There has been some relatively new analysis concerning how the slab attenuation factor (AF) is calculated for vapor intrusion (VI) investigations. Depending on the regulator’s view of this development, it could be a significant factor in assessing vapor. In general, this is favorable news for those who have been looking for a more robust scientific approach to assessment of VI. For details and a more academic view, I will refer you to Groundwater Monitoring and Review, “Estimation of Generic Subslab Attenuation Factors for Vapor Intrusion Investigations” DOI: 10.1111/gwmr.12086 (Brewer, et. al 2014).
Attenuation Factor and Vapor Intrusion
The AF is the multiplier used for estimating indoor air concentrations from measured sub-slab concentrations in the absence of indoor air data. Most regulatory guidance recommends an AF based on a statistical analysis of the Environmental Protection Agency’s (EPA’s) VI database. The recent analyses point out some flaws in the database and the statistical analysis conducted on it to calculate EPA’s default AF.
In both a presentation and in the previously-referenced paper, Roger Brewer, Ph.D., State of Hawaii, Department of Public Health, outlines a completely new method for calculating default slab attenuation factors for VI investigations. The AFs presented are based on climatic data and typical HVAC equipment in different parts of the country. Note that the AFs suggested are significantly lower than those used by EPA and most state regulatory agencies.
Vapor Intrusion and Climate Zones Source: Brewer et al
Statistical “dive” into data
It is also worth noting that recent VI guidance from Oregon takes a deep, statistical dive into the EPA database used to calculate the default AF presented in the EPA VI guidance. The AFs suggested in the Oregon guidance are also significantly lower (actually higher, the way they are presented in the guidance) than the EPA default. The Oregon AFs are consistent with those in the Brewer, et al. paper.
Based on conversations with regulators and industry contacts, the EPA has acknowledged the limitations of their database, but they aren’t ready to revise their recommended AF.
Again, it appears that these developments are based on good science and less controversial than those we have reported on in the last year or so. As we learn more, we will share our observations and insights with you via this forum (blog).
In the meantime, if you have an upcoming VI investigation, you may want to consider proposing AFs consistent with these new approaches. And if you have any questions regarding VI, feel free to send me a note (mschroeder@dragun.com), or call me at 248-932-0228, ext 117.