Radon is a colorless, odorless, radioactive gas that is naturally present in the environment and seeps into homes through the foundation. It is the second leading cause of lung cancer in the United States, making its presence a serious health concern. Because radon originates from the ground, its concentration is directly influenced by environmental factors, including the weather. Heavy rainfall often creates conditions that temporarily elevate the amount of radon gas infiltrating a structure.
The Origin of Radon Gas
Radon gas originates from the natural radioactive decay of uranium found in trace amounts within the Earth’s soil and rocks. Uranium decays into radium-226, which then decays to form radon-222, the most common isotope. Because radon is a noble gas, it does not chemically bond to the soil matrix. This allows it to move freely through porous spaces in the soil profile. The gas travels upward, either dispersing into the atmosphere or finding a path into a home’s lower level. The amount of radon available depends on the local geology and the concentration of its parent elements.
How Soil Saturation Influences Radon Concentration
Heavy rain significantly influences indoor radon levels by altering the dynamics of gas movement in the surrounding soil. The primary mechanism involves the saturation of the ground around the foundation, creating the “soil sealing” or “capping effect.” As rainwater fills the air pockets and pores in the soil, it effectively blocks the normal path for radon to escape into the outdoor air.
This saturation redirects the flow of the trapped gas. Instead of diffusing upward into the atmosphere, the radon is forced to migrate laterally toward the lower pressure area beneath the house’s sub-slab. A second factor is the increase in hydrostatic pressure caused by the heavy water table. A large volume of water pressing down on the soil physically pushes the concentrated radon gas into the structure.
Rainfall is frequently accompanied by a drop in atmospheric pressure. This change lowers the air pressure inside the house relative to the soil beneath it. This pressure difference causes the house to act like a slight vacuum, actively drawing soil gases, including radon, through foundation cracks, utility penetrations, and sumps. Radon levels can temporarily spike during or immediately following significant precipitation.
Timing Radon Testing for Accuracy
Given the temporary fluctuations caused by weather, homeowners must understand the difference between the two main types of radon measurement devices. Short-term tests typically run for two to seven days, providing a quick snapshot of the radon concentration. Because these short periods are highly susceptible to being skewed by events like heavy rain, they may not accurately reflect the home’s typical exposure level.
Industry standards advise avoiding short-term testing during periods of severe weather to prevent an artificially high reading. Long-term tests, which monitor levels for a minimum of 90 days, are generally recommended for determining a home’s true annual average. These extended periods account for seasonal and weather-related changes, providing a much more reliable indicator of the long-term health risk. If a short-term test shows an elevated result, a long-term follow-up test is the best next step to confirm the average exposure level.