New York State Department of Health Updates Soil Vapor Intrusion Guidance

By Sahana Rao

The N.Y. Department of Health (“NYSDOH”) recently revised its guidance on soil vapor intrusion, tightening guidelines on certain volatile chemicals and loosening guidelines on others. The revisions consist of an updated set of Soil Vapor/Indoor Air Decision Matrices, which prescribe recommended actions based on the concentrations of a specified chemical vapor inside a building and below it.

The updated matrices operate as part of NYSDOH’s 2006 Guidance for Evaluating Soil Vapor Intrusion in the State of New York (“2006 Guidance”) and provide guidance for identifying and addressing current or potential human exposure to subsurface vapors associated with certain volatile chemicals. The updated matrices replace a prior set of matrices included with the 2006 Guidance, as discussed further below.

Generally, “soil vapor intrusion” refers to the process by which volatile chemicals move from an underground source into the air inside of a building. Soil vapor (also known as sub-slab vapor) is the air that exists in the spaces between particles of soil. Due to the difference in pressure above and below the ground, soil vapor can enter a building’s indoor air through small cracks and openings in its foundation. Volatile chemicals, or chemicals that readily evaporate, can contaminate soil vapor and thereby make their way into the indoor air of buildings above, with varying effects on human health depending on the chemical, the concentration present in the air, the length of exposure, and other factors.

The decision matrices formulated by NYSDOH provide recommended actions based on the concentration of certain chemicals in the indoor air in conjunction with the concentration found in sub-slab vapor. These include:

  • “No Further Action” at the lowest range of indoor air and sub-slab vapor concentrations.
  • “Identify Source(s) and Resample or Mitigate” at certain higher concentration ranges (particularly for indoor air); this entails determining the source of chemical contamination and taking reasonable and practical action to reduce the concentration levels.
  • “Monitor” at certain higher concentration ranges; this involves sampling to evaluate whether indoor air or sub-slab vapor concentrations change, and NYSDOH considers this an interim measure prior to remediation of contaminated environmental media, rather than a substitute for remediation in the form of removal, treatment, or containment.
  • “Mitigate” at the highest concentration ranges to minimize current or potential exposure; NYSDOH lists common mitigation methods, including sealing preferential pathways in conjunction with installing a sub-slab depressurization system, or changing the pressurization of a building and monitoring the indoor air and sub-slab vapor concentrations. NYSDOH also considers this a temporary measure prior to remediation of contaminated environmental media.

The table below lists the eight volatile chemicals of concern for which guidelines are provided in the guidance and denotes the impact of the updates to the decision matrices relative to the original 2006 matrices. Where the impact is that the recommended actions in the 2017 update are mainly less intensive for a particular chemical, this means that the recommended action in the 2006 matrix was changed to a less intensive recommended action in the 2017 matrix for the same concentration range—for instance, from “Identify Source(s) and Resample or Mitigate” to “No Further Action,” or “Mitigate” to “Monitor.” The converse is true for chemicals where the recommended action became mainly more intensive. Where the recommended action increased in intensity with indoor air concentration but decreased in intensity for sub-slab vapor concentration ranges, the impact of the 2017 update is characterized as “mixed.”

Volatile Chemical Assigned Matrix (2006) Assigned Matrix (2017) Impact of 2017 Update
Carbon tetrachloride Matrix 1 Matrix A Mainly less intensive
1,1-dichloroethene Matrix 2 Matrix A Mainly more intensive
Cis-1,2-dichloroethene Matrix 2 Matrix A Mainly more intensive
Trichloroethene (TCE) Matrix 1 Matrix A Mainly less intensive
Methylene chloride N/A (new addition) Matrix B N/A
Tetrachloroethene (PCE) Matrix 2 Matrix B Mainly less intensive
1,1,1-trichloroethane Matrix 2 Matrix B Mainly less intensive
Vinyl Chloride Matrix 1 Matrix C Mixed impact


For additional information on the impact of these updates, please contact Christine Leas or Mark Chertok.