Radon (Rn) is a naturally occurring, colorless, odorless, and tasteless noble gas. It is radioactive and forms as a decay product in the decay chain of uranium and thorium found in soil and rock. The physical property of density dictates how radon moves and accumulates indoors. Comparing the density of radon gas to the air we breathe provides the scientific basis for understanding the health risk it poses.
The Measured Density of Radon Gas
The density of pure radon gas at standard temperature and pressure (STP) is approximately \(9.73 \text{ kg/m}^3\). This value positions radon as the densest noble gas and one of the heaviest gases known. In comparison, the density of normal dry air at STP is significantly lower, typically around \(1.29 \text{ kg/m}^3\). This means a volume of radon gas is roughly seven to eight times heavier than an equal volume of ambient air. The substantial disparity in mass governs how the gas behaves once it enters an indoor environment.
Environmental Factors That Influence Density
The density of any gas, including radon, is directly influenced by changes in its environment. According to the Ideal Gas Law, external conditions like temperature and pressure cause fluctuations in density. Density decreases as temperature increases, meaning warmer gas is slightly lighter and tends to rise more easily. Conversely, an increase in surrounding pressure causes the gas molecules to be packed into a smaller volume, thereby increasing density. While these environmental factors mean the exact density of radon gas in a home is constantly changing, the ratio to air remains consistent.
How Density Affects Radon Accumulation Indoors
The high density of radon gas causes it to exhibit a strong tendency to settle due to gravity. This gravitational pooling means that radon concentrations are consistently highest in the lowest accessible areas of a building. The gas enters a home primarily through the foundation, moving from the soil into the structure due to a difference in air pressure. Once inside, its weight causes it to stay close to the floor, accumulating in basements, crawl spaces, and ground-level rooms. This natural settling behavior is the reason why testing for the gas is focused on the lowest occupied level of a building.
Addressing the Health Risk and Mitigation
The accumulation of this dense gas poses a significant health risk because radon is radioactive and decays into solid, radioactive particles called progeny. When inhaled, these decay products can become lodged in the lung tissue, where they continue to release energy that damages the cells. Long-term exposure to high concentrations of radon is recognized as the second leading cause of lung cancer, accounting for an estimated 21,000 deaths annually. Professional testing is the first step in addressing this threat to determine the concentration level within the home. If testing reveals elevated levels, a process known as mitigation is necessary to reduce the risk.
Mitigation Techniques
Mitigation typically involves sealing the entry points in the foundation and utilizing sub-slab depressurization. This system actively draws the gas from beneath the structure and vents it safely outside. Because the gas is so much denser than air, the venting pipe must terminate above the roofline to ensure proper dispersion into the atmosphere and prevent the gas from re-entering the building.