Radon is a naturally occurring radioactive gas produced by the decay of uranium found naturally in soil, rock, and groundwater.
It is invisible, odorless, and tasteless, which means you cannot determine whether a home has elevated radon by sight or smell.
Radon can accumulate inside buildings when gas from the soil enters through openings in the foundation.
Long-term exposure to elevated radon increases the risk of lung cancer. Because individual buildings can have very different radon concentrations, the only way to determine the radon level in a particular home is to test it.
Radon forms naturally as uranium in soil and rock undergoes radioactive decay.
Because small amounts of uranium occur naturally in geological materials, radon can be generated beneath buildings in many different geographic areas.
Once produced in the soil, radon gas can migrate through pore spaces and fractures and enter buildings through foundations.
The amount entering a particular building depends on geology, soil conditions, foundation design, building pressure, ventilation, weather, and other factors.
This is why two neighboring homes can have very different radon concentrations even when they were built at approximately the same time.
Yes. Homes in California can have elevated radon.
California's geology varies significantly, and radon potential differs between regions and even between individual properties.
A radon map can provide useful regional information, but the EPA specifically advises that radon-zone maps should not be used to determine whether an individual home needs testing. The EPA recommends testing homes regardless of geographic zone because elevated radon can occur in properties located outside designated high-potential areas.
The only way to know the radon concentration in a particular California home is to test it.
Radon primarily enters buildings from the soil beneath and around the foundation.
Common entry pathways can include:
Buildings can also develop slightly lower air pressure indoors than the surrounding soil, which can draw soil gas containing radon into the structure.
Because entry pathways and pressure conditions vary between buildings, radon concentrations can differ substantially even between neighboring homes.
Radon testing involves placing an approved measurement device in an appropriate location within the home for a specified period.
Devices can include continuous radon monitors, charcoal devices, electret ion chambers, or long-term alpha-track detectors.
For a typical real-estate or short-term measurement, the device is placed in the lowest level of the home that is suitable for occupancy and operated under appropriate testing conditions.
Short-term testing generally requires closed-building conditions so normal ventilation changes do not artificially influence the measurement.
After the measurement period, the result is reported in picocuries per liter (pCi/L).
A Continuous Radon Monitor, or CRM, is an electronic instrument that measures radon over time rather than providing only a single integrated result.
Depending on the instrument, a CRM may record hourly radon concentrations along with environmental parameters such as temperature, barometric pressure, or movement.
The time-resolved data can be particularly useful during real-estate transactions because the tester can evaluate how radon concentrations changed throughout the measurement period and identify unusual conditions that may have affected the test.
CRMs must be properly placed and operated according to applicable measurement protocols to obtain reliable results.
Short-term radon tests generally measure radon for several days to several weeks and are commonly used when results are needed quickly, such as during a real-estate transaction.
Long-term tests remain in place for more than 90 days and can provide a better estimate of the home's average radon concentration over changing weather and seasonal conditions.
Radon levels naturally fluctuate over time, so a short-term measurement represents conditions during the testing period.
The appropriate testing method depends on whether the goal is a rapid screening measurement, real-estate evaluation, confirmation of an earlier result, or a better estimate of long-term exposure.
pCi/L stands for picocuries per liter of air and is the standard unit commonly used to report radon concentrations in the United States.
A picocurie is a unit of radioactivity. When a radon report shows a result such as 2.0 or 4.0 pCi/L, the number represents the amount of radioactive decay occurring from radon within a liter of air.
The unit may sound technical, but homeowners primarily need to understand how their measured concentration compares with EPA recommendations.
EPA recommends mitigation at 4 pCi/L or higher and recommends considering mitigation when concentrations are between 2 and 4 pCi/L.
The EPA recommends that a home be fixed when the radon concentration is 4.0 pCi/L or higher.
The 4.0 pCi/L value should not be interpreted as a line between "safe" and "unsafe." EPA states that there is no known safe level of radon exposure.
Because radon risk decreases as exposure decreases, EPA also recommends that homeowners consider mitigation when radon concentrations are between 2.0 and 4.0 pCi/L.
Results should be evaluated together with the type and duration of the measurement and whether proper testing conditions were maintained.
A radon concentration between 2 and 4 pCi/L is below the EPA's 4 pCi/L action level, but it does not represent zero risk.
