Radon is a radioactive gas that is colorless, odorless, and tasteless, making it impossible to detect without specialized testing. It originates from the breakdown of uranium found in soil, rock, and groundwater everywhere in the world. Understanding whether radon sinks or rises is central to knowing where it accumulates inside a home and how to manage exposure risk. As the second leading cause of lung cancer after smoking, understanding radon’s behavior is a significant public health concern.
The Physical Properties of Radon Gas
Radon is an extremely dense element, which determines its movement inside a building. Air has an average molecular weight of approximately 29 atomic mass units (AMU). In contrast, the most stable isotope of radon, Radon-222, has a molecular weight of about 222 AMU, making it roughly 7.5 times heavier than air.
This density difference means that radon does not rise; it tends to sink and settle in the lowest available spaces due to gravity. However, this sinking behavior is counteracted by air movement within a home. While air currents, temperature variations, and ventilation patterns cause the gas to mix throughout the indoor environment, radon’s heavier nature remains the primary reason for higher concentrations being found in basements and crawl spaces.
Entry Pathways and Concentration Zones
The gas enters a home from the soil beneath the structure, primarily driven by pressure differences between the indoor air and the surrounding ground. This pressure differential is often caused by the “stack effect,” where warmer indoor air rises and escapes through the upper levels of a building. This escaping air creates a negative pressure, or suction, in the lower levels of the house, which effectively draws soil gas, including radon, into the foundation.
The soil gas is pulled in through any opening that connects the house interior to the earth. Common entry points include:
- Cracks in the concrete foundation slab or walls.
- Construction joints.
- Openings around utility pipes.
- Floor drains.
- Sump pits and crawl spaces, which offer direct access to the soil underneath the home.
Because radon is denser than air and is pulled in from the ground, it accumulates most readily in the lowest occupied level of a structure. This typically means basements, slab-on-grade first floors, and crawl spaces are the concentration zones where the highest levels are found. While it can migrate to upper floors due to the stack effect and household air circulation, concentrations are generally highest at the foundation level.
Testing and Mitigation Strategies
The fact that radon accumulates in the lowest areas dictates the proper testing location and mitigation strategy. Testing is the only way to determine a home’s radon level. The test device must be placed in the lowest lived-in level of the house. There are two main types of tests: short-term tests, which measure levels for 2 to 90 days, and long-term tests, which measure for more than 90 days, providing a more accurate year-round average.
The unit of measurement for radon concentration is picocuries per liter (pCi/L) of air. The action level set by the Environmental Protection Agency (EPA) is 4.0 pCi/L. Remediation is recommended if the average level is at or above this threshold. If a short-term test shows an elevated result, a follow-up test, preferably a long-term one, is recommended to confirm the reading before proceeding with mitigation.
The most effective method for remediation in homes with a concrete slab foundation is Active Soil Depressurization (ASD). This system works by reversing the pressure relationship that draws radon into the home. A specialized fan is installed to continuously draw the soil gas from beneath the foundation through a vent pipe and safely exhaust it above the roofline. Sealing large openings in the foundation is a secondary measure that enhances the effectiveness of the ASD system.