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What is Vapor Intrusion?

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Vapor intrusion occurs when chemicals in the shallowest groundwater (the water table) evaporate and move through the soil as a gas. These vapors can potentially make their way indoors, affecting air quality in buildings above the ground.

Volatile organic compounds (VOCs) like trichloroethene (TCE) and vinyl chloride in groundwater can release small amounts of vapor into the soil. The vapors rise toward the ground surface through gaps between soil particles. Often there is no noticeable odor because the concentrations are so low.

Under certain conditions, soil vapors that reach building foundations can pass from soil into interior building spaces. They build up in indoor air or other enclosed areas, such as crawl spaces.
Whether the vapors enter the building – and if they do, how much they affect indoor air quality – depends on several factors. Factors include:

  • How contaminated the groundwater is at the water table.
  • Soil conditions above the water table.
  • The type of foundation a building has – slab, crawl space, or basement.
  • The building’s ventilation.
Vapor intrusion is investigated by focusing on buildings in an area near contaminated groundwater. Investigators can determine what effects, if any, vapor intrusion is having on indoor air quality by inspecting buildings and collecting indoor air samples. Some buildings are more susceptible to vapor intrusion due to their foundation types and construction.