Radon is a naturally occurring, invisible, and odorless radioactive gas that poses a public health concern. This gas is produced from the natural decay chain of uranium, which is present in nearly all soil and rock formations. Once formed, radon gas can seep up through the ground and enter buildings through cracks in foundations, construction joints, or openings around utility penetrations. Because it is trapped and accumulates in enclosed spaces, particularly homes, indoor radon concentration becomes a major factor in determining a person’s exposure.
Defining Long-Term Exposure Duration
The concept of “long-term exposure” to radon is defined by the accumulation of risk over many years, not by a specific number of days or weeks. Health organizations assess the danger based on cumulative exposure, which is the product of the radon concentration level and the total duration spent in that environment.
The danger from radon is chronic, not acute, meaning there is no immediate illness caused by a brief exposure to high levels. The concern is the consistent, low-dose radiation exposure that occurs repeatedly over decades. Therefore, long-term exposure refers to the average annual concentration a person is exposed to over a substantial portion of their life. This duration-based assessment is necessary because indoor radon levels fluctuate significantly day-to-day and season-to-season, making a short-term reading an unreliable indicator of lifetime risk.
The Cumulative Health Effects of Duration
Long-term exposure is harmful because of the way radon and its decay products interact with lung tissue. Once inhaled, the radon gas itself is largely exhaled, but its solid, short-lived decay products attach to airborne dust particles. These radioactive particles are then inhaled and become lodged in the airways of the lungs.
The lodged particles decay further, emitting bursts of alpha radiation, a highly energetic form of ionizing radiation. This repeated alpha particle bombardment directly damages the deoxyribonucleic acid (DNA) of the cells lining the respiratory tract. Over prolonged periods, this cumulative DNA damage increases the likelihood of uncontrolled cell growth, which can lead to lung cancer. Radon is recognized as the leading cause of lung cancer in non-smokers and is the second leading cause overall.
Measuring Chronic Radon Levels
Accurately determining chronic exposure requires long-term testing, which is designed to measure the average concentration over an extended period. This methodology accounts for the natural fluctuations in radon levels caused by changes in weather, barometric pressure, and ventilation patterns. Short-term tests, which typically last between 2 and 90 days, are primarily used for initial screening or quick real estate transactions.
To establish a reliable estimate of annual average exposure, long-term testing devices are employed for a minimum of 90 days, and sometimes for an entire year. The most common long-term device is the alpha track detector, which consists of specialized plastic housed in a container. When radon decay products emit alpha particles, they strike the plastic, leaving microscopic tracks that are later counted in a laboratory to determine the average concentration. Other devices, like electret ion chamber tests, also provide a long-term average by measuring the discharge of an electrically charged disk over time.
Action Levels and Reducing Ongoing Exposure
Once chronic exposure levels have been accurately measured, they can be compared against established regulatory thresholds known as action levels. In the United States, the level established by the Environmental Protection Agency (EPA) is 4 picocuries per liter (pCi/L) of air, the point at which mitigation is strongly recommended. However, health organizations like the World Health Organization (WHO) recommend a lower reference level for national action, equivalent to approximately 2.7 pCi/L.
Mitigation systems are designed to reduce the long-term average concentration and thus lower the cumulative health risk. The most common and effective technique is Sub-Slab Depressurization (SSD). This system uses a vent pipe and an electric fan to create a negative pressure field beneath the foundation. This continuously draws radon gas from the soil before it can enter the home and safely vents it outside above the roofline, reducing the ongoing exposure to occupants.