Radon is a naturally occurring, radioactive gas that is colorless, odorless, and tasteless. It originates from the natural breakdown of uranium in soil and rock, seeping into homes and buildings from the ground. While generally harmless outdoors, once trapped inside an enclosed space, it can accumulate to dangerous concentrations. Radon exposure is the second leading cause of lung cancer overall in the United States, and the leading cause among people who have never smoked.
How Radon Exposure Causes Lung Cancer
Radon itself is chemically inert, but its decay process creates the danger. As radon gas breaks down, it forms short-lived, solid radioactive particles known as radon progeny or decay products, which include elements like polonium, bismuth, and lead. These decay products easily attach themselves to dust, smoke, and other aerosol particles floating in the indoor air.
When a person breathes in air contaminated with these particles, the radioactive solids lodge onto the delicate lining and airways of the lungs. Once fixed in the lung tissue, the decay products continue their radioactive breakdown, releasing high-energy alpha particles. Alpha particles are a type of ionizing radiation that deposits intense energy within a very short range, affecting only the cells immediately surrounding the lodged particle.
This localized, high-energy impact causes severe damage to the lung cell DNA, often resulting in complex double-strand breaks. This clustered damage is difficult for cells to fix accurately. Over time, repeated exposure and subsequent DNA damage lead to genetic mutations that promote uncontrolled cell growth, which is the hallmark of cancer.
Statistical Likelihood of Developing Lung Cancer
The Environmental Protection Agency (EPA) estimates that radon is responsible for approximately 21,000 lung cancer deaths each year in the United States. The concentration of radon directly relates to the risk of developing lung cancer over a lifetime of exposure.
The EPA has set an “Action Level” at 4.0 picocuries per liter (pCi/L), which is the concentration where homeowners are strongly advised to take steps to reduce the radon level. For people who have never smoked, exposure to this 4 pCi/L level results in an estimated 7 deaths per 1,000 people over a lifetime. Because any exposure carries some risk, the EPA also recommends considering mitigation when levels are between 2.0 pCi/L and 4.0 pCi/L.
Risk projections show that a significant portion of radon-induced lung cancers occur even below the 4.0 pCi/L action threshold. For instance, a lifetime exposure at 10 pCi/L increases the estimated risk of death for never-smokers to 18 per 1,000 people. While the action level is a benchmark for intervention, the goal of mitigation is to reduce the concentration to the lowest achievable level, preferably below 2.0 pCi/L.
Factors That Increase Individual Risk
The single most significant factor that multiplies the risk of lung cancer from radon exposure is smoking. The combination of smoking and radon creates a synergistic effect much greater than the sum of the individual risks. This compounding danger is why the vast majority—more than 85%—of radon-related lung cancer deaths occur in current or former smokers.
Cigarette smoke damages the protective lining of the lungs and impairs the cilia that help clear inhaled particles. This existing damage makes the lung tissue more vulnerable to the radiation from radon progeny. Smoke particles also act as carriers, transporting the radioactive solids deeper into the lungs, where they can lodge and inflict radiation damage more effectively.
A smoker exposed to the EPA’s action level of 4 pCi/L faces a lifetime lung cancer risk of approximately 62 per 1,000, nearly ten times the risk for a non-smoker at the same concentration. Other risk-modifying elements include the duration of exposure and the age at which the exposure began. The risk is directly tied to the total cumulative dose of radiation received by the lung tissue over a person’s lifetime.
Testing and Reducing Radon Levels
Testing is the only method to determine if a home has an elevated radon concentration. Testing devices are generally placed in the lowest lived-in area of the home, such as a basement or first floor, where radon is most likely to enter and accumulate. Test kits fall into two main categories: short-term and long-term.
Short-term tests are conducted over two to seven days and offer a quick initial screening, often used in real estate transactions. Long-term tests, which measure radon levels for a minimum of 90 days, provide a much more accurate representation of the home’s annual average concentration. Since radon levels can fluctuate daily and seasonally due to weather and ventilation changes, long-term results are considered more reliable for determining the true health risk.
If testing reveals a high radon concentration, the primary method for reduction is the installation of a mitigation system, typically handled by certified professionals. The most common technique is an Active Soil Depressurization (ASD) system. This system uses a vent pipe and a fan to draw radon gas from beneath the foundation and safely exhaust it outside, preventing it from entering the living space.