United States
Environmental Protection
Agency
VAPOR INTRUSION SCREENING LEVEL (VISL) CALCULATOR
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF SOLID WASTE AND EMERGENCY RESPONSE
OFFICE OF SUPERFUND REMEDIATION AND TECHNOLOGY INNOVATION
WASHINGTON, D.C. 20460
May 2014
USER'S GUIDE
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VISL Calculator
User's Guide
DISCLAIMER
This document describes the calculation of recommended, but not mandatory, screening levels for use
in evaluating the vapor intrusion pathway at Comprehensive Environmental Response, Compensation
and Liability Act (CERCLA) and Resource Conservation and Recovery Act (RCRA) sites. These screening
levels are calculated using the recommended approaches in existing guidance. The information provided
in this document does not impose any requirements or obligations on the U.S. Environmental Protection
Agency (EPA) or on the owner/operators of sites that may be contaminated with volatile and toxic
compounds. Alternative approaches for evaluating the vapor intrusion pathway may be found to be
more appropriate at specific sites (for example, where site circumstances do not match the underlying
assumptions used in calculating the screening levels).
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VISL Calculator
User's Guide
VAPOR INTRUSION SCREENING LEVEL (VISL) CALCULATOR
USER'S GUIDE
Overview
The Vapor Intrusion Screening Level (VISL) Calculator is a spreadsheet tool that (1) lists chemicals
considered to be volatile and known to pose a potential cancer risk or noncancer hazard through the
inhalation pathway; (2) provides generally recommended screening-level concentrations for
groundwater, soil gas (exterior to buildings and sub-slab) and indoor air for default target risk levels and
exposure scenarios; and (3) allows calculation of site-specific screening levels based on user-defined
target risk levels and exposure scenarios. The VISL Calculator can assist Comprehensive Environmental
Response, Compensation and Liability Act (CERCLA) remedial project managers (RPMs) and on-scene
coordinators (OSCs), as well as Resource Conservation and Recovery Act (RCRA) project managers in
determining whether the vapor intrusion (VI) pathway has the potential to pose an unacceptable level of
risk to human health by: (1) identifying whether chemicals that can pose a risk through VI are present;
(2) determining if those chemicals are present at explosive levels; and (3) comparing subsurface or
indoor data against screening levels provided in the Calculator. The screening level concentrations in
the Calculator are not intended to be used as cleanup levels, nor are they intended to supersede existing
criteria of the lead regulatory authority.
The VISLs are calculated using the recommended approaches in existing guidance and are based on
current understanding of the vapor intrusion pathway. Target indoor air concentrations are calculated
according to the guidance provided in Risk Assessment Guidance for Superfund (RAGS) F (EPA 2009),
which does not support the route-to-route extrapolations that were used in the now outdated screening
tables in the EPA's November 2002 draft vapor intrusion guidance (EPA 2002). The screening levels for
groundwater and soil gas (either sub-slab gas or soil gas collected exterior to buildings) are calculated
from the target indoor air concentrations using empirically-based conservative "generic" attenuation
factors that reflect generally reasonable worst-case conditions as described in the EPA's draft vapor
intrusion guidance (EPA 2002). The default, generic VISLs are based on default exposure parameters and
factors that represent Reasonable Maximum Exposure (RME) conditions for long-term/chronic
exposures. Site-specific criteria that can be input to the calculator include exposure scenario (either
residential or commercial), target risk for carcinogens, target hazard quotient for noncarcinogens and
average in situ groundwater temperature (stabilized temperature measured during well purging prior to
groundwater sampling). The VISL Calculator incorporates the latest toxicity values in the Regional
Screening Levels (RSL) tables (EPA, "Regional Screening Levels") and will be updated as new versions of
the RSL tables are released1.
Applicability of the VISL Calculator
The purpose of the VISL calculator is to provide a tool to assist RPMs, OSCs, risk assessors and others
involved in decision-making concerning CERCLA hazardous waste sites and to determine whether levels
of contamination found at the site may warrant further investigation or site cleanup, or whether no
further investigation or action may be required.
