VISL Frequently Asked Questions

This page presents many questions asked by site users . Please search this page for answers to your questions prior to contacting technical support staff. Researching the questions and answers posted here will greatly reduce the time it takes for you to solve many problems that arise from calculating and using this VISL site.

1. What are VISLs? How are they used?
2. How does this calculator relate to the previously provided VISL spreadsheet tool?
3. How often do you update the VISL calculator?
4. How does the VISL calculator relate to the RSL calculator?
5. How can I adjust groundwater temperatures?
6. How can I change attenuation factors?
7. Why can't I calculate VISLs for bulk soil samples?
8. How can I access the "forward calculator" function?
9. Why won't the calculator report a screening level for my chemical of interest?
10. How are acute exposures addressed?
11. What is the recommended practice for significant figures in reporting risk estimates and screening levels?

1. What are VISLs? How are they used?

   VISLs are screening level concentrations for groundwater, soil gas (target sub-slab and near-source), and indoor air. VISLs assist Agency staff with making a vapor intrusion determination based on limited, initial data.

2. How does this calculator relate to the previously provided VISL spreadsheet tool?

   The web calculator is an expansion of the previously provided VISL Spreadsheet for Modeling Subsurface Vapor Intrusion capabilities. The  web calculator is linked to the Regional Screening Level databases of toxicity and chemical parameters to provide continuity between the two tools for determining air screening levels and risks. If all input parameters are identical, the spreadsheet and the web calculator should give the same results.

3. How often do you update the VISL calculator?

   The VISL calculator will be updated following every Regional Screening Level update. The RSLs are typically updated around Memorial Day and Thanksgiving.

4. How does the VISL calculator relate to the RSL calculator?

   The calculator is linked to the Regional Screening Level databases of toxicity and chemical parameters to provide continuity between the two tools for determining air screening levels and risks. The inhalation screening level equations are identical.

5. How can I adjust groundwater temperatures?

   The main page of the calculator has a cell where the temperature can be changed from the default of 25 °C.

6. How can I change attenuation factors?

   To change the attenuation factors, the calculator must be run in site-specific mode. On the site-specific page, the last section has editable cells with the default factors presented.

7. Why can't I calculate VISLs for bulk soil samples?

   Consistent with the 2015 Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway from Subsurface Vapor Sources to Indoor Air (268 pp, 3 MB) "bulk soil (as opposed to soil gas) sampling and analysis is not currently recommended for estimating the potential for vapor intrusion to pose unacceptable human health risk in indoor air, because of the potential for vapor loss due to volatilization during soil sampling, preservation, and chemical analysis. In addition, there are uncertainties associated with soil partitioning calculations."

8. How can I access the "forward calculator" function?

   In addition to calculating screening levels, the calculator allows the user to input data collected from their site. The data are used to model the indoor air concentration, calculate a chronic daily intake, and present both cancer risk and hazard quotients. On the calculator main page, select "Yes" under the section titled "Predict indoor air concentrations, and risk, from measured media concentrations?". A choice will be presented for the user to select between air, soil gas, and groundwater data. "Site-specific" mode is automatically activated and after selecting the analytes, the user will be prompted to enter her or his site data.

9. Why won't the calculator report a screening level for my chemical of interest?

   VISLs are only calculated for chemicals that meet the volatility criteria (Henry's Law Constant (atm m³/mol) > 10^-5 or Vapor Pressure (mmHg) > 1) and have inhalation toxicity data.

10. How are acute exposures addressed?

    The VISL calculator considers and incorporates noncancer toxicity values for chronic exposures. It is recommended that a regional risk assessor be consulted when in any medium less than chronic exposure scenarios are considered.

11. What is the recommended practice for significant figures in reporting risk estimates and screening levels?

    EPA recommends reporting and communicating risk estimates for vapor intrusion (i.e., lifetime cancer risk and non-cancer hazard quotient or hazard index) with one significant digit, when generated by the Calculator, consistent with general recommendations in Risk Assessment Guidance for Superfund, Volume I, Part A and other EPA risk assessment guidance documents. EPA’s rationale includes the general reasons cited previously by EPA (e.g., the precision of non-cancer toxicity values is “no better than an order of magnitude”) and a recognition that relying upon a generic attenuation factor represents a blunt tool for predicting vapor intrusion exposures and risks.

    Likewise, EPA recommends that the screening levels derived using the VISL Calculator be reported and implemented with one significant digit. The automated calculator displays more than one digit solely to make it possible for others to independently verify the calculations.