What is vapor intrusion, and should my property be assessed for it?

(Part 1 of a 3-Part Series)

What is vapor intrusion?

Vapor intrusion (VI) is the migration of vapor-forming chemicals from any subsurface contamination into an overlying enclosed space that can pose a threat to indoor air quality. Vapor-forming chemicals may originate from contaminated soil, groundwater, sewer, or other sources. These sources of contamination may be onsite, near the building’s foundation, or from a migrating offsite source.

How do vapors from soil and groundwater end up inside buildings?

Vapors can enter buildings through cracks in foundations, utility penetrations, or other openings (drains and sumps).  Vapors can dissipate, depending on the number of fresh air exchanges in the building’s heating, ventilating, and air conditioning (HVAC) equipment.  In cold-weather climates, heating may exacerbate VI through “stack effects” — vapors are “pulled up” through cracks as warm air rises through the building.

“How vapor intrusion happens: a complex path” © Minnesota Pollution Control Agency

Is vapor intrusion dangerous?

The chemicals that most commonly enter buildings are volatile organic compounds (VOCs), measured in parts-per-billion-volume or micrograms per cubic meter in air.  VOCs, such as trichloroethylene, benzene, and other petroleum compounds and solvents, can pose a threat to human health, even at low concentrations.

For example, petroleum hydrocarbons seeping from an underground storage tank can release vapor phase hydrocarbons that mix with air and water vapor. If these vapors in soil gas migrate to confined spaces like basements and sewers, they can accumulate and create an immediate threat of explosion or asphyxiation or a long-term exposure threat. Because chemicals can enter a building without worker (or resident) knowledge, exposures can occur over the long term.

Which regulations apply to vapor intrusion?

VI risks are evaluated based on EPA standards, not OSHA standards. EPA’s screening levels are significantly lower than the Permissible Exposure Limits or Threshold Limit Values typically used for comparing monitoring results in occupational settings.  As a result, VI investigations and response actions are typically managed as an environmental corrective action, not as an industrial hygiene issue.

Could your property need to be assessed for vapor intrusion?

If your property has had a response action implemented for VOC contamination, you may have received a request from regulators to assess the potential for VI.  In some states, this process is still underway (i.e., requests are still being issued), while other states are adopting a more reactive approach to what triggers a request for VI evaluation.  Properties with VOC-related contamination may need to be assessed for VI-related liabilities (a) to maintain regulatory compliance and (b) to assess potential future environmental actions and associated costs.

Even if regulators have not yet addressed VI-related issues, you should consider evaluation VI for any kind of property transaction (sale, refinancing), since it may pose significant risk and liability for the parties involved.  ASTM E1527-13 Standard Practice for Conducting Phase I Environmental Site Assessments specifies that both on- and offsite sources of VI need to be considered.  In short, either an onsite VOC source (or offsite VOC source that has migrated onsite) that presents the potential for migration of hazardous substances or petroleum products in vapor form may meet the ASTM definition of a Recognized Environmental Condition and/or suggest the need for further VI consideration/evaluation.

How will a vapor intrusion assessment benefit you?

VI factors can affect redevelopment of a parcel including soil and site management, groundwater use/classification, building design, and even building placement.  Identification of potential risks early in the process will allow the appropriate level of VI investigation/mitigation to be integrated into development.

This is the first of a three part series on VI.  Part 2 will discuss VI sampling and both qualitative and quantitative data evaluation.  Part 3 will discuss VI mitigation options.