Radon

Formation and Origin of Radon Gas

Radon is a product of the natural radioactive decay chain of Uranium-238 (U−238), which is universally present in trace amounts within the Earth's crust, including in rock, soil, and water.

1. The Decay Chain

The formation of radon is a multi-step process:

• Source Material: The decay chain begins with long-lived U−238.

• Precursor: U−238 decays into a series of other radioactive elements, including Radium-226 (Ra−226).

• Radon Generation: Radon is a direct decay product of Ra−226. Ra−226 has a half-life of 1,600 years. When an atom of Ra−226 decays, it emits an alpha particle and forms an atom of Radon-222 (Rn−222).

• Gas Phase: Since radon is a gas, it can escape the solid mineral matrix of the soil or rock and migrate through porous materials. Its half-life is relatively short, approximately 3.8 days, meaning it must be generated continuously in the immediate vicinity of a structure to accumulate to hazardous levels.

2. Pathways into Buildings

Radon is drawn into a building from the soil largely due to the stack effect and pressure differential. Buildings are almost always under negative pressure relative to the soil directly beneath the foundation, particularly during the heating season. This negative pressure acts like a vacuum, pulling soil gas (including radon) into the building.

The most common entry points into the conditioned space are:

• Cracks in Concrete Slabs and Foundation Walls: The primary pathway, created by concrete shrinkage, settling, or structural stress.

• Pores in Concrete: Even intact concrete is somewhat permeable to gas flow.

• Openings around Utilities: Unsealed gaps around plumbing, electrical conduits, and sewer line penetrations.

• Sumps and Drains: Unsealed floor drains or sump pits offer direct, low-resistance pathways for soil gas entry.

Effects on the Human Body

Radon is classified as a Group 1 human carcinogen by the World Health Organization (WHO) and is the second leading cause of lung cancer overall (and the leading cause among non-smokers).

1. The Real Hazard: Radon Progeny

Radon itself is inert, and most of the inhaled gas is quickly exhaled before it can decay. The primary hazard comes from its solid, short-lived decay products, known as radon progeny (Polonium, Bismuth, and Lead isotopes).

• Mechanism of Damage: Once inside the building, the radon gas decays into these solid, electrically charged particles. These particles attach readily to airborne aerosols, dust, and particulate matter. When this contaminated particulate matter is inhaled, it lodges deep within the lung tissue.

• Alpha Particle Emission: The lodged radon progeny continue to decay, emitting highly energetic alpha particles. Because alpha particles are massive and carry a strong electrical charge, they cannot penetrate the skin, but they are highly ionizing over a very short range. When emitted inside the lungs, they directly damage the DNA of the sensitive cells lining the bronchial tubes and lungs.

• Cancer Risk: This cellular DNA damage is the first step in the formation of cancerous cells, leading to an elevated risk of lung cancer over years of chronic exposure.

2. Measurement and Mitigation

The health risk is directly proportional to the concentration and duration of exposure. Radon is measured in picoCuries per liter (pCi/L) or Becquerels per cubic meter (Bq/m3).

• The U.S. Environmental Protection Agency (EPA) recommends mitigation when levels are 4.0 pCi/L or higher.

• Mitigation involves Sub-Slab Depressurization (SSD), a building science technique that uses a fan to draw soil gas from beneath the slab and safely vent it above the roofline before it can enter the living space, creating a zone of lower pressure beneath the foundation to reverse the flow direction.