1 EPA Regional Screening Level (RSL) tables are updated semi-annually with the latest version available online at
http://www.epa.gov/reg3hwmd/risk/human/rb-concentration table/index.htm. The Vapor Intrusion Screening
Level (VISL) Calculator will be revised accordingly to reflect the most recent information available.
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Users within and outside the CERCLA program should use the calculator results at their own discretion,
and they should take care to understand the assumptions incorporated in these results and to apply the
screening levels appropriately.
The screening levels presented in the calculator are chemical-specific concentrations for individual
contaminants in air that may warrant further investigation or site cleanup. The screening levels
generated from the calculator may be site-specific concentrations for individual chemicals in air. It
should be emphasized that screening levels are not cleanup standards. It is not recommend that the
VISLs be used as cleanup levels for Superfund Sites until the recommendations in EPA's Supplemental
Guidance to Risk Assessment Guidance for Superfund, Volume I, Part A, Community Involvement in
Superfund Risk Assessments (EPA 1999) have been addressed. Screening levels should not be used as
cleanup levels for a CERCLA site until the other remedy selections identified in the relevant portions of
the National Contingency Plan (NCP), 40 CFR Part 300, have been evaluated and considered.
The subsurface target concentrations in the VISL Calculator are based on the generic conceptual model
for vapor intrusion described in EPA's draft vapor intrusion guidance. This conceptual model assumes a
groundwater or vadose zone source of volatile vapors that diffuse upwards through unsaturated soils
towards the surface and into buildings. In this model, the soil in the vadose zone is considered to be
relatively homogeneous and isotropic, though horizontal layers of soil types can be accommodated. The
receptors are assumed to be occupants in buildings with poured concrete foundations (for example,
basement or slab on grade foundations or crawlspaces with a liner or other vapor barrier). The
underlying assumption for this generic model is that site-specific subsurface characteristics will tend to
reduce or attenuate vapor concentrations as vapors migrate upward from the source and that site-
specific building characteristics will tend to further dilute the vapors as they mix with the air in the
building. The generic attenuation factors were developed based on analysis of VI data in EPA's VI
database (EPA 2002, 2008).
In general, it is recommended that the user consider whether the assumptions underlying the generic
conceptual model are applicable at each site, and use professional judgment to make whatever
adjustments (including not considering the model at all) are appropriate. Specific factors that may result
in unattenuated or enhanced transport of vapors towards a receptor, and consequently are likely to
render the VISL screening target subsurface concentrations inappropriate, include:
• Very shallow groundwater sources (for example, depths to water less than 5 ft below
foundation level);
• Shallow soil contamination vapor sources (for example, sampled at levels within a few feet of
the base of the foundation)
• Buildings with significant openings to the subsurface (for example, sumps, unlined crawlspaces,
earthen floors) or significant preferential pathways, either naturally-occurring or anthropogenic
(not including typical utility perforations present in most buildings).
Description and Use of VISL Calculator
The VISL Calculator contains nine worksheets:
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User's Guide
Navigation Guide: Provides information regarding the Calculator's contents and structure, including a
data field dictionary (see "Navigation Guide" tab in the VISL Calculator) that describes each data field
and any formulas used to calculate the contents of the data field.
VISL: Main worksheet where the user can enter the exposure scenario and risk information and select
chemicals to calculate the screening-level values for groundwater, soil gas (sub-slab and exterior soil
gas) and indoor air.
SG IA calc: Worksheet where the user can enter the exposure scenario and site soil gas (sub-slab and
exterior soil gas) concentrations to calculate the indoor air concentrations and the resulting risk values
for indoor air.
GW IA calc: Worksheet where the user can enter the exposure scenario and site groundwater
concentrations to calculate the indoor air concentrations and the resulting risk values for indoor air.
IA risk calc: Worksheet where the user can enter the exposure scenario and site indoor air
concentrations to calculate resulting risk values for indoor air.
ChemProps: Supporting worksheet that contains a compilation of physical and chemical properties of
each chemical, which are used in the calculations for the VISL worksheet.
Version Notes: Worksheet to track the changes made in each version of the VISL calculator workbook.
Tox Summary: Supporting worksheet that provides a list of contaminants, toxicity values, maximum
contaminant levels (MCLs) and the lesser (more protective) of the cancer and noncancer screening levels
(SLs) for resident soil, industrial soil, resident air, industrial air and tap water from EPA's RSL website
(EPA, "Regional Screening Levels").
