Radon is a naturally occurring radioactive gas formed from the breakdown of uranium, which is present in trace amounts in nearly all rock and soil. As a noble gas, radon is chemically inert, colorless, odorless, and tasteless, making it undetectable by human senses. Understanding its physical properties, such as its density, is important for homeowners concerned about indoor air quality and potential health risks.
The Physical Reality of Radon Gas
The direct answer to whether radon is heavier than air is yes, and significantly so. The primary components of air, nitrogen and oxygen, have an average molecular weight of approximately 29 atomic mass units (AMU). In contrast, the most stable isotope of radon, Radon-222, has an atomic mass of about 222 AMU, making it nearly 7.5 times denser than the air around it. This substantial difference in density dictates that in a static environment, radon will settle and accumulate in the lowest areas due to gravity. However, household air currents and ventilation patterns prevent a perfect layering, allowing the gas to move and mix throughout the entire home.
How Radon Enters and Accumulates in Homes
While radon’s density encourages it to settle low, its primary movement into a home is driven by the stack effect. This effect describes how warmer indoor air rises and escapes through upper levels, creating a negative pressure differential at the lower levels. This pressure difference draws air from the soil beneath the foundation into the house, pulling radon gas along with it. Radon enters a home through any opening that connects the living space to the soil below.
Common entry points include:
- Cracks in the concrete foundation slab or walls.
- Floor-to-wall joints.
- Openings around utility penetrations like pipes and wires.
- Sump pumps and crawlspaces.
The negative pressure pulls soil gas inward, trapping it and allowing concentrations to build over time, leading to the highest levels typically found in the lowest occupied levels of the structure.
Health Implications and Exposure Risk
The danger of radon gas comes from what it becomes as it radioactively decays. Radon-222 has a relatively short half-life of 3.8 days and quickly transforms into short-lived, radioactive solid particles known as radon progeny. These decay products include isotopes of polonium, lead, and bismuth.
These solid progeny attach to airborne dust particles, which are then inhaled and lodged in the lining of the lungs. Once deposited, these particles continue to decay and emit alpha radiation. Alpha radiation damages the DNA in the lung cells, leading to mutations that may cause cancer over many years. Radon exposure is recognized as the second leading cause of lung cancer overall, and the leading cause among people who have never smoked. The risk increases with the concentration and duration of exposure.
Testing and Practical Mitigation Strategies
Measuring radon levels is the only way to determine the extent of the risk in any given home. Testing can be done using short-term kits (two to seven days) or long-term devices (90 days or more). Long-term testing offers a more accurate representation of the home’s annual average exposure.
The Environmental Protection Agency (EPA) recommends mitigation if the long-term average concentration is 4 picocuries per liter (pCi/L) or higher. Even at lower concentrations between 2 pCi/L and 4 pCi/L, the EPA suggests considering corrective measures.
The most common and effective technique for reducing high radon concentrations is Active Soil Depressurization (ASD). ASD systems install a vent pipe and a continuously running fan that draws air from beneath the foundation slab or crawlspace membrane. This creates a slight vacuum under the home, reversing the pressure differential that pulls radon inward. The radon-laden air is then safely exhausted through the piping to the outside, typically above the roofline, where it rapidly disperses into the atmosphere. Sealing major entry points like foundation cracks and improving general home ventilation are secondary measures that contribute to lower indoor radon levels.