Radon is a naturally occurring, odorless, colorless, and invisible radioactive gas. It forms from the natural decay of uranium found in nearly all soil and rock, producing radium as an intermediate step. Because it is a gas, radon easily migrates up through the ground and into enclosed structures like homes. This infiltration makes radon a significant environmental health concern, as it is the leading cause of lung cancer among non-smokers. Extreme radon concentration measurements can reach levels far exceeding standard safety recommendations.
Defining Action Levels and Measurement
Radon concentrations are primarily measured using two different units. In the United States, the standard unit is the picocurie per liter of air (pCi/L), which represents the number of radioactive decay events occurring per liter. Globally, the becquerel per cubic meter (Bq/m³) is the more common unit, signifying one radioactive disintegration per second per cubic meter of air. For conversion, 1 pCi/L is roughly equivalent to 37 Bq/m³.
Regulatory bodies establish “action levels” defining the threshold at which mitigation efforts are recommended. The U.S. Environmental Protection Agency (EPA) advises action if indoor radon concentration is 4 pCi/L or higher (approximately 148 Bq/m³). The World Health Organization (WHO) suggests a lower reference level, recommending steps be taken when concentrations exceed 100 Bq/m³, or about 2.7 pCi/L. These intervention points protect occupants from the cumulative risk associated with long-term exposure.
The Highest Documented Radon Levels
The highest indoor radon levels documented surpass regulatory action points. While readings in uranium mines are exponentially higher, the record for a residential structure in the United States is 7,879.3 pCi/L. This measurement was documented in a community-tested building in Dallas County, Texas. This single residential reading is more than 1,900 times the WHO’s recommended action level of 100 Bq/m³.
This extreme concentration translates to approximately 291,534 Bq/m³. Historically, a famous extreme reading occurred in Boyertown, Pennsylvania, in 1984, measuring around 2,700 pCi/L in the home of Stanley Watras. That case brought widespread public attention to the issue of radon in homes. The Texas record exceeds that measurement by nearly three times, underscoring that extreme, localized readings remain a threat.
Geological and Structural Causes of Extreme Readings
Extreme radon levels result from a specific combination of geological and structural factors.
Geological Factors
Extreme concentrations begin with underlying geology, specifically bedrock rich in uranium, such as granite, shale, and phosphate rock. As the uranium in these rocks decays, it produces a high volume of radon gas that collects in the soil.
Gas Movement
The movement of this gas is facilitated by soil permeability and the presence of fractured or porous rock formations, such as karst topography. These fractures and channels act as conduits, allowing radon to flow quickly and directly from the source rock toward the surface and building foundations. Without permeable pathways, the gas would dissipate more slowly.
Structural Factors
The building’s structural design must trap and concentrate the incoming gas. A primary factor is the air pressure differential, often called the “stack effect.” Warm air rising and exiting the upper levels of a home creates a negative pressure at the foundation level. This negative pressure actively draws soil gas, including concentrated radon, into the lowest level through foundation cracks, sump pits, and unsealed utility penetrations, allowing it to accumulate.