On this page
airgroundwatersoil

Ohio Vapor Intrusion Evaluation Guide

How to evaluate vapor intrusion in Ohio under VAP and DERR programs. EPA VISL calculator, sampling methods, and screening.

Verified March 22, 2026 Source: Ohio EPA DERR VI Guidance (March 2020)

What Is Vapor Intrusion?

Vapor intrusion (VI) is the migration of volatile chemicals from contaminated subsurface sources - soil, groundwater, or soil gas - through the vadose zone and into the indoor air of overlying buildings. The chemicals move upward through soil pore spaces, enter buildings through cracks in foundations, utility penetrations, sump pits, and other openings, and accumulate in indoor air at concentrations that may pose a health risk to occupants.

This pathway matters because it can produce the most restrictive cleanup standards at a site. A groundwater plume that doesn’t exceed drinking water standards might still cause indoor air concentrations above acceptable risk levels through vapor intrusion. Sites with chlorinated solvents (TCE, PCE, vinyl chloride) and petroleum compounds (benzene, naphthalene) are the most common candidates for VI evaluation, but any volatile and toxic chemical in the subsurface requires consideration.

Ohio EPA’s Approach

Ohio EPA’s Division of Environmental Response and Revitalization (DERR) published its current vapor intrusion guidance in March 2020, titled “Sample Collection and Evaluation of Vapor Intrusion to Indoor Air.” This guidance applies to all DERR cleanup programs: the Voluntary Action Program (VAP), CERCLA, and RCRA.

The guidance recommends a stepwise approach to evaluating vapor intrusion, which allows you to exit the evaluation at any point where the data demonstrates the pathway is not complete or does not pose an unacceptable risk.

Step 1: Conduct the Site Assessment

The site assessment (Phase I for VAP sites) determines whether volatile and toxic chemicals were used, stored, or released at the property. This step is about identifying whether there’s a reason to suspect volatile compounds in the subsurface - not about collecting samples yet.

Step 2: Determine If Volatile and Toxic Chemicals May Be Present

Chemicals must be both sufficiently volatile and sufficiently toxic to pose a vapor intrusion risk. If the site history and available data indicate no volatile compounds were released, document that finding and the VI pathway does not need further evaluation.

Step 3: Determine If There Is a Potentially Complete Pathway

If volatile chemicals are or may be present, evaluate whether the pathway to indoor air is potentially complete. Key factors include:

  • Current and future land use - are there existing or planned buildings?
  • Distance - how far is the contamination from buildings, both laterally and vertically?
  • Preferential pathways - are there utility corridors, sewers, or other conduits that could transport vapors?
  • Source characteristics - is the plume at steady state? Is NAPL present?

Develop a Conceptual Site Model (CSM) for the vapor intrusion pathway. Ohio EPA provides a CSM checklist in Appendix A of the VI guidance.

Step 4: Collect Data and Screen Against VISLs

If the pathway is potentially complete, collect subsurface data (soil gas, sub-slab vapor, groundwater, and/or indoor air) and screen results against applicable screening levels using the EPA VISL calculator.

Step 5: Evaluate Risk and Determine Remedy

If screening levels are exceeded, conduct a more detailed risk evaluation and determine whether mitigation is needed.

The EPA VISL Calculator

The U.S. EPA Vapor Intrusion Screening Level (VISL) calculator is the primary tool for generating screening levels for the vapor intrusion pathway. It’s an online calculator that produces risk-based screening concentrations for:

  • Indoor air - target breathing-zone concentrations
  • Sub-slab and near-source soil gas - screening levels derived by dividing indoor air targets by an attenuation factor
  • Groundwater - screening levels for dissolved-phase concentrations that could result in unacceptable indoor air concentrations via volatilization

How It Works

The VISL calculator uses the same toxicity values and chemical parameters as EPA’s Regional Screening Levels (RSLs). It’s updated semi-annually when the RSL database is updated. The calculator takes user inputs and generates screening levels based on the following parameters:

User inputs:

  • Exposure scenario - residential or commercial/industrial worker
  • Target cancer risk - default is 1 x 10⁻⁶ (one in a million)
  • Target hazard quotient - default is 0.1 for non-carcinogens
  • Groundwater temperature - affects Henry’s Law constant and volatilization rates; default is 25°C but should be adjusted to site-specific measured temperature

Default attenuation factors:

  • Sub-slab/near-source soil gas to indoor air: 0.03 (meaning indoor air is assumed to be 3% of the sub-slab concentration)
  • Groundwater to indoor air: 0.001 (with Henry’s Law constant multiplier)

These default attenuation factors are conservative - they represent generally reasonable worst-case conditions. Site-specific attenuation factors can be developed with paired indoor air and sub-slab or soil gas data, but this requires multiple sampling events and careful evaluation.

What the Calculator Outputs

For each chemical selected, the calculator determines:

  1. Whether the chemical is sufficiently volatile and toxic to pose an inhalation risk from a soil source and/or a groundwater source
  2. Target indoor air concentration (the screening level for indoor air)
  3. Target sub-slab and near-source soil gas concentrations (derived from the indoor air target using the attenuation factor)
  4. Target groundwater concentration (derived from the indoor air target using the groundwater attenuation factor and Henry’s Law constant)

The calculator selects the lower of the carcinogenic and non-carcinogenic target concentrations as the final screening level for each chemical.

