Last month, the California Department of Toxic Substances Control (“DTSC”) issued its long-awaited “Vapor Intrusion Update” (“Advisory”) which is intended to guide the selection of attenuation factors (“AFs”) at contaminated sites overseen by DTSC.

For folks frantically Googling the term “attenuation factors,” I’ll save you some time: attenuation factors are merely estimates of the decimal fraction of vapor phase chemicals that pass through a barrier and enter the indoor space of a structure. So, for example, an attenuation factor of 0.03 means that an estimated 3% of vapor phase chemicals pass through a barrier and enter an indoor structure.

DTSC’s one-page Advisory is broken up into three sections: “Site Screening,” “Delineation,” and “Mitigation and Remediation.” I’ll underline the good stuff:

• With respect to “Site Screening,” the Advisory specifies that an AF of 0.03 can be used for the initial evaluation of the potential for vapors to enter a building, especially to support a decision to move directly to an indoor-air evaluation. Critically, however, DTSC states that “[a]n alternative AF may also be proposed using multiple lines of evidence.” 

• With respect to “Delineation,” the Advisory emphasizes that “Soil vapor plume delineation should be conducted using site-specific screening criteria that incorporates either a 0.001 AF or a site-specific AF that has been developed using “multiple lines of evidence.”

• With respect to “Mitigation and Remediation,” the Advisory focuses on the fact that “multiple lines of evidence should guide the development of an AF” and clarifies that the use of a 0.03 AF is not a default requirement for developing cleanup goals or determining whether mitigation or remediation is required.

The upshot of all of this is that despite previous fears, DTSC is allowing some flexibility in developing site-specific attenuation factors in connection with vapor intrusion investigation and remediation. This is a positive development overall, as default attenuation factors have been proven to be un-tethered to actual attenuation levels and are often overly conservative. However, the Advisory makes it clear that a robust (aka, costly) dataset based on “multiple lines of evidence” must substantiate a site-specific AF. We query whether the cost to develop a site-specific AF will be so prohibitive that folks will continue to rely on default AFs. TBD.