Environmental Engineering Reference
In-Depth Information
important, indoor concentrations are more directly affected by soil/ground
emanation power. The emanation rate is influenced by both radon produc-
tion and soil porosity. Typical radon emanation rates for U.S. soils have been
reported in the range of 1 to 4
becquerels (37 becquerels =
1 pCi)/kg/sec, with radon in soils in the concentration range of 20 to 30 to
>100,000 pCi/L. Most soils in the U.S. have radon concentrations between
200 and 2000 pCi/L. Buildings on sandy/gravelly soils typically have sig-
nificantly higher radon levels than those on clay soils.
Radon entry into buildings occurs through substructures. In basements,
radon-laden soil gas flows through cracks in the floor slab and walls, block
wall cavities, plumbing connections, and sump wells. In slab-on-grade build-
ings, cracks in the slab and penetrations associated with plumbing are the
primary avenues of soil gas flow. In houses constructed on crawlspaces, soil
gas must move through the airspace between the ground and building floor.
Crawlspaces are of two types. The most common is nominally isolated
from living spaces above it. Such crawlspaces are usually provided with
screened vents in perimeter walls. The second crawlspace type is open to
livable areas such as adjoining basements; it is not generally provided
with vents.
Crawlspaces nominally isolated from living spaces and potentially ven-
tilated with outdoor air are not generally decoupled from living spaces. The
degree of decoupling depends on the presence of openings such as vents
and cracks in the foundation wall, opening/closure condition of crawlspace
vents, leakage potential between crawlspace and living spaces, and presence
of leaky forced air heating ducts in the crawlspace (particularly cold air
return ducts). Tracer gas studies have shown that crawlspaces are a signifi-
cant source of air which infiltrates living spaces (on the order of 30 to 92%).
Highest infiltration rates and radon flows are associated with closed vents
and leaky air return ducts.
Assuming equal radon emanation potentials under dwellings with base-
ment, slab-on-grade, and crawlspace substructures, highest indoor radon
concentrations are expected to occur in those constructed on basements. This
is so because basements have the highest surface area in contact with the
ground and are more commonly constructed in regions with porous, well-
drained soils. Basement radon concentrations are typically twice those in
upstairs living spaces. Slab-on-grade dwellings are generally expected to
have higher radon concentrations than those on crawlspaces because they
have a greater surface area in direct contact with the ground. Crawlspaces
without adequate ventilation, or with vents closed to conserve heat under
cold climatic conditions, would have radon levels similar to those of slab-
on-grade houses in which there are cracks sufficient to allow soil gas move-
ment into the building interior.
The typical substructure in nonresidential buildings such as schools,
commercial, and office buildings is slab-on-grade. Radon levels in such
buildings are affected by pressure-driven flows through cracks/penetrations
in the slab. These natural, pressure-driven flows are associated with meteo-
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