How does
radon enter a home?
-Information from the US Geological Survey
Radon entry
into buildings
Radon moving through soil pore spaces and rock fractures
near the surface of the earth usually escapes into the atmosphere.
Where a house is present, however, soil air often flows toward its
foundation for three reasons: (1) differences in air pressure
between the soil and the house, (2) the presence of openings in the
house's foundation, and (3) increases in permeability around the
basement (if one is present).
- Heated air rising inside the home (stack
effect).
- Wind blowing past a home (downwind draft
effect).
- Air used by fireplaces, wood stoves, and
furnaces (vacuum effect).
- Air vented to the outside by clothes dryers
and exhaust fans in bathrooms, kitchens, or attics (vacuum
effect).
In constructing a house with a basement, a hole is dug, footings are
set, and coarse gravel is usually laid down as a base for the
basement slab. Then, once the basement walls have been built, the
gap between the basement walls and the ground outside is filled with
material that often is more permeable than the original ground. This
filled gap is called a disturbed zone.
Radon moves into the disturbed zone and the gravel bed underneath
from the surrounding soil. The backfill material in the disturbed
zone is commonly rocks and soil from the foundation site, which also
generate and release radon. The amount of radon in the disturbed
zone and gravel bed depends on the amount of uranium present in the
rock at the site, the type and permeability of soil surrounding the
disturbed zone and underneath the gravel bed, and the soil's
moisture content.
The air pressure in the ground around most houses is often greater
than the air pressure inside the house. Thus, air tends to move from
the disturbed zone and gravel bed into the house through openings in
the house's foundation. All house foundations have openings such as
cracks, utility entries, seams between foundation materials, and
uncovered soil in crawl spaces and basements.
Most houses draw less than one percent of their indoor air from the
soil; the remainder comes from outdoor air, which is generally quite
low in radon. Houses with low indoor air pressures, poorly sealed
foundations, and several entry points for soil air, however, may
draw as much as 20 percent of their indoor air from the soil. Even
if the soil air has only moderate levels of radon, levels inside the
house may be very high.
Because radon is a gas, it has much greater mobility than uranium
and radium, which are fixed in the solid matter in rocks and soils.
Radon can more easily leave the rocks and soils by escaping into
fractures and openings in rocks and into the pore spaces between
grains of soil.
The ease and efficiency with which radon moves in the pore space or
fracture effects how much radon enters a house. If radon is able to
move easily in the pore space, then it can travel a great distance
before it decays, and it is more likely to collect in high
concentrations inside a building.
The method and speed of radon's movement through soils is controlled
by the amount of water present in the pore space (the soil moisture
content), the percentage of pore space in the soil (the porosity),
and the "interconnectedness" of the pore spaces that determines the
soil's ability to transmit water and air (called soil permeability).
Radon moves more rapidly through permeable soils, such as coarse
sand and gravel, than through impermeable soils, such as clays.
Fractures in any soil or rock allow radon to move more quickly.
Radon in water moves slower than radon in air. The distance that
radon moves before most of it decays is less than 1 inch in
water-saturated rocks or soils, but it can be more than 6 feet, and
sometimes tens of feet, through dry rocks or soils. Because water
also tends to flow much more slowly through soil pores and rock
fractures than does air, radon travels shorter distances in wet
soils than in dry soils before it decays.
For these reasons, homes in areas with drier, highly permeable soils
and bedrock, such as hill slopes, mouths and bottoms of canyons,
coarse glacial deposits, and fractured or cavernous bedrock, may
have high levels of indoor radon. Even if the radon content of the
air in the soil or fracture is in the "normal" range (200-2,000 pCi/L),
the permeability of these areas permits radon-bearing air to move
greater distances before it decays, and thus contributes to high
indoor radon.
Major Radon Entry
Routes
- Cracks in concrete slabs.
- Spaces behind brick veneer walls that rest on uncapped
hollow-block foundations.
- Pores and cracks in concrete blocks.
- Floor-wall joints.
- Exposed soil, as in a sump or crawl space.
- Weeping (drain) tile, if drained to an open sump.
- Mortar joints.
- Loose fitting pipe penetrations.
- Open tops of block walls.
- Building materials, such as brick, concrete, rock.
- Well water.