EPA recommends considering radon reduction when concentrations fall within this range because there is no known completely safe level of radon exposure.
Whether a homeowner chooses to mitigate may depend on the measured concentration, test duration, additional testing, occupancy patterns, and personal risk considerations.
A follow-up or longer-term measurement can sometimes provide additional information about the home's average radon concentration before a mitigation decision is made.
Short-term professional radon measurements commonly run for a minimum of approximately 48 hours, depending on the device and testing protocol.
During the measurement, proper closed-building conditions must be maintained so the result reflects standardized testing conditions.
Long-term radon tests remain in place for more than 90 days and provide a measurement that incorporates changing weather and seasonal conditions.
For real-estate transactions, short-term measurements are commonly selected because results are needed within the inspection period.
The appropriate test duration depends on the purpose of the measurement and the type of device being used.
Closed-building conditions are standardized conditions used during short-term radon testing to help produce a representative and reproducible measurement.
Generally, exterior doors and windows should remain closed except for normal entry and exit. Normal heating or cooling systems may typically operate, but equipment that brings substantial amounts of outside air into the building should not be operated unless it is part of normal building operation.
Closed-building conditions generally need to begin before the test and continue throughout the measurement period.
Failure to maintain proper conditions can affect the validity of a short-term radon measurement.
Radon devices are generally placed in the lowest level of the home that is suitable for occupancy, such as a basement, first-floor living area, bedroom, family room, or other regularly occupiable space.
Devices should not normally be placed in bathrooms, kitchens, closets, garages, crawl spaces, or locations with unusual heat, humidity, drafts, or direct sunlight.
Placement should also avoid exterior walls, windows, fans, and other conditions that could interfere with the measurement.
Proper device placement is one of the most important factors in obtaining a reliable radon test result.
Yes. Radon concentrations can change with weather and atmospheric conditions.
Changes in temperature, barometric pressure, rainfall, wind, soil moisture, and heating-system operation can influence how soil gas moves beneath and into a building.
This natural variability is one reason radon concentrations can change from hour to hour, day to day, and season to season.
Short-term testing provides a measurement for a limited period, while long-term testing provides a better estimate of average exposure across changing conditions.
Professional testing protocols are designed to reduce avoidable influences and help ensure the measurement period is appropriate.
A basement does not automatically mean a home will have elevated radon, but below-grade areas have more direct contact with surrounding soil and can provide additional pathways for soil gas to enter.
Cracks, slab joints, sump pits, utility penetrations, foundation walls, and other openings can allow radon-containing soil gas to migrate indoors.
Because of this, radon testing is generally performed on the lowest level of the home that is suitable for occupancy.
Homes built on slabs or crawl spaces can also have elevated radon, so the absence of a basement does not eliminate the need for testing.
Yes. A real-estate transaction is an excellent opportunity to determine the radon concentration in a home before purchase.
Radon cannot be detected by a normal home inspection, and neighboring homes can have very different radon levels.
A professional short-term test can generally be completed within the inspection period and can provide useful information before closing.
EPA guidance specifically addresses radon testing during real-estate transactions and notes that buyers may request a new test when previous testing is outdated, building conditions have changed, or the planned occupancy differs from the area previously tested.
Radon should not necessarily be treated as a one-time measurement for the lifetime of a property.
EPA recommends considering retesting over time, particularly when living patterns change, a lower level becomes occupied, significant renovations are completed, or previous test results are no longer recent. During real-estate transactions, EPA notes that buyers may request new testing when previous results are more than approximately two years old.
Retesting is also appropriate after installation of a radon mitigation system to confirm that the system is reducing concentrations as intended.
Because building and soil conditions can change, periodic testing provides additional confidence about long-term conditions.
Radon mitigation is the process of reducing radon concentrations inside a building.
One of the most common methods is active soil depressurization. A pipe is installed through or beneath the foundation and connected to a continuously operating fan. The system creates negative pressure beneath the building and vents radon-containing soil gas safely outdoors before it can enter the occupied space.
Other approaches may be used depending on the foundation type and building configuration.
After a mitigation system is installed, follow-up radon testing should be performed to verify that indoor radon concentrations have been reduced.
EPA recommends fixing homes at 4 pCi/L or higher and considering mitigation between 2 and 4 pCi/L.