Parameters Summary: Supporting worksheet that contains physical and chemical properties from EPA's
RSL website (EPA, "Regional Screening Levels"). The properties in this worksheet are linked to the
ChemProps worksheet.
The list of chemicals included in the VISL calculator includes all chemicals denoted as volatile in the RSL
tables by the "voc" field, and for which either an inhalation unit risk (IUR) or a reference concentration
(RfC) is listed in the RSL tables.
The VISL Calculator allows the user to enter the exposure scenario (residential or commercial), target
risk for carcinogens, target hazard quotient for noncarcinogens and average in situ groundwater
temperature, if appropriate. Default values are provided for all these entries. For site-specific exposure
scenarios, the worksheet can be unprotected to change the exposure parameters.
The Calculator uses chemical property and toxicity information to determine whether a chemical, if
present in soil, is sufficiently volatile and toxic to pose an inhalation risk through vapor intrusion and
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User's Guide
whether a chemical, if present in groundwater, is sufficiently volatile and toxic to pose an inhalation risk
through vapor intrusion at the selected cancer risk or hazard quotient levels.
For a soil source, a chemical is considered sufficiently volatile and toxic to pose an inhalation risk
through vapor intrusion if the vapor pressure (expressed in terms of vapor concentration using the ideal
gas law) of the pure component is greater than the target indoor air concentration. The soil gas
attenuation factor is not considered in this comparison, although it is considered in the subsequent
calculation of the target soil gas concentration.
For a groundwater source, a chemical is considered sufficiently volatile and toxic to pose an inhalation
risk through vapor intrusion if the vapor concentration (calculated using the chemical's Henry's Law
Constant at the groundwater temperature) corresponding to the chemical's solubility limit in water, is
greater than the target indoor air concentration. The groundwater attenuation factor is not considered
in this comparison, although it is considered in the subsequent calculation of the target groundwater
concentration.
The soil gas attenuation factor used in the calculations is 0.1, which is the generic attenuation factor
used for sub-slab gas in the 2002 draft vapor intrusion guidance. This attenuation factor is used for all
soil gas data, as additional information collected after development of the 2002 draft vapor intrusion
guidance (EPA 2002) suggests that external soil gas samples need to be evaluated as conservatively as
sub-slab gas samples (EPA 2010).
The groundwater attenuation factor used in the calculations is 0.001, which is the generic attenuation
factor used for groundwater concentrations (expressed in terms of vapor concentration in equilibrium
with groundwater concentration) in the 2002 draft vapor intrusion guidance (EPA 2002). Additional
information gathered since release of the 2002 draft vapor intrusion guidance (EPA 2002) suggests the
2002 generic groundwater attenuation factor remains valid (EPA 2010).
Data Sources
The chemical property and toxicity information used in the VISL Calculator is obtained from the EPA's
RSL website at http://www.epa.gov/reg3hwmd/risk/human/rb-concentration table/index.htm (EPA,
"Regional Screening Levels").
Target Indoor Air Concentration to Satisfy Both the Prescribed Cancer Risk Level and the Target
Hazard Index (Column F).
The target breathing zone indoor air concentrations in the VISL Calculator are risk-based screening levels
for ambient air. The indoor air concentrations for noncarcinogens and carcinogens are calculated
following an approach consistent with the EPA's RAGS F (EPA 2009). The toxicity values used as the basis
for the calculations are from the EPA's RSLs, which also include the source of the toxicity data. Separate
carcinogenic and noncarcinogenic target concentrations are calculated for each compound when both
unit risks and reference concentrations are available. For carcinogens, target indoor air concentrations
are based on an adult residential exposure scenario and assume exposure of an individual for 350 days
per year over a period of 70 years. For noncarcinogens, target indoor air concentrations are based on
the corresponding reference concentration.
