Radon-222 is an important indoor air pollutant because it forms naturally underground and can accumulate inside buildings. Understanding where it comes from and the pathways it uses to enter homes explains why exposure varies widely by location and building design.

What Radon-222 Is (and Why It Exists Indoors)

Radon is unusual among common air pollutants because it is naturally produced and invisible and odourless, so people cannot detect it without testing.

Because it is a gas, radon can migrate from the ground into indoor air.

Cross-section diagram of a house showing radon/soil gas rising from the ground and entering through lower-level openings, then moving into indoor living spaces. It visually reinforces why radon levels are typically highest near basements and other rooms in contact with soil. Source

In enclosed spaces, it may build up to higher concentrations than outdoors, where it disperses quickly.

Origin: Uranium Decay in Rocks and Soils

Where the uranium is

• Uranium occurs naturally in many types of bedrock and soil (distribution varies regionally).

• As uranium breaks down over time, it produces a series of decay products; radon-222 is one of these products.

Uranium-238 decay-series diagram showing radon-222 produced after radium-226, with half-lives for major steps in the chain. This connects the geology idea (“uranium in rocks and soils”) to the source of indoor radon as a specific decay product. Source

How radon forms and moves underground

• Radon is generated within mineral grains and can escape into soil pore spaces.

• It then travels through:

  • Permeable soils (sands, gravels) more easily than compact clays

  • Cracks and fractures in bedrock

  • Groundwater, where it can dissolve and be transported

Even when the radon source is deep, fractures and porous layers can connect it to the surface.

How Radon-222 Enters Homes

The AP focus is that radon enters through basements, cracks, or well water. These pathways all rely on radon moving from higher concentrations in the ground (or water) into lower concentrations indoors.

1) Entry through basements and lower levels

• Basements are in direct contact with soil and often contain:

  • Unsealed joints (floor-to-wall seams)

  • Penetrations for plumbing, sump pits, or drains

  • Porous materials or unfinished surfaces

• Radon-laden soil gas can seep into these openings and spread into living spaces.

2) Entry through cracks and openings in the building envelope

• Common entry points include:

  • Cracks in concrete slabs and foundation walls

  • Gaps around service lines (pipes, cables)

  • Construction joints and crawlspace openings

• Radon entry is often strongest at the lowest part of a structure because that is where it contacts soil gas.

3) Entry through well water

• Groundwater can contain dissolved radon, especially when drawn from aquifers in uranium-bearing rock.

• Radon can move into indoor air when water is used for:

  • Showering and bathing

  • Washing dishes and laundry

  • Cooking (to a lesser extent)

• This is most relevant for private wells; surface water supplies typically allow more radon to escape before reaching homes.

Why Some Homes Have More Radon Than Others

Soil, rock, and moisture conditions

• Uranium-rich geology increases radon production.

• Permeable soils allow faster soil-gas movement toward foundations.

• Soil moisture can change gas flow paths and pressure relationships, sometimes increasing radon entry.

Pressure differences that pull soil gas indoors

Homes can act like low-pressure zones relative to surrounding soil, drawing in soil gas through openings.

This effect can be stronger in colder seasons when indoor-outdoor temperature differences are larger.

Building design and ventilation

• More foundation contact with soil (basements, crawlspaces) can increase entry opportunities.

• Tighter construction can reduce outdoor air exchange, allowing accumulation if radon enters.

• Ventilation patterns influence how quickly radon is diluted or concentrated indoors.

Essential APES Points to Retain

• Radon-222 is radioactive and originates from uranium decay in rocks and soils.

• It can enter homes through basements, foundation cracks/openings, and well water.

• Entry is driven by pathways (openings) and forces (soil gas movement and pressure differences).