Radon is a naturally occurring radioactive gas that forms from the breakdown of uranium, which is present in nearly all soils and rock formations. This invisible, odorless substance can move up through the ground and accumulate inside any building. Understanding how this gas behaves physically is key to protecting indoor air quality, determining where the highest concentrations are likely to be found, and testing for it effectively.
The Physics of Radon Movement
Radon gas is significantly heavier than the average ambient air mixture. The molecular weight of radon is approximately 222 atomic mass units, compared to about 29 atomic mass units for dry air, making radon roughly 7.5 to eight times denser than air.
Because of this high density, radon tends to sink and pool in the lowest accessible areas of a structure when air conditions are still. This gravitational effect is why basements and crawl spaces typically show the highest concentrations. However, its movement is heavily influenced by air currents and temperature gradients.
Indoor air circulation, such as that caused by a forced-air heating and cooling system, can easily distribute radon throughout the home. While its inherent density dictates a tendency to remain low, the gas is readily mixed and moved by convection and ventilation. Changes in atmospheric pressure can also affect the rate at which it is drawn from the soil into the building.
Pathways for Radon Entry into Structures
Radon enters a home primarily through a pressure-driven mechanism called soil suction. The air pressure inside a house is often slightly lower than the pressure in the soil beneath the foundation, which effectively causes the house to act like a vacuum. This pressure differential draws the radon-laden soil gas directly into the structure.
One significant factor creating this suction is the “stack effect,” where warmer indoor air rises and escapes through the upper levels of the home. This rising air must be replaced, and the replacement air is often pulled from the soil through the foundation, bringing radon along with it. Combustion appliances and exhaust fans also contribute to this negative pressure.
The gas enters through any opening where the house meets the soil. Common entry points include cracks in the foundation slab, unsealed construction joints, gaps around utility penetrations like pipes and wires, and openings around sump pump systems. The pores and cracks in concrete blocks also provide pathways for entry. Radon entry is concentrated at the lowest level because this area is closest to the soil source and experiences the greatest pressure differential.
Determining the Correct Testing Location
The physical behavior and entry mechanism of radon gas dictate the correct protocol for testing. Since the gas is heaviest and enters from the ground, the highest concentrations are generally found at the lowest occupied level of the home. Testing should be conducted in the lowest level of the home that is used regularly, such as a finished basement, den, or bedroom.
The testing device should be placed in a central, regularly used room, away from exterior doors, windows, and heating or cooling vents that can cause drafts or airflow changes. It is recommended to place the monitor at least 20 inches above the floor and 12 inches away from any wall. Avoiding testing in high-humidity areas like laundry rooms or bathrooms is also advised, as moisture can interfere with some test devices. Placement should represent the air quality people are breathing, not an unused area like a closet or storage space, ensuring a representative measurement of radon concentration.