A Citizen’s Guide to 

Vapor Intrusion Mitigation

What Is Vapor Intrusion Mitigation?

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 or workplace, 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. 

How Does It Work?

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. 

Vapors Rise 

Through Soil

Contaminated

Groundwater

Vapor intrusion into a home.
United States Office of Solid Waste and EPA 542-F-12-021

Environmental Protection Emergency Response September 2012

Agency (5102G) www.epa.gov/superfund/sites

www.cluin.org

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.

For More Information

For more information about this 

and other technologies in the 

Citizen’s Guide Series, visit:

www.cluin.org/remediation

www.cluin.org/products/

citguide

www.cluin.org/vi

www.epa.gov/oswer/ 

vaporintrusion/

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.

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?

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.