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Vapor Intrusion Assessment Calculator

Evaluate the vapor intrusion pathway using Johnson-Ettinger model attenuation factors and EPA screening levels.

About this calculator

Vapor intrusion happens when volatile chemicals in soil gas or groundwater migrate upward and seep into a building's indoor air through slab cracks, and this calculator estimates how much actually gets inside using a simplified Johnson-Ettinger (J&E) model. It first computes the building's ventilation rate from footprint, mixing height, and air exchange rate, then models the pathway for vapor as a competition between two flows: soil gas diffusing through slab cracks (using an EPA default diffusion coefficient and slab thickness) versus outdoor air being exchanged through ventilation. The ratio of crack-flow to crack-flow-plus-ventilation gives the attenuation factor, alpha — the fraction of sub-slab concentration that shows up indoors. A tighter, less-ventilated building or a larger crack fraction raises alpha and worsens indoor exposure; more ventilation dilutes it.

Groundwater contamination is converted to an equivalent soil-gas concentration via Henry's law (concentration times the dimensionless Henry's constant, which varies significantly by chemical — benzene's 0.228 vs. TCE's higher volatility), then run through the same attenuation factor alongside any directly measured sub-slab data, and the two contributions are summed for a total indoor air estimate. That total is compared against a risk-based screening level to flag whether mitigation is warranted, with a parallel calculation using EPA's generic default attenuation factor (0.03) shown for comparison. This is a screening-level simplification, not a full multi-zone or Petroleum Vapor Intrusion (PVI) fate-and-transport model — it doesn't account for biodegradation of petroleum vapors, temporal/seasonal variability, or preferential pathways like utility corridors, so a result near the screening threshold should trigger actual indoor air or sub-slab sampling rather than a final decision.

Inputs

sq ft
ft

Results

J&E attenuation factor

0

Total indoor air (µg/m³)

0.22

Risk ratio (J&E model)

0.71

Soil gas from GW (µg/m³)11,400
Indoor air from sub-slab (µg/m³)0
Indoor air from GW (µg/m³)0.22
Exceeds screening (0/1)0
Risk ratio (EPA generic)1,112.9

Figures current as of 2015. Source: U.S. EPA, Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway into Buildings, OSWER Publication 9200.2-154, June 2015

How to Use This Calculator
  1. Enter sub-slab soil gas concentration (µg/m³) from vapor probes or building surveys.
  2. Set groundwater concentration (µg/L) and Henry's Law constant for your contaminant.
  3. Enter building footprint (m²) and mixing height (m) for indoor air dilution modeling.
  4. Review calculated indoor air concentrations from both sub-slab and groundwater pathways.
  5. Compare results to indoor air screening levels to determine if mitigation is required.

What each input means

Sub-slab soil gas (µg/m³)
Measured or modeled contaminant concentration in soil gas beneath the building slab.
Groundwater concentration (µg/L)
Dissolved contaminant concentration in the shallow aquifer.
Henry's law constant (dimensionless)
Dimensionless Henry's law constant. Benzene = 0.228, TCE = 0.403, PCE = 0.723.
Building footprint (m²)
Ground floor slab area of the building.
Mixing height (m)
Height of the indoor air mixing zone (typically ceiling height of lowest occupied floor).
Crack fraction of slab
Fraction of slab area occupied by cracks/penetrations. EPA default 0.0005-0.001.
Air exchange rate (ACH)
Building air changes per hour. Residential 0.3-0.5, commercial 1-5.
Indoor air screening level (µg/m³)
Target indoor air concentration. Benzene 0.31, TCE 0.48 µg/m³ at 1e-6 risk.

What each result means

J&E attenuation factor
Calculated Johnson-Ettinger attenuation factor (sub-slab to indoor).
Soil gas from GW (µg/m³)
Estimated soil gas concentration from groundwater via Henry's law.
Indoor air from sub-slab (µg/m³)
Indoor air contribution from measured sub-slab soil gas.
Indoor air from GW (µg/m³)
Indoor air contribution from groundwater pathway.
Total indoor air (µg/m³)
Combined predicted indoor air concentration from all pathways.
Risk ratio (J&E model)
Indoor concentration / screening level. Values >1 indicate potential concern.
Exceeds screening (0/1)
1 = predicted indoor air exceeds screening level, further action needed.
Risk ratio (EPA generic)
Risk ratio using EPA generic attenuation factor of 0.03 for comparison.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Sub-slab soil gas (µg/m³) = 100, Groundwater concentration (µg/L) = 50, Henry's law constant (dimensionless) = 0.228, Building footprint (m²) = 200 = 8 input(s) provided
  2. Calculate J&E attenuation factor
    J&E attenuation factor = ventilationRate > 0 ?
    0.00002 = 0.00002
  3. Calculate Total indoor air
    Total indoor air = indoorFromSubSlab + indoorFromGW
    0.221 = 0.221
  4. Calculate Risk ratio
    Risk ratio = totalIndoorConc / screeningLevel
    0.71 = 0.71
  5. Calculate Soil gas from GW
    Soil gas from GW = gwConc * henryConstant * 1000
    11400 = 11400
  6. Calculate Indoor air from sub-slab
    Indoor air from sub-slab = subSlabConc * alphaSoilGas
    0.002 = 0.002

Figures and sources

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.

Frequently Asked Questions

Why does raising the air exchange rate lower the risk ratio?

The attenuation factor alpha is soil-gas flow through cracks divided by ventilation rate plus that same crack flow. Raising the air exchange rate increases ventilation rate in the denominator without changing the crack flow in the numerator, which shrinks alpha — physically, more fresh-air exchange dilutes whatever vapor seeps in through the slab, lowering the modeled indoor concentration and the resulting risk ratio.

Why does the calculator ask for both a sub-slab concentration and a groundwater concentration?

It sums two independent pathways: the sub-slab contribution uses your measured soil gas value directly, while the groundwater contribution first converts your dissolved concentration into an equivalent soil-gas concentration via Henry's law before applying that same attenuation factor. If you only have one type of data, set the other concentration to zero so it doesn't add a fabricated exposure estimate.

What does the "EPA generic" risk ratio show that the main result doesn't?

The EPA generic calculation substitutes a fixed default attenuation factor of 0.03 in place of the Johnson-Ettinger value the model derives from your building's actual crack fraction and ventilation rate. Comparing the two shows how much the main result depends on your building-specific inputs versus a simplified regulatory screening default — a large gap between them usually means your crack fraction or air exchange rate is far from typical.

Why does the calculator say this isn't sufficient for petroleum vapor intrusion sites?

The Johnson-Ettinger model implemented here treats vapor transport as pure diffusion through slab cracks with no chemical decay term, but petroleum hydrocarbons biodegrade aerobically in the vadose zone in ways chlorinated solvents largely don't. Because the math has no biodegradation term, applying it to a gasoline or diesel release will overstate indoor air risk relative to EPA's separate Petroleum Vapor Intrusion guidance, which explicitly accounts for that attenuation. The generic attenuation factor (0.03) used for the EPA-comparison output comes from EPA's 2015 Technical Guide for Assessing and Mitigating the Vapor Intrusion Pathway (OSWER 9200.2-154), which also documents the Johnson-Ettinger model this calculator's main pathway is based on.

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