Radon is a naturally occurring, colorless, odorless, and tasteless radioactive gas produced by the decay of uranium in soil and rock. This gas constantly seeps up from the ground and accumulates in enclosed spaces like homes and buildings, posing a significant health risk. When inhaled, the radioactive particles can damage lung tissue, making radon the second leading cause of lung cancer after smoking. Since the gas is undetectable by human senses, measurement is the only reliable method to determine the exposure risk.
Short-Term and Long-Term Testing Strategies
Measuring indoor radon concentration involves choosing between two primary testing durations. Short-term testing is conducted over two to seven days, providing a rapid snapshot of the concentration. This quick assessment is preferred for time-sensitive situations, such as real estate transactions, where a fast determination of elevated levels is necessary.
Radon levels naturally fluctuate significantly based on weather changes, seasonal variations, and the operation of heating or cooling systems. Long-term testing, which extends for a minimum of 90 days and often up to a full year, is generally recommended for establishing a reliable annual average. This extended duration smooths out temporary spikes and dips, yielding a more accurate representation of the building’s true long-term exposure level. Long-term test results are the most appropriate basis for determining the need for and effectiveness of permanent radon mitigation systems.
Passive Measurement Devices
The most common and cost-effective methods for homeowners rely on passive devices that require later analysis in a laboratory. Activated charcoal canisters are a popular short-term method, utilizing charcoal granules to adsorb radon gas from the air over a two-to-four-day period. Because charcoal continuously absorbs and desorbs the gas, the final result is weighted toward the concentration present in the last 12 to 24 hours of exposure. Charcoal is also sensitive to humidity, which can interfere with the adsorption process, though some models use a diffusion barrier to minimize this effect.
The alpha track detector is primarily used for long-term measurements spanning 90 days to one year. This device contains a small, specialized plastic chip, such as CR-39, sealed within a container. As radon gas and its decay products enter and emit alpha particles, these particles strike the chip, creating microscopic damage tracks. After the test, the chip is sent to a lab where it is chemically etched and the tracks are counted using high-power microscopy to calculate the time-weighted average radon concentration.
Active and Continuous Monitoring Systems
For professional testing, especially during real estate transactions, certified professionals use active devices known as Continuous Radon Monitors (CRMs). These electronic instruments operate by drawing air into an ionization chamber or scintillation cell, counting the number of alpha particles emitted by decaying radon over set time intervals. Unlike passive devices that yield a single average, CRMs record and log radon concentration readings hourly, providing a detailed data profile of fluctuations throughout the test period.
CRMs are used for short-term, high-stakes testing because of their advanced features designed to detect interference. Many units include built-in motion sensors, power-interruption monitors, and sensors that record ambient temperature and barometric pressure. These features help ensure the required “closed-house conditions” are maintained and provide tamper-proof data, which is necessary for validating the test results to all involved parties. The immediate on-site data retrieval from these devices also reduces the time required to complete the testing process.
Interpreting Radon Measurement Results
Radon concentration is reported in picocuries per liter of air (pCi/L), the standard unit used in the United States to describe the rate of radioactive decay. One picocurie represents 2.2 radioactive disintegrations per minute within a liter of air. Internationally, the unit Becquerels per cubic meter (Bq/m³) is used, where 1 pCi/L is approximately equal to 37 Bq/m³.
The U.S. Environmental Protection Agency (EPA) recommends that homeowners take action to mitigate radon levels if the measurement result is 4.0 pCi/L or higher. This level is an action level, above which the agency advises steps to reduce the concentration, rather than a safety threshold. While mitigation is recommended at or above 4.0 pCi/L, the EPA also encourages homeowners to consider action for levels between 2.0 pCi/L and 4.0 pCi/L. There is no known safe level of radon, and the goal of any mitigation system is to reduce the indoor concentration as far below the action level as achievable.