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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.
               >   Vapors Rise
                   Through Soil
^  Gr
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.

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?
                                        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

                                                   Or visit:

United States
Environmental Protection
Office of Solid Waste and
Emergency Response
EPA 542-F-12-023
September 2012