The Fundamentals of Vapor Intrusion – EPA’s VISL Calculator

October 2021 Vapor Intrusion News

The Fundamentals of Vapor Intrusion – EPA’s VISL Calculator

By: Henry Stahl, CPG

In a previous issue of Focus on the Environment, we discussed the decline in chemical concentration (attenuation) as vapor migrates from subsurface to indoor air.  The ratio of indoor air to subsurface concentration is the attenuation factor.  EPA combines default attenuation factors with Regional Screening Levels (RSLs) for indoor air to generate SLs for soil gas and groundwater.  EPA’s Vapor Intrusion Screening Level (VISL) Calculator is an essential tool for vapor intrusion (VI) work.

The earliest version of the VISL Calculator was based on November 2011 RLSs and was generally a great improvement over SL tables in EPA’s Draft 2002 Vapor Intrusion Guidance.  Toxicity levels used to generate SLs in the 2002 guidance were long outdated, whereas the VISL Calculator is updated semiannually, following updates to EPA’s RSLs. Additionally, SLs in the 2002 guidance were tied to residential settings and overestimated VI risk in commercial industrial (CI) settings where exposure times are shorter.  Furthermore, the 2002 groundwater SLs were based on a temperature of 25 degrees centigrade, which works in Miami, Florida, but exaggerated VI risk in Ohio and other locations. 

The latest version of the VISL Calculator is in the form of an online calculator. The VISL Calculator requires user input to generate relevant screening levels for either a list of individually selected chemicals or all available chemicals (this option limits some calculator features). Additional inputs are chosen by the user on the VISL Calculator page prior to generating the output.

The four main user inputs on the VISL Calculator form are:  

  • Hazard Quotient for Non-Carcinogens [default = 0.1],
  • Target Risk for Carcinogens [default = 10-6],
  • Exposure Scenario [default = Residential], and
  • Average Groundwater Temperature [default = 25 Centigrade],

The hazard quotient and target risk vary by region and program, and the appropriate settings are specified in the VI guidance for some states, including Ohio EPA’s 2021VI Guidance.  The exposure scenario differentiates between residential and commercial exposure assumptions. The Groundwater Temperature setting affects only groundwater SLs, but the effect can be significant.  Changing the temperature from 25° C to 11°C, as per Ohio guidance, increases the groundwater SL for benzene from 16 to 31 micrograms/liter (μg/l).  Groundwater temperature is more or less constant seasonally and normally equals the average annual outdoor air temperature.  

Once you have retrieved the VISL Calculator results, you can choose to view the results online or export the output as a PDF or Excel spreadsheet.  The spreadsheet’s first sheet, “Air Inputs”, describes the variables used in the calculations to generate SLs.  Additional documentation and explanation of the sources of chemical properties and toxicity data used in the VISL Calculator can be found in the EPA’s VISL User’s Guide.  

The heart of the VISL Calculator is on the spreadsheet’s second sheet, “Air VISL” (or second page of the PDF). If using the exported Excel spreadsheet, first click the “Enable Editing” button at the top of the page. Now, a user can select the “Filter” option in Excel, allowing many of the outputs to be filtered by touching the small gray box at the top of each column.   For example, someone working at a petroleum hydrocarbon site might wish to display only benzene, toluene, ethylbenzene, and xylenes (BTEX).  Some outputs can be filtered mathematically, such as presenting only compounds with Target Indoor Air concentrations below 5 micrograms/cubic meter (μg/m3).  

To the right of the column of Target Indoor Air Concentrations is a column labeled “Toxicity Basis”.  The calculator automatically selects the lower target level—Cancer versus Non-Cancer—to determine SLs, and the Toxicity Basis column indicates which was lower.  Other output columns include Target Soil Gas Concentrations, Target Groundwater Concentrations, and Lower Explosive Limits (LELs).  The VISL’s default attenuation factor for near-source/sub-slab soil gas and groundwater are 0.03 and 0.001, respectively.  Accordingly, the near-source/sub-slab soil gas target levels are 33 times higher than indoor-air target levels, and groundwater target levels are 1,000 times higher than indoor targets (after allowing for Henry’s Law partitioning from water to air).  Previous iterations of the calculator used the highly conservative 0.1 soil gas attenuation factor, but it is now consistent with the 0.03 factor recommended in EPA’s 2015 OSWER VI Technical Guidance.

If using the site-specific risk prediction mode, you will enter media specific concentrations for each compound in the online VISL Calculator form.  This form is designed for entering near-source/sub-slab soil gas, groundwater, or indoor air concentrations, respectively, for various chemicals to determine risk to building occupants.  After specifying the Exposure Scenario (residential or commercial), Target Risk for Carcinogens, and Target Hazard Quotient for Non-Carcinogens, select “Yes” under the “Predict indoor air concentrations, and risk, from measured media concentrations?” section of the form and select the media type. Once you have completed this part of the form and selected “Retrieve”, you will need to enter the concentration for each compound.  Using the default attenuation factors of 0.03 for near-source/sub-slab soil gas and 0.001 (with the Henry’s Law multiplier) for groundwater, the Calculator estimates the indoor air concentration and subsequent risks for each chemical.  Under the “VISL Risk” sheet or page of the spreadsheet or PDF output, if the calculated Carcinogenic Risk or VI Hazard (Non-Carcinogenic) in indoor air exceeds specified risk levels, the compound is highlighted yellow.  

The remaining sheet/page in the VISL Calculator output is “Chemical Properties”. This sheet/page is an invaluable source of information for molecular weights, solubility, and other chemical properties.

Having a single source for chemical properties in an easy-to-use document and the availability of indoor air target levels based on regularly updated toxicity data makes this tool very helpful.  The VISL Calculator will undoubtedly continue to be an essential tool for VI assessments.