Johnson & Ettinger model: modeling vapor intrusion into buildings

Written by Eric Azulay — SSP Engineer (contaminated land), Managing Director of SARL G.M.E.P · Published on July 3, 2026

Principle of the Johnson & Ettinger model

Initially developed by Paul Johnson and Robert Ettinger in 1991 and later formalized in the US EPA's 2004 guide ("User's Guide for Evaluating Subsurface Vapor Intrusion into Buildings"), the Johnson & Ettinger model is a steady-state analytical model that simulates the transport of volatile compounds from a subsurface contamination source (contaminated soil or groundwater) to the indoor air of a building located at the surface.

The model represents two successive transport mechanisms: diffusion/advection through the unsaturated zone (between the source and the foundation), then passage through the building foundation (slab, cracks, joints), followed by dilution in the volume of indoor air renewed by ventilation.

The attenuation factor α

The central result of the model is the attenuation factor α (alpha), defined as the ratio between the indoor air concentration and the source concentration (soil gas or groundwater, depending on the calculation point):

α = Indoor air concentration (Ci) / Source concentration (Cs)

This factor, generally between 10⁻² and 10⁻⁴, depends on the depth of the source, the properties of the soil traversed (porosity, permeability, water content), and the characteristics of the building (foundation surface in contact with the soil, air exchange rate, air entry rate through cracks). Once α is determined, the indoor air concentration is simply: Ci = α × Cs, a value then fed into the calculation of the inhalation exposure dose for the EQRS.

Transport equation

The Johnson & Ettinger model solves a one-dimensional diffusion-advection equation across the unsaturated zone, coupled with a mass balance in the building's indoor air volume. In simplified form, the flow of vapors entering the building (Jf) is expressed as:

Jf = Deff × Cs / Lt (diffusive flux through the unsaturated zone)
Ci = Jf × Ab / (Qbuilding) (mass balance in the building volume)

where Deff is the effective diffusion coefficient of the compound in the soil, Lt the total diffusion distance (source depth), Ab the foundation surface in contact with the contaminated soil, and Qbuilding the building's air exchange rate. The full model (EPA 2004) actually incorporates a diffusion/advection coupling through foundation cracks, via a soil gas permeability parameter.

Key calculation parameters

Substances of concern

The Johnson & Ettinger model applies to compounds sufficiently volatile to migrate as a gas through the unsaturated zone. The most frequently assessed families of substances are:

• Chlorinated solvents — trichloroethylene (TCE), tetrachloroethylene (PCE), vinyl chloride, 1,1-dichloroethylene: priority substances for vapor intrusion due to their toxicity and high volatility
• Petroleum volatile organic compounds (VOCs) — benzene, toluene, ethylbenzene, xylenes (BTEX)
• Light polycyclic aromatic hydrocarbons (PAHs) — particularly naphthalene, for its relatively high volatility among PAHs

Metals and most heavy PAHs, being of low or no volatility, are on the other hand not relevant to this transfer pathway and fall under other modeling approaches (ingestion, dermal contact).

Limitations and precautions

Like any simplified analytical model, Johnson & Ettinger relies on assumptions that should be kept in mind when interpreting results:

• Steady-state conditions — the model does not capture seasonal variations in groundwater levels, soil water content, or barometric pressure, which can nonetheless significantly affect measured indoor air concentrations
• Homogeneous soil — actual soil heterogeneity (sand lenses, cracks, buried utility networks) can create preferential pathways not represented by the 1D model
• No biological degradation in the conservative version of the model, whereas certain compounds (particularly petroleum hydrocarbons) can be partially biodegraded in the unsaturated zone
• High sensitivity to input parameters — small variations in permeability or source depth can change the α factor by several orders of magnitude

Comparison with field measurements

Due to these limitations, methodological guidance recommends, whenever possible, comparing the model's results with direct measurements: sub-slab soil gas sampling, indoor air measurements and reference ambient air measurements (to distinguish the site's contribution from indoor sources), or even surface flux measurements. This multiple lines of evidence approach (modeling + measurements) is particularly recommended when the results of the Johnson & Ettinger model are close to the QD or ERI acceptability thresholds, or when the building under study is already occupied.

Modeling nonetheless remains an essential tool during initial diagnostic (IEM) and measurement campaign sizing, as well as for evaluating prospective scenarios (construction projects on contaminated sites) where no direct measurement is yet possible.

Typical application scenarios

The Johnson & Ettinger model is used in several operational contexts encountered by SSP engineering firms:

Diagnosis of an existing building on a contaminated site

When a building is already constructed on or near an identified volatile pollution source (contaminated groundwater or soil), the model allows for an initial estimate of vapor intrusion risk before undertaking, if necessary, an indoor air or sub-slab soil gas measurement campaign.

Development or construction project

As part of a construction project on a site with residual soil or groundwater contamination, the model makes it possible to anticipate, from the design stage, the risk of vapor intrusion into future buildings and to size appropriate management measures (vapor barrier membrane, crawl space ventilation, sub-slab depressurization system).

Reassessment after remediation works

The model is also used to verify, based on newly measured residual concentrations, that the vapor intrusion risk remains acceptable after remediation or containment works have been implemented, as part of management plan monitoring.

Integration into the EQRS

The indoor air concentration calculated by the Johnson & Ettinger model feeds directly into the calculation of the daily inhalation exposure dose, then into the calculation of the QD and ERI according to the methodology described in our article on QD and ERI calculation in EQRS. This linkage makes it possible to cover the entire source-transfer-target chain, from the pollution source to the health risk for the occupants of the building.

FAQ — Johnson & Ettinger model and vapor intrusion

Methodological references

• US EPA (2004) — Johnson & Ettinger model guidance
• US EPA — Vapor Intrusion Screening Level (VISL) calculator
• ANSES — Opinion on vapor intrusion into indoor air
• INERIS — Guide on accounting for the vapor inhalation exposure pathway

Written by Eric Azulay — SSP Engineer (contaminated land), Managing Director of SARL G.M.E.P