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What Is Vapor Intrusion Mitigation? How Does It Work?
Vapor intrusion is the movement of chemical vapors from
contaminated soil and groundwater into nearby buildings.
Vapors primarily enter through openings in the building
foundation or basement walls — such as cracks in the
concrete slab, gaps around utility lines, and sumps. It also
is possible for vapors to pass through concrete, which is
naturally porous. Once inside the home orworkplace, vapors
may be inhaled posing immediate or long-term health risks
for the occupants. In rare cases, the buildup of vapors, such
as those from gasoline, may cause explosive conditions.
Risks will depend on the types of chemical vapors and their
concentrations, how much time people spend in the building,
and the building's ventilation. Vapor concentrations will be
higher indoors when windows and doors remain closed.
Mitigation methods, which lessen the effects of vapor
intrusion, may be needed until contaminated soil or
groundwater is cleaned up. Mitigation methods are available
for both existing buildings and those planned for construction
near the contaminated area.
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Vapor intrusion mitigation methods are classified as either
"passive" or "active." Passive methods prevent the entry
of chemical vapors into the building, while active methods
change the pressure difference between the sub-slab
and the inside of the building to keep vapors out. Passive
mitigation methods tend to be cheaper, while active methods
tend to be more effective. Examples of each include:
Passive Vapor Intrusion Mitigation Methods:
• Sealing openings involves filling in cracks in the floor slab
and gaps around pipes and utility lines found in basement
walls. Concrete can be poured over unfinished dirt floors.
• Installing vapor barriers involves placing sheets of
"geomembrane" or strong plastic beneath a building to
prevent vapor entry. Vapor barriers are best installed
during building construction, but can be installed in existing
buildings that have crawl spaces.
• Passive venting involves installing a venting layer beneath
a building. Wind or the build-up of vapors causes vapors
to move through the venting layer toward the sides of the
building where it is vented outdoors. A venting layer can
be installed prior to building construction as well as within
existing buildings. It is usually used with a vapor barrier.
Active Vapor Intrustion Mitigation Methods:
• Sub-slab depressurization involves connecting a blower
(an electric fan) to a small suction pit dug into the slab in
order to vent vapors outdoors. (Most common method.)
• Building over-pressurization involves adjusting the
building's heating, ventilation, and air-conditioning system
to increase the pressure indoors relative to the sub-slab
area. This method is typically used for office buildings and
other large structures.
How Long Will It Take?
Mitigation will be needed to prevent vapor migration into
buildings as long as vapor intrusion poses a health risk to
occupants. This may be several years, or even decades,
until cleanup of soil and groundwater is complete.
Vapor intrusion into a home.
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Is It Safe?
Vapor intrusion mitigation systems are quite safe to use and will improve the quality
of the indoor air by removing chemical vapors due to vapor intrusion as well as radon
(another health risk) and moisture, which may lead to mold growth. However, mitigation
systems will not reduce vapors from indoor sources of chemicals, such as paints,
plastic items, and hobby supplies.
Until the threat of vapor intrusion is gone, mitigation systems should be inspected
regularly to make sure they are working correctly. For example, floors and walls are
checked to see that no new cracks develop, a geomembrane in a crawlspace is
checked for rips and holes, and electric fans are checked to ensure they are working
correctly. Homeowners should not turn off the electric fans until EPA or state agency
notifies them that it is appropriate to do so. Homeowners should report broken fans
and vent pipes to the lead agency.
How Might It Affect Me?
Example
Mitigation is reducing possible
risks from vapor intrusion at
43 homes near the Nyanza
Superfund site in Massachusetts.
Dye manufacturing from the
1910s to 1978 contaminated
groundwater with trichloroethene
(TCE) and other chemicals.
By the 1980s, a plume of
groundwater contamination
was found to extend beneath a
nearby neighborhood. Sampling
of indoor air, sub-slab air, and
groundwater showed that vapor
intrusion was occurring, and
TCE concentrations posed a risk
to some homeowners. Vapor
intrusion also had the potential to
occur at several other homes.
As a result, EPA installed
depressurization systems in
homes located above the most
contaminated groundwater where
vapor intrusion is most likely to
be a problem. Before installing
the systems in 2007, EPA sealed
cracks in basement walls and
floors, and covered sump pits.
In homes with dirt basements,
they poured a concrete floor
or installed a vapor barrier.
Following installation, each
depressurization system was
tested to ensure that it worked
properly. The systems are
inspected annually to ensure that
they continue to work.
An occupant of a home or office constructed with a vapor mitigation system will not
likely notice it. However, the installation of systems in existing homes typically takes
one or two days, and workers may need to access crawl spaces or indoor living
areas. They may need to pull back carpet or move furniture to find and seal cracks
or to drill holes in the foundation for sub-slab pipes. They typically place these pipes
near the basement walls, in closets, and in low-traffic areas for the convenience of
the homeowner. The vent pipes and fan may be visible on the outside of the house.
However, in some cases, the pipes may be run through a closet to the attic and
vented through the roof. Later, workers may need to visit homes periodically to inspect
mitigation systems to ensure the systems are working properly.
Homeowners may notice the hum of the electric fans, if they have a depressurization
system. These fans use less electricity than an LED television; electric bills will
rise slightly.
Why Use Vapor Intrusion Mitigation?
Vapor intrusion mitigation systems are installed to
reduce health risks in buildings where chemical vapors
from contaminated soil and groundwater may be inhaled
by indoor occupants. They also may be installed as a
precaution where vapor intrusion might occur in the
future. Installing a system during building construction
typically is cheaper, more effective, and less disruptive
than waiting until after construction. Depressurization
systems offer the added benefit of reducing radon,
moisture, and mold inside the building.
Mitigation systems have been installed and operated
at hundreds of homes near Superfund sites and other
contaminated sites across the country.
Typical fan and vent pipe.
NOTE: This fact sheet is intended solely as general information to the public. It is not intended, nor can it be relied upon, to create any rights
enforceable by any party in litigation with the United States, or to endorse the use of products or services provided by specific vendors. The
Agency also reserves the right to change this fact sheet at any time without public notice.
For More Information
For more information on this and
other technologies in the Citizen's
Guide Series, contact:
U.S. EPA
Technology Innovation &
Field Services Division
Technology Assessment Branch
(703)603-9910
Or visit:
www.cluin.org/vi
www.epa.gov/oswer/
vaporintrusion/
United States
Environmental Protection
Agency
Office of Solid Waste and
Emergency Response
(5102G)
EPA 542-F-12-023
September 2012
www.epa.gov/superfund/sites
www.cluin.org
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