In February, staff from the California Department of Toxic Substances Control (DTSC) published an article in a technical journal (Groundwater Monitoring & Remediation) that suggested that vapor attenuation in California is different from what is observed nationwide by the United States Environmental Protection Agency (USEPA). Publication of the data and the findings is an important step in revising the current vapor intrusion guidelines that stalled the development of brownfield properties in the state.

Since 2019, California has implemented USEPA’s recommended environmental screening levels of buildings for vapor intrusion. USEPA’s empirically derived and generic factors are considered by many as very conservative and in many impractical. As a result, property owners have experienced decision-making challenges due to uncertainties when performing soil vapor intrusion studies for indoor air quality evaluation or to support their property transaction due diligence. With no approved mathematical models, it became financially unfeasible for many developers to acquire and improve the impacted properties.

In California, there are over 200,000 listed impacted brownfields sites in the state, such as dry cleaners, manufacturing, and old gas stations, and many are within urban areas, disadvantage communities, and close to public transportation.

Vapor migration in the subsurface, in building foundations, and within buildings is complex and influenced by many natural and human-caused factors.

What is vapor attenuation?

Vapor attenuation refers to the reduction in volatile chemical concentrations that occurs during vapor migration in the subsurface, coupled with the dilution that can occur when the vapors enter a building and mix with indoor air.


It is anticipated that the publication will lead to generating new scientific based guidance and regulations in the state that developers can effectively use for their projects.

In the interim, it is anticipated that the State agencies will be more receptive to presentations of mathematically modeled attenuation factors that are based on theoretical chemical behavior.

William O’Braitis, CEG
Environmental Business Unit Leader
Oneida Engineering Solutions (OES)
[email protected]