Radon Detection and Mitigation: A Complete Homeowner Guide

Radon is a colorless, odorless radioactive gas produced by uranium decay in soil and rock beneath every home. It seeps through foundations, accumulates indoors, and contributes to lung cancer when inhaled over years. The EPA estimates radon causes about 21,000 lung cancer deaths per year in the US, second only to smoking. Roughly one in fifteen US homes has radon levels above the 4.0 picocurie per liter action threshold. Testing is cheap (15 to 30 dollars for a mail-in kit) and mitigation, when needed, is a well-understood engineering fix. This guide walks through how radon enters homes, how to test for it, and how mitigation systems work.

Where radon comes from

Uranium-238 occurs naturally in trace amounts in essentially all soils. As uranium decays through a chain of radioactive intermediates, it produces radon-222 gas. Radon-222 has a half-life of 3.8 days and decays into a series of solid radioactive isotopes (polonium-218, lead-214, polonium-214) that are themselves alpha emitters with much shorter half-lives.

The danger is not the radon gas itself but the decay products. Radon gas inhaled is mostly exhaled. The decay products, which are charged particles, stick to dust and aerosols in the air, are inhaled, and lodge in the bronchial epithelium. The alpha radiation from their decay damages DNA in lung cells over years and decades, leading to lung cancer.

Soil radon concentrations vary by geology. Granite, shale, and uranium-rich sedimentary rocks produce more radon. The EPA’s Radon Zone Map identifies counties by predicted indoor radon levels (Zone 1: high potential, average above 4 pCi/L; Zone 2: moderate; Zone 3: low). Living in a low-risk zone reduces the probability of elevated radon but does not eliminate it. Individual homes within any zone can have any level.

How radon enters homes

Radon seeps from soil into homes through every gap in the foundation. The driving force is the stack effect: warm air inside the home rises and exits through upper-floor leaks, creating slight negative pressure at the basement and slab level. This pulls soil gas (including radon) through cracks, plumbing penetrations, sump pits, and bare-soil crawl spaces.

Common entry points:

Slab-to-wall joints in basements and slab-on-grade construction. The joint between a poured slab and concrete or block walls is rarely fully sealed.

Plumbing and electrical penetrations. Holes around pipes and conduit are often unsealed or poorly sealed.

Sump pits and floor drains. Open pits provide direct soil gas pathways.

Crawl space soil. Bare-dirt crawl spaces are the largest entry point in homes with this construction.

Building materials. Granite countertops, concrete, and certain stone products contain trace uranium and emit small amounts of radon. The contribution from building materials is usually small relative to soil gas entry.

Well water. Radon dissolves in groundwater and outgasses into the home when water is used (showering, washing, toilet flushing). Well-water radon is a separate testing concern; the EPA has not established a federal action level for water radon but several states have (typically 4,000 to 10,000 pCi/L water radon contributes about 1 pCi/L air radon).

Short-term and long-term testing

Short-term tests measure radon over 2 to 7 days. Charcoal canisters adsorb radon and decay products onto activated charcoal; you mail the canister back to a lab for gamma spectroscopy. Alpha track detectors record alpha decay events on a plastic film over similar time periods. Cost: 15 to 30 dollars per test.

Long-term tests measure radon over 90 days to one year. Alpha track detectors and electret ion chambers are common. Cost: 30 to 60 dollars per test. Long-term tests provide a more accurate annual average because they integrate over seasonal and daily variation.

Continuous radon monitors are electronic devices that report hourly radon levels. The Airthings Wave Plus, Corentium Home, and Ecosense RadonEye are common consumer models. Cost: 150 to 250 dollars for the device, with no recurring lab fees. Continuous monitors show daily and seasonal patterns, which is useful for understanding mitigation performance and for repeated testing in a single home over years.

For real estate transactions and initial home evaluation, the standard is a short-term test in the lowest occupied level of the home with closed-building conditions (windows and exterior doors closed for 12 hours before and during the test). If the result is above 4.0 pCi/L, a follow-up long-term test or mitigation discussion is warranted.

Test placement and conditions

Test in the lowest lived-in level of the home. For a finished basement, test in the basement. For an unfinished basement that is not occupied, test on the lowest occupied level (typically first floor).

Place the test 20 inches to 6 feet above the floor, away from drafts, exterior walls, heat sources, and high humidity. The middle of an interior room on a shelf or table is ideal.

Maintain closed-building conditions: keep windows and exterior doors closed for at least 12 hours before the test starts, and during the test except for normal entry and exit. HVAC systems run normally. Bathroom and kitchen exhaust fans run normally but should not be left running continuously.

Avoid testing during severe weather or major HVAC changes. Stable weather and normal occupancy produce the most representative results.

Mitigation systems

The standard residential mitigation system is sub-slab depressurization (SSD). A small fan creates negative pressure beneath the basement slab or crawl space barrier, exhausting soil gas to the outdoors before it can enter the living space.

Components of a typical SSD system:

A suction point cut through the foundation slab into the gravel or soil beneath. A 4-inch PVC pipe extends from this point.

A continuous-duty radon fan (50 to 150 watts) installed in the PVC pipe, typically outside the basement (in an attic or on an exterior wall) to keep negative-pressure piping in conditioned space.

PVC piping routed to the exterior, terminating above the roof line (or at least 10 feet above grade and away from windows).

A monitoring manometer that visually confirms the fan is creating suction.

Sealing of major foundation gaps (slab-to-wall joints, sump pit covers, plumbing penetrations) to direct the negative pressure where it is needed.

A correctly installed SSD system typically reduces basement radon by 80 to 95 percent. Pre-mitigation 8.0 pCi/L drops to 0.8 to 1.6 pCi/L after mitigation. Verification testing 24 to 48 hours after installation confirms the result.

Crawl spaces require encapsulation before SSD can work. The crawl space floor is covered with 6 to 20 mil polyethylene vapor barrier sealed at seams and to the foundation walls. The fan then evacuates the space beneath the barrier. Encapsulation adds 1,000 to 2,500 dollars to a mitigation project.

Slab-on-grade homes use the same SSD approach as basements but require a suction point through the living-area slab. Cosmetic patching of the slab penetration is needed.

After mitigation

Re-test 24 to 48 hours after fan startup. A correctly designed system reduces levels well below 4.0 pCi/L; many achieve below 1.0 pCi/L.

Re-test every 2 years thereafter, or whenever there are significant home changes (HVAC replacement, foundation work, additions). Fans last 10 to 15 years and gradually lose suction. The visual manometer should always show a pressure differential; if it reads zero, the fan has failed.

Replace the radon fan when it fails (60 to 200 dollars for the fan, 200 to 400 dollars installed by a contractor).

For more on home environmental hazards see our whole-house water filtration explained and our methodology at /methodology.

Frequently asked questions