example: A home with a basement at 12 pCi/L might have:

• Finished basement bedroom: 10–11 pCi/L
• First floor living room: 3–5 pCi/L
• Second floor bedroom: 1–2 pCi/L

Where to Test

EPA testing protocol: Place the test in the lowest occupied level — the lowest area of the home where people spend 4+ hours per week.

If you have a finished basement with a TV room or bedroom, test there — that's your highest-exposure space and the legally relevant number for mitigation decisions.

If your basement is purely unfinished storage and you never spend time there, test on the first floor. That's where your actual exposure happens.

Where NOT to place the test:

• Kitchen or bathroom (humidity and ventilation skew results)
• Closet or under a staircase (poor air circulation)
• Against an exterior wall (influenced by outdoor air)
• Near a sump pit or drain (artificially elevated)

Health Risk Is Floor-Dependent

Radon cancer risk accumulates with exposure: hours per day × radon level × years. Floor level affects both the radon level and how much time you spend there.

Highest-risk scenario: A basement bedroom used nightly. If the basement tests at 12 pCi/L and someone sleeps there 8 hours a night, their personal exposure is much higher than the first-floor resident of the same home.

The EPA action level of 4.0 pCi/L is measured at the lowest occupied level. It's calibrated to account for the fact that people spend varying amounts of time at various levels of the house.

Children in basements: Children spend more time at lower levels (playrooms, family rooms, finished basements) and have longer life expectancy ahead of them — compounding long-term exposure. If children spend significant time in a basement, test that level and treat 4.0 pCi/L as a hard action threshold.

Apartments and Condos: Same Principle

Ground-floor units in multi-family buildings have significant radon risk — they sit on the same soil as single-family homes, and radon concentrates in the ground-floor unit. Second-floor units have lower levels; third floor and above is generally low risk.

If you live in a ground-floor apartment, particularly in a high-radon state, testing is worthwhile.

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