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User's Guide
For carcinogens, the equation for Cia c, the target indoor air concentration based on cancer toxicity
(Column V), is:
TCR X ATC X
/365 days\ /24 hours\
\ year J \ day J
C¦ =
ia'c EF xED xET x IUR
where ATC, EF, ED and ET are the default exposure parameters for residential or commercial exposure as
appropriate, and:
TCR = Target cancer risk (for example, 1.0 x 10 s)
IUR = Inhalation unit risk (ng/m3)"1
For carcinogens with mutagenic mode-of-action (MMOA), the equation is slightly modified in
accordance with the RSL User's Guide (EPA, "Regional Screening Levels").
G65 days'] f2Ahours\
C¦ =
wrr r
TCR x ATC x (— / ) x
year
) v day )
'ia'c EF x EDmmoa xETx IUR
where EDMM0A is calculated with the equation:
EDmmoa = (ED0_2 x AF0_2) + (ED2.e x AF2.e) + (ED6_16 x AF6.16) + (ED16.30 x AF16.3Q)
and
EDx_y = Exposure duration for the age cohort from age x to y
AFx_y = Age-dependent adjustment factor, which is 10 for ages 0 to 2, 3 for ages 2 to 6
and 6 to 16 and 1 for ages 16 to 30
For trichloroethylene, which has mutagenic and non-mutagenic components of the IUR, two Cia c values
are calculated using the mutagenic and non-mutagenic components of the IUR. The reciprocal of the
sum of the reciprocals of those Cia c values is used to calculate the final Cia c value, in accordance with the
EPA RSL guidance (EPA, "Regional Screening Levels").
For vinyl chloride, a slightly modified version of the equation for Cia c is used (EPA, "Regional Screening
Levels"):
C¦ =
uia,c
TCR
EF x ED x ET x IUR
/365 days\ {24 hours')
c X \ year ) X \ day
For non-carcinogens, the equation for Cia nc, the target indoor air concentration based on non-cancer
toxicity (Column W), is:
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THQ x RfC x ATnc x
1000 jig
mg
ia,nc
EF X ED X ET
where ATnc, EF, ED and ET are the default exposure parameters for residential or commercial exposure
as appropriate, and:
The more stringent (minimum) of the cancer- and noncancer-based contaminant concentrations is
chosen as the target indoor air concentration (Ctarget ia) that satisfies both the prescribed cancer risk level
and the target hazard quotient.
If Qarget,ia exceeds the maximum possible pure chemical vapor concentration, the designation NVT (not
sufficiently volatile and/or toxic to pose inhalation risk) is entered in the target indoor air concentration
column of the table (Column F).
The vapor concentrations in the VISL Calculator are given in units of micrograms per cubic meter
(Hg/m3). Although the VISL Calculator uses these units for all vapor concentrations, some users may
have results in parts per billion by volume (ppbv). The conversion from ppbv to ng/m3 is:
Target Sub-Slab and Exterior Soil Gas Concentration Corresponding to Target Indoor Air Concentration
(Column H)
The target soil gas concentration corresponding to a chemical's target indoor air concentration at the
selected target cancer risk or hazard quotient is calculated by dividing the indoor air concentration by
the generic attenuation factor of 0.1.
THQ
RfC
Target hazard quotient (for example, 1.0)
Reference concentration (mg/m3)
where:
R
T
MW
Universal gas constant (0.082057 L-atm/mole-degrees Kelvin)
Absolute temperature (298.15 K)
Molecular weight (grams [g]/mole)
where:
Target soil gas concentration [ng/m3]
Attenuation factor (ratio of indoor air concentration to sub-slab or soil gas
concentration; default value is 0.1)
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If Qoii-gas exceeds the maximum possible pure chemical vapor concentration at 25^C, but Ctarget ia does not
exceed the maximum possible pure chemical vapor concentration, then NVT is entered in the table
under target sub-slab or soil gas concentration (Column H).
