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EPA Updates Vapor Intrusion Screening Level Calculator


In February 2018, US EPA updated its Vapor Intrusion Screening Level (VISL) Calculator in a number of ways. The VISL Calculator now consists of an online calculator, instead of a downloadable spreadsheet, and it is in most respects, more powerful and easier to use.

To use the VISL Calculator, one enters the chemicals of interest, and other inputs including Hazard Quotient for non-carcinogens (typically 1), Target Cancer Risk (10-6, 10-5, 10-4, or other), Exposure Scenario (residential vs commercial), and chemical names. The user then hits the Retrieve button and a new window opens up with the results. One can opt to:

  • View the results online
  • Output the files to an Excel worksheet
  • Output a series of tables in PDF format

The revised VISL calculator represents a considerable improvement over the earlier VISL Calculator, and selecting the chemicals of concern is easier in most respects. On the plus side, unlike the earlier spreadsheet-based versions, the chemical names include a number of synonyms and Chemical Abstracts Services (CAS) numbers. One can select benzene, for example, by typing in “benzene”, “benzol”, “phenyl hydride”, “71432”, etc. The earlier VISL calculator provided only a few synonyms, and while you could select chemicals via name or CAS numbers, you couldn’t do both. If you selected a number of chemicals via name and then selected one via CAS number, all of the earlier selections were lost. Now you can mix and match. Unfortunately, the new VISL Calculator displays only 7 or 8 chemicals at a time, and scrolling down through the window to pick a chemical from the very long list is time consuming. In general, it’s easier to select a chemical by typing in part of the chemical’s name, but the entry “benzene”, for example, brings up more than 130 chemicals, and they’re not in alphabetical order. It might be easier if the VISL Calculator opened up a larger window in which more chemicals could be seen at once.

As discussed earlier, the new calculator is not a downloadable spreadsheet, but it can output spreadsheets showing the results. But because the spreadsheets contain no hidden information or calculations, it’s impossible to add to the chemicals of concern or to modify risk settings, groundwater temperature, etc., after leaving the website. If you wish to add chemicals or change settings, you’ll have to return to the online calculator and start over.

A potentially useful feature of the revised VISL Calculator is the ability to modify the soil-gas or groundwater attenuation factors. As discussed in the July 2014 issue of Focus on the Environment, attenuation factors represent the ratio between indoor air and subsurface vapor concentrations. The default soil-gas attenuation factor is 0.03, but the measured attenuation factor, according to EPA’s Vapor Intrusion Database report, (2012) is 0.003. The risk is intentionally exaggerated by a factor of 10 to allow for a safety margin. This makes sense to a point, but the higher ceilings, greater air-exchange rates, and other conditions characteristic of commercial/industrial settings result in even lower actual attenuation factors. Cox-Colvin routinely sees actual soil-gas attenuation factors in commercial/industrial settings of 0.0001 or lower, where indoor air is okay, even though soil gas concentrations greatly exceed soil-gas VISLs. So the ability to match attenuation factors to site-specific conditions will enable the use of more realistic – but still protective – VISLs. However, convincing regulators to accept less conservative attenuation factors for commercial/industrial settings might be difficult, and you or your consultant will need a good understanding of attenuation factors to justify their use.

Another advantage of the revised VISL Calculator is that it’s now tied directly to EPA’s Regional Screening Level (RSL) database, and VISLs are updated automatically with RSLs. The older spreadsheet-based calculator was updated periodically, but screening levels lagged the latest RSLs. Additionally, the list of substances in the new VISL Calculator includes more constituents, including Total Petroleum Hydrocarbons (TPHs), which are useful at petroleum sites. And finally, the new VISL Calculator outputs metadata, including inhalation Unit Risk Factors, Inhalation Reference Concentrations, and other information that risk assessors may find useful.

In all, the revised VISL Calculator is a step forward, but you might be wise to have CAS numbers handy when selecting chemicals, and don’t leave the website until you’re done selecting.

Author: Mort Schmidt is a Senior Scientist with Cox-Colvin & Associates, Inc. He received his BS and MS degrees in Geology and Mineralogy from The Ohio State University, and has been a Cox Colvin & Associates employee since 1997. His areas of expertise include vapor intrusion and contaminant investigation and analysis, and he currently serves as Cox Colvin’s Practice Leader – Vapor Intrusion Services. Mort is a Certified Professional Geologist with AIPG and is a registered Geologist in Indiana.