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The Department of Toxic Substances Control (DTSC) Human and Ecological Risk Office (HERO) released an update to its Human Health Risk Assessment Note (referred to as HERO Note 3 and available here) this week but did not provide more clarity on which soil vapor intrusion screening levels should be used to evaluate redevelopment of brownfield sites in California. In short, HERO Note 3 now recommends comparing soil vapor concentrations to both DTSC’s current screening levels (based on building specific attenuation factors) and screening levels based on the same generic attenuation factor that was used by the San Francisco Bay Regional Water Quality Control Board (SF Regional Board) to develop its new controversial environmental screening levels (ESLs). HERO Note 3 applies statewide to sites under the DTSC’s jurisdiction.

Screening levels are generally used as a first step in the investigation of potentially contaminated sites. In the context of redevelopment, they are often used by potential buyers to either “screen out” sites that may present additional time and expense to redevelop based on required interaction with governmental agencies, potential investigation and clean-up costs, cost of remediation and mitigation, and the time and cost to obtain contributions from any potentially responsible parties.

The SF Regional Board’s new ESLs dramatically reduced the screening levels for certain Volatile Organic Compounds (VOCs) in soil vapor and groundwater. The SF Regional Board’s soil vapor ESLs are based on a generic attenuation factor, which does not take into account site specific information such as soil type or type/quality of construction. For this reason, the new ESLs are very conservative and may not be representative of actual conditions at a given site. In contrast, DTSC’s current Vapor Intrusion Guidance (available here) recommends using default attenuation factors based on six different building scenarios, which makes them more tailored to actual conditions. DTSC’s new update to HERO Note 3 recommends that in addition to using the default attenuation factors based on building scenarios, screening assessments should also evaluate soil vapor concentrations based on the same generic attenuation factor that was used by the SF Regional Board in developing its new ESLs. The good news is that DTSC’s has not removed its more tailored attenuation factors and these should still be considered in evaluating sites under DTSC’s jurisdiction.

The table below illustrates that the soil vapor screening levels for Trichloroethene (TCE) in micrograms per cubic meter (µg/m3) are significantly lower than the prior screening levels:


New Residential

New Commercial
Prior Current Prior


SF Regional Board


16 3,000




480 and 16 6,000

6,000 and 100

The impact of these reduced ESLs is already playing out on property transactions throughout California – with potential buyers either prolonging the time to close a transaction or walking away from deals because of the uncertainty as to what level of remediation may be necessary. This uncertainty should be resolved when the DTSC, the State Water Board and the SF Regional Board issue their highly anticipated joint statewide guidance for vapor intrusion evaluations, expected this Spring.