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Vapor Intrusion Guidance

Vapor Intrusion Pathways

"Vapor intrusion" refers to the process by which volatile chemicals move from a subsurface source into the indoor air of overlying or adjacent buildings. The subsurface source can either be contaminated groundwater or contaminated soil which releases vapors into the pore spaces in the soil.

vapor intrusion graphicVapors can enter buildings in two different ways. In rare cases, vapor intrusion is the result of groundwater contamination which enters basements and releases volatile chemicals into the indoor air. In most cases, vapor intrusion is caused by contaminated vapors migrating through the soil directly into basements or foundation slabs. Although the Department historically has evaluated soil gas pathways, improvements in analytical techniques and the knowledge gained from remedial sites in New York and other states has increased our understanding of how vapor intrusion occurs.

Historically, we thought that vapor intrusion was only an issue where the source of the contaminants was very shallow and the magnitude of the contamination was very great. We now know that our previous assumptions about the mechanisms that could lead to exposure to vapor intrusion were not complete. The result is that additional work may be required to investigate or remediate sites that are in the operational or monitoring phase, or that have already been closed. Separate ranking systems have been developed to account for the two different sources of contaminated vapors. Because we now recognize the need to take a different sampling approach, when the Department evaluates a site for vapor intrusion, both sources can now be effectively considered.

Contaminated soil vapor is not the only possible source of volatile chemicals in indoor air. Chemicals are part of our everyday life. Volatile chemicals are found in many household products, such as paints, glues, aerosol sprays, new carpeting or furniture, refrigerants and recently dry-cleaned clothing. Volatile chemicals are also emitted by common industrial and commercial activities such as drycleaners (see the "A Citizen's Guide To Drycleaner Cleanup" a publication of the State Coalition for Remediation of Drycleaners - see the link on the right of this page to download that and other publications and resources) . Indoor air may also become affected through the infiltration of outdoor air containing volatile chemicals.

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