Radon is a colorless, odorless, and tasteless gas that occurs naturally as a product of uranium decay in soil and rock. It is also radioactive, making it an environmental health concern when it accumulates indoors. Radon is significantly heavier than air. This density difference is why the gas builds up to hazardous concentrations in certain parts of a home.
Radon’s Physical Properties
The reason radon is heavier than air is due to its atomic mass. Radon-222, the most stable and common isotope, has an atomic mass of approximately 222 atomic mass units (AMU). Dry air, primarily composed of nitrogen and oxygen, has an average molecular weight of only about 29 AMU.
This substantial difference means that radon is roughly 7.5 to 8 times heavier than the surrounding air. Under conditions with minimal airflow, this density difference causes the gas to settle and pool in the lowest accessible areas, such as basements and crawl spaces. While air currents and temperature gradients prevent a perfect separation, the heaviness of radon dictates its tendency to remain concentrated at lower elevations.
Mechanisms of Indoor Accumulation
Radon gas originates from the natural radioactive breakdown of uranium found in the earth’s crust. As uranium decays, it produces radium, which then decays into radon gas. This gas moves freely through pores in the soil until it finds a path into a structure.
Structural entry mechanisms include cracks in concrete slabs, gaps around utility pipes, floor-wall joints, and sumps. Once inside, the “stack effect” often exacerbates the problem. Warmer air inside a home rises and escapes through upper openings, creating a slight negative pressure at the foundation level.
This lower pressure actively draws soil gases, including radon, into the home through any opening. Because the gas is denser than air, it remains trapped and concentrated at the lowest level of the home, especially in unventilated basements.
Health Impact and Necessary Mitigation
The health risk posed by radon is not from the gas itself, but from its decay products, known as “progeny.” Radon-222 has a short half-life of 3.8 days and quickly decays into tiny, solid, radioactive particles like polonium-218 and polonium-214. When inhaled, these particles attach to dust or aerosols and lodge in the tissue lining the lungs.
Once deposited, the progeny emit alpha radiation, a highly energetic form of ionizing radiation. This radiation directly damages the DNA in lung cells, which can lead to mutations and uncontrolled cell growth. Long-term exposure makes radon the second leading cause of lung cancer overall and the leading cause among non-smokers.
Because radon is invisible and odorless, the only way to determine the risk level in a home is through testing. Simple, inexpensive testing kits are available for monitoring.
If test results show elevated levels, mitigation is necessary, typically involving sub-slab depressurization. This system uses a fan and sealed piping to draw the radon gas from beneath the foundation and vent it above the roofline before it can enter the living space.