Important Notes on Using the Calculator

  • The VISL calculator generates screening levels, not cleanup standards. Exceedance of a VISL does not automatically mean there is unacceptable risk - it means further evaluation is warranted.
  • The default attenuation factors assume diffusion-dominated transport. Sites with advective transport (landfills with methane generation, pressurized vapor sources, or preferential pathways) may require different approaches.
  • Groundwater temperature significantly affects screening levels. Always use the measured in-situ groundwater temperature from your site rather than the default 25°C.
  • The air VISLs generated by the calculator are identical to the air RSLs for resident and composite worker scenarios.

Access the VISL calculator: EPA VISL Calculator

Ohio VAP-Specific Considerations

Generic Indoor Air Standards (OAC 3745-300-08)

For VAP sites, generic indoor air standards due to vapor intrusion are listed in OAC 3745-300-08 Appendix A. These standards were calculated deterministically for volatile compounds and represent the VAP’s codified screening levels. They may differ from the current VISL calculator output because the VISL calculator is updated semi-annually while the OAC standards are updated with rule revisions.

When the OAC standard and the current VISL differ, consult with Ohio EPA or the program-specific guidance to determine which applies. For DERR remedial program sites, the current VISLs typically apply. For VAP sites, the OAC Appendix A standards are the codified standards, though CPs often reference current VISLs for comparison.

Petroleum Vapor Intrusion (PVI)

Ohio EPA’s VI guidance includes a dedicated section (Section 9) on petroleum vapor intrusion. Petroleum compounds undergo aerobic biodegradation in the vadose zone, which significantly reduces vapor concentrations between the source and the building foundation. This means petroleum sites may have less vapor intrusion risk than chlorinated solvent sites at comparable source concentrations.

Key considerations for PVI evaluation:

  • Lateral inclusion zone - buildings within a defined lateral distance of the source should be evaluated
  • Vertical separation distance - sufficient clean soil between the source and building foundation allows biodegradation to attenuate vapors
  • LNAPL presence - free product can serve as a continuing source of volatile emissions
  • BUSTR compliance - petroleum UST sites managed under BUSTR may follow BUSTR-specific vapor intrusion procedures

The EPA’s PVI database and guidance provide additional screening tools specifically for petroleum sites.

Sampling Methods

Ohio EPA’s guidance describes procedures for collecting the following types of vapor intrusion samples:

Soil Gas Sampling

Soil gas samples are collected from probes installed in the vadose zone to characterize the nature and extent of volatile compounds in soil gas. Probes can be permanent (dedicated) or temporary (driven or augered). Samples are typically collected in Summa canisters or other passivated stainless steel canisters and analyzed by EPA Method TO-15 or TO-15 SIM for lower detection limits.

Sub-Slab Vapor Sampling

Sub-slab samples are collected through probes drilled through the building foundation slab. These provide the most direct measurement of vapor concentrations immediately below the building. Sub-slab data paired with indoor air data allows calculation of site-specific attenuation factors.

Indoor Air Sampling

Indoor air samples measure the actual breathing-zone concentrations inside buildings. These require careful planning because indoor air is affected by both subsurface vapors and indoor sources (cleaning products, paints, stored chemicals, attached garages). Ohio EPA’s guidance recommends:

  • A thorough building inspection and product inventory before sampling
  • Sampling during conditions biased toward worst-case vapor intrusion (heating season, closed-building conditions)
  • Collecting background/outdoor air samples for comparison
  • Evaluating potential indoor sources that could confound results

Leak Testing

When collecting sub-slab or soil gas samples, leak testing is critical. A tracer compound (typically helium or isopropanol) is placed around the probe seal, and the sample is analyzed for the tracer. If the tracer is detected above a threshold, the sample may have been compromised by ambient air leaking into the probe, and the results may not be representative.

Mitigation Options

If the vapor intrusion pathway is confirmed and poses unacceptable risk, several mitigation technologies are available:

  • Active sub-slab depressurization (SSD/ASD) - the most common and effective approach. Creates negative pressure beneath the slab to prevent vapor entry. Requires ongoing operation, monitoring, and maintenance.
  • Soil vapor extraction (SVE) - removes volatile compounds from the vadose zone, addressing the source rather than just blocking the pathway.
  • Passive barriers - vapor barriers and membrane systems installed during new construction or renovation. Less reliable than active systems for existing contamination.
  • HVAC modifications - building pressurization, enhanced ventilation, or air filtration. Can be effective in commercial settings but typically not a standalone remedy.
  • Environmental media remediation - removing or treating the subsurface source. The most permanent solution but often the most expensive and time-consuming.

Ohio EPA requires monitoring to verify the effectiveness of engineering controls and may require environmental covenants and operation and maintenance agreements for ongoing mitigation systems.

Common Chemicals of Concern for Vapor Intrusion

The following volatile compounds are frequently evaluated for vapor intrusion at Ohio sites:

Chlorinated solvents: TCE (trichloroethylene), PCE (tetrachloroethylene), vinyl chloride, 1,1-dichloroethylene, cis-1,2-dichloroethylene, carbon tetrachloride, chloroform, methylene chloride

Petroleum compounds: Benzene, toluene, ethylbenzene, xylenes (BTEX), naphthalene

Other: Methyl tert-butyl ether (MTBE), 1,4-dioxane (limited volatility but increasingly evaluated)

Not all volatile compounds in groundwater or soil are sufficiently volatile and toxic to pose a VI risk. The VISL calculator automatically screens chemicals for both volatility (vapor pressure > 1 mm Hg or Henry’s Law constant > 1 x 10⁻⁵ atm-m³/mol) and toxicity before generating screening levels.

Future Enhancement: Built-In VISL Calculator

We’re developing a built-in vapor intrusion screening level calculator for this page that will let you generate screening levels directly on SiteRef and download the results as a CSV or PDF. This will use the same EPA methodology and toxicity values with a cleaner interface. Check back for updates.