Target Groundwater Concentration Corresponding to Target Indoor Air Concentration (Column I)
The target groundwater concentration corresponding to a chemical's target indoor air concentration is
calculated by dividing the target indoor air concentration by an attenuation factor of 0.001 and then
converting the vapor concentration to an equivalent groundwater concentration, assuming equilibrium
between the aqueous and vapor phases at the water table. The equilibrium partitioning is assumed to
obey Henry's Law so that:
r
„ _ ^target,ia
Lnu; —
'9W rrrr /1000L>
HLC x AFm„ x
m3
where:
Cgw = Target groundwater concentration (micrograms per liter [|j.g/L])
3\
Qarget,ia = Target indoor air concentration (ng/m
AFgw = Attenuation factor (ratio of indoor air concentration to groundwater
concentration; the default value is 0.001)
HLC = Dimensionless Henry's Law Constant at the specified groundwater temperature
[(milligrams per liter (mg/L) - vapor)/(mg/L - H20)]
The following equation can be used to calculate the Henry's Law Constant at a groundwater
temperature of 25 degrees Celsius:
„ __ /1000 L\
Hc2S x
TTf nc — V III '
R X (298.15 K)
where:
H'25 = Dimensionless Henry's Law Constant at 25 degrees Celsius
Hc25 = Henry's Law Constant at 25 degrees Celsius (atm-m3 per mole)
R = Universal gas constant (0.082057 L-atm/mol-degrees Kelvin)
For groundwater temperatures other than 25 degrees Celsius, the following equation is used to
calculate Henry's Law Constant:
H'Tgw = Hc2S x exp
DH.
v,b
RC
x
I1
1
(1
Tboil\
Tcrit'.
x
lgw
298.15 K
where:
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n
Hc25
DHv,b
RC
Dimensionless Henry's Law Constant at the groundwater temperature
Henry's Law Constant at 25 degrees Celsius (atm-m3 per mole)
Enthalpy of vaporization at the normal boiling point (cal/mol)
Universal gas constant (1.9872 cal/mol-K)
Groundwater temperature (degrees Kelvin)
Critical temperature (degrees Kelvin)
Normal boiling point (degrees Kelvin)
If (Tboii/Tcrit < 0.57), n = 0.3
If (Tboii/Tcnt > 0.71), n = 0.41
If (0.57 < Tboil/TCnt < 0.71), n = (0.74 x Tboil/Tcrit - 0.116)
For some chemicals, the parameters required to calculate H'Tgw are not available. In these cases, the
Henry's Law Constant at 25 degrees Celsius is used instead. The temperature assumed for each chemical
is listed as the temperature for groundwater vapor concentration (Column M).
If Ctarget ia is determined to be NVT, NVT is entered as the target groundwater concentration (Column I). If
Cg„ exceeds the aqueous solubility of the pure chemical, but Ctarget ia does not exceed the maximum
possible pure chemical vapor concentration, then NVT is entered in the table under this column
(Column I).
For informational purposes, the calculated groundwater target concentration is compared with the MCL
for the compound in Column J.
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References
EPA. (1999). "Risk Assessment Guidance for Superfund, Volume I: Human Health Evaluation Manual
Supplement to Part A: Community Involvement in Superfund Risk Assessments." EPA 540-R-98-042.
March, http://www.epa.gov/oswer/riskassessment/ragsa/pdf/ci ra.pdf
EPA. (2002). "OSWER Draft Guidance for Evaluating the Vapor Intrusion to Indoor Air Pathway from
Groundwater and Soils (Subsurface Vapor Intrusion Guidance)." Office of Solid Waste and Emergency
Response. Washington, DC. EPA 530-D-02-004. November.
www.epa.gov/osw/hazard/correctiveaction/eis/vapor/complete.pdf
EPA. (2008). "Draft US EPA's Vapor Intrusion Database: Preliminary Evaluation of Attenuation Factors."
Office of Solid Waste and Emergency Response. March 4.
iavi.rti.org/OtherDocuments.cfm?PagelD=documentDetails&AttachlD=369
EPA. (2009). "Risk Assessment Guidance for Superfund, Volume I: Human Health Evaluation Manual
(Part F, Supplemental Guidance for Inhalation Risk Assessment)." EPA 540-R-070-002. January.
www.epa.gov/oswer/riskassessment/ragsf/pdf/partf 200901 final.pdf
EPA. (2010). Review of the Draft 2002 Subsurface Vapor Intrusion Guidance. Office of Solid Waste and
Emergency Response.
www.epa.gov/oswer/vaporintrusion/documents/review of 2002 draft vi guidance final.pdf
EPA. "Regional Screening Levels for Chemical Contaminants at Superfund Sites." Latest edition.
www.epa.gov/reg3hwmd/risk/human/rb-concentration table/index.htm
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