What is Vapor Intrusion?

Vapor intrusion occurs when there is a migration of vapor-forming chemicals from any subsurface source-soil or groundwater-into indoor air in an overlying structure. These chemicals can include Volatile Organic Compounds (VOCs) like benzene, organic solvents, and toluene. These VOCs are or have been used in many industrial products and processes and have been inadvertently released to the environment as a result.

Vapor intrusion is the process by which these chemicals can enter a structure where exposure to occupants creates potential health concerns.

How Does Vapor Intrusion Occur?

Imagine a house built on a potentially contaminated site. While all may look normal above ground, underground there may be soil or groundwater plumes contaminated with VOCs or other chemicals. These vapors can travel up through the soil and water table and eventually enter through cracks in the home’s foundation or openings in a utility line. Atmospheric conditions and building ventilation may also affect vapor intrusion into a structure.

Is Vapor Intrusion Harmful?

The primary concern with vapor intrusion is whether the chemicals could pose an unacceptable risk to occupants of a structure. Vapor forming chemicals can accumulate in indoor air spaces and present health hazards unknown to the building occupants. Health effects ranging from asthma to cancer can result from exposure to different indoor air pollutants. In rare instances vapor intrusion can also pose an explosion risk when flammable vapors (e.g., gasoline or methane) accumulate in indoor air and are exposed to an ignition source.

Vapor intrusion is of particular concern during redevelopment projects for properties with changing land use. Properties transitioning from commercial or industrial use to residential use require additional attention as residential standards for vapor intrusion are more stringent. Changes in building type, which affect a building’s ventilation rate, will also change the degree to which vapors accumulate in structures and the associated health risks to occupants of those buildings.

How Can Vapor Intrusion be Monitored or Mitigated? 

Once a vapor intrusion concern has been identified, ongoing monitoring and mitigation is required. Monitoring includes the collection of indoor air samples over time as a direct measure of the inhalation exposure experienced by building occupants. Vapor mitigation systems (VMS) are used to reduce or eliminate vapor intrusion into a building. VMS systems can range from simple modifications to a building’s heating ventilation and cooling (HVAC) system, to installation of vapor barriers or venting systems beneath a building. While existing buildings can be retrofitted with below-ground systems to intercept or cut off vapor flow, it is far more cost-effective to install these systems as a component of new building construction.

Vapor intrusion assessments should be performed whenever a threat of vapor intrusion exists for either new or existing buildings. Historically, vapor intrusion was widely performed using fate and transport models to predict indoor air concentrations, rather than measuring them. Regulatory support for this approach has eroded and many sites have been required to reassess vapor intrusion using current regulatory guidance. If a property has received "no further action” status for vapor intrusion, it is important to understand the basis for that determination to evaluate the potential that reassessment will be required.


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