What Causes False Positive and False Negative Radon Test Results?

Knowing the causes of false positive and false negative radon results will ensure both financial peace of mind and prevent long-term health risks.

Technically, every home or building is “positive” for radon. Radon is naturally occurring and is present in both our indoor and outdoor environments. A false positive refers to radon test results that are at or above the action levels of 4.0 pCi/L (United States) or 200 Bq/m³ (Canada) when the actual radon levels are lower.

The causes of false positive and false negative radon results can be divided into three categories: environmental factors, placement of the radon detectors, and device tampering.

Environmental Factors

Environmental conditions can significantly influence radon test results, sometimes causing readings that are higher or lower than a home or building’s actual long-term average. Rain, high winds, soil conditions, and barometric pressure affect how radon enters a structure during a test period. Understanding these environmental factors is essential for interpreting results accurately and avoiding false conclusions.

• Rain: Heavy rain can force radon into homes. When rain saturates the soil it displaces moisture in the soil, enabling radon gas in the soil to follow the path of least resistance and be transported up to the surface and into a home or building. Rain can cause a false positive or false negative result, but in most cases rain temporarily increases radon in a home leading to a false high/positive.

• Wind: Rain is frequently accompanied by high winds. Wind blowing against the side of a home with doors and windows can create positive pressure in the house that “pushes” the radon out. High winds hitting the side of a house without doors or windows can create negative pressure inside the home which then draws the radon into the home. Wind can cause both false positive and false negative radon results.

• Barometric pressure: Drops in barometric pressure associated with weather fronts can alter pressure differentials between the soil gas and the building interior. Low barometric pressure that occurs during rainfall increases radon levels indoors. The reduction in pressure on the earth’s surface (low pressure) allows more radon gas to rise from the soil where it can move into homes through cracks in the foundation. Also, when the air pressure in the home is lower than the air pressure outdoors, the home acts like a vacuum and pulls in air and radon gas from the soil around your home.

Extreme weather conditions can cause both false positive and false negative radon test results. For a short-term radon test (less than 72 hours in duration), avoid radon testing during severe weather. Most likely, it could lead to a false positive result.

Radon Test Placement

Improper radon detector or instrument placement can skew radon test result data. Factors like proximity to drafts and high-moisture areas lead to results that don't reflect the actual radon in your living space.

• Placement Errors: Placing the device too close to a sump pump, furnace, or near a fireplace (on a mantel) can yield false positive/high radon readings. Placing the radon detector near windows, doorways, and exterior walls can introduce too much fresh air and dilute the accurately, causing lower than normal readings (false negative).

• Room Location: Testing for radon in kitchens, bathrooms, or laundry rooms should be avoided. Steam and humidity can interfere with short-term charcoal-based detectors, causing false negatives. Consult the ANSI/AARST or Health Canada standards for testing locations in multi-family housing, schools, and workplaces. For example, radon test guidelines for these buildings call for testing 100% of the ground floor occupied rooms and 10% of upper floors.

• Unoccupied Spaces: Placing the radon detector inside closets, drawers, or tight corners where the air is stagnant can lead to unnaturally high readings (false positives).

Radon Detector Device Tampering

Intentional or accidental interference with a radon detector, known as tampering, can lead to misleading data. Whether motivated by a desire to "pass" a real estate inspection or simply a misunderstanding of how the sensors function, changing the environment around the device prevents it from capturing an accurate radon result.

• Moving the test device: Moving a radon to a higher floor or a breezy garage or porch during the test period will artificially lower the reading. Conversely, moving it near a sump pump or a crawlspace where radon enters the home creates a false positive that doesn't reflect actual living conditions. Electronic continuous radon monitors, such as the SPIRIT continuous radon monitor, have a feature that reports movement or tampering with the device.

• Opening doors and windows: Increasing ventilation and airflow dilutes radon gas inside the home with fresh air. Also, open doors and windows allow accumulated radon gas in the home to escape, causing false negative/low results.

What to Do About Suspected False Positive or False Negative Radon Results

• Retest: If your first test was a short-term test then conduct another test, ideally a long-term one (90+ days). A long-term radon test (90 to 365 day duration) provides better long-term exposure average and is less affected by seasonal and environmental factors.

• Test with a long-term radon test. Because radon can fluctuate greatly over time, a long-term measurement, such as with the Radtrak, provides a much more stable and reliable result than short-term testing.

• Hire a qualified measurement professional: An NRPP (U.S.) or C-NRPP (Canada) certified radon measurement professional will provide proper test placement, timing, and use reliable methods to minimize false negatives or positives.

Accurate radon testing is the foundation of radon risk management and lung cancer prevention. By carefully following the placement instructions located in your Radonova radon test kit you will minimize the changes of false positive/high or false negative/low results.

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