vapor intrusion assessment

February 28, 2022

Rapidly evolving vapor intrusion health risk guidance requires more diligence by your Environmental Engineer.



Contamination at thousands of shopping centers across California from previous business operations presents problems for property owners who wish to continue commercial use, redevelop, and maintain property value. Commercial property remediation targets returning these buildings and land to predevelopment conditions, presenting opportunities for reuse and redevelopment.

One property owner discovered that securing adequate funding and working closely with state and regional regulatory agencies leads to success despite changing regulations and oversimplifying regulatory health risk assessment methods. The Draft Cal-EPA Supplemental Vapor Intrusion Guidance (DSVIG) suggests changes to the methods in which vapor phase transport and potential health risks are modeled and calculated for occupants of buildings with known soil or groundwater contamination beneath them. These changes, the result of a multi-year working group collaboration, recommend an arguably more conservative calculation of indoor air quality. The changes rely on EPA work and guidance, with empirically derived attenuation factors (AFs), which will increase the number of sites requiring additional environmental assessment and mitigation to achieve health risk standards. Although the DSVIG is currently draft guidance, there is evidence that regional regulatory agencies have already adopted AFs in calculating indoor air quality.


Diamond Bar Commercial Center Assessment and Mitigation

Drucker Survivors Trust owns and operates a multi-tenant commercial building in Diamond Bar, California, including a dry cleaner at one time. The former cleaners caused an unauthorized release of dry cleaning solvent containing chlorinated volatile organic compounds to the subsurface during its operation.

Financing for this all too common situation requires environmental due diligence in the form of research commonly completed in a Phase I Environmental Site Assessment followed by an assessment to characterize potential liabilities associated with chlorinated solvent releases before lenders provide funding.

Regulatory oversight in California can either be voluntarily engaged or involuntarily if assessment activities on an adjacent or nearby property indicate the presence of chlorinated volatile organic compounds in the subsurface linked to dry cleaning operations in the vicinity.

The Drucker Survivors Trust sought approval from the applicable regulatory agency, Los Angeles Regional Water Quality Control Board (LARWQCB), to assess and mitigate the chlorinated solvent release to ensure the protection of human health and reduce environmental liabilities associated with the property.

Regulatory closure is the acceptance of assessment and remediation activities by the governing regulatory entity to bring the site into compliance. Compliance, in this case, required assessment and mitigation of beneficial use groundwater underlying the property impacted by the solvent release and completing soil vapor assessment and health risk screening calculations under current state and federal guidelines.

Guidance on vapor assessment and associated health risk screening methods have changed rapidly in California state environmental regulations. As environmental engineers and consultants, SCS professionals manage an extensive list of vapor assessment, health risk assessment, and vapor intrusion mitigation projects resolving these vapor–related issues.

To start this project, the SCS team prepared a successful grant application securing more than $650,000 in funding from the California State Water Resources Control Board’s Site Cleanup Subaccount Program (SB 445, established in 2014). This state-provided grant money enables the assessment and mitigation necessary to close with the LARWQCB.

Subsurface assessment activities defined the extent and scale of chlorinated solvent impacts to soil vapor, soil, and groundwater, enabling the design of a remediation program. To reduce the groundwater contamination to cleanup levels set by the LARWQCB, SCS Engineers designed and implemented an injection program to deliver engineered chemicals directly to the groundwater plume. The injected chemicals destroy the chlorinated solvents via in situ chemical reduction and stimulation of biological degradation.

While challenging drilling conditions precluded previous consultants from attempting groundwater remediation, SCS industry experts safely achieved up to a 99 percent concentration reduction within the groundwater plume. SCS designed a soil vapor assessment that relied more on site-specific data collection and less on conservative default assumptions while conforming to the most current regulatory guidance targeted at minimal impact on the building tenants.

SCS managed all aspects of the project, including grant requirements and communication between the client, regional and state water board staff, city staff, and subcontractors. Obtaining and managing entrance under state waste discharge requirements is necessary, and SCS completed all necessary permitting and reporting requirements to facilitate the groundwater mitigation activities. Careful planning and experience with similar projects minimized impacts on tenants and kept the project on a strict timeline with no missed regulatory deadlines. SCS continues working with the LARWQCB to conclude the client’s final closure requirements and is in the process of applying for an additional $900,000 in SCAP funding to implement the final stages of the project targeted at obtaining final regulatory closure.


Changes Coming to Regulatory Guidance

Recent changes to regulatory guidance in California are arguably making obtaining closure on sites with vapor intrusion health risk concerns more difficult to achieve. The Draft Cal-EPA Supplemental Vapor Intrusion Guidance (DSVIG) suggests changes to the methods in which vapor phase transport and potential health risks are modeled and calculated for occupants of buildings with known soil or groundwater contamination beneath them. These changes, which result from a multi-year working group collaboration, recommend a more extensive and site-specific data collection effort. They include indoor air quality calculation methods relying on EPA work and guidance and empirically derived attenuation factors (AFs) which some would argue lead to overestimating potential health risks.

The consequences of the DSVIG are potentially significant if adopted as is and appear likely to result in more sites being “screened in” with vapor intrusion issues and more sites requiring mitigation. The impact, resultant costs, and possibly detrimental secondary effects such as decreases in affordable housing production, particularly in urban infill areas. And while none would argue with appropriate protection of health risk, the question is whether the studies and empirical data used to support the DSVIG represents the best available science and is truly representative and predictive of risk.

The DSVIG adopts an attenuation rate of 0.03 for the flux of both soil and sub-slab vapor to indoor air based on a previous 2012 EPA Study comprised of empirical data collected from buildings arguably not representative of modern construction in California.

The development of a reliable screening level attenuation factor for California based on high-quality, recent, California-specific data:

1) Will be protective of human health, as no toxicological imperative or basis supports a call for accelerated or immediate action (as evidenced by the fact that the DSVIG workgroup commenced its work in 2014 and issued the review draft in 2020).

2) Will ensure California’s environmental policy satisfies the gold standard for data quality and insightful analysis in which the state once took pride.

3) Will not unnecessarily decimate the California housing development market. The empirically derived screening level AF in the DSVIG is overly conservative based on the available data. More accurate empirical data and measurement methods for site-specific measurement are available.

With respect, oversimplifying the VI health risk assessment methods has constrained the environmental community’s ability to apply science-based health risk screenings, often resulting in costs associated with additional environmental assessment and mitigation. An additional revision to the DSVIG to utilize a screening level AF more reflective of the current California data and building specifications could save state resources, increase infill development by reducing urban sprawl, promote housing development, all while protecting human health.


Keith EtchellsAbout the Author: Keith Etchells is a professional geologist and hydrogeologist with over two decades of experience assisting clients in managing environmental risks associated with ownership, transfer, or operation of commercial, industrial, and waste disposal properties. His particular technical expertise involves aspects of groundwater science and engineering relevant to contaminated sites and landfills, including supervision and conduct of subsurface data acquisition, remedial design and implementation, conceptual site model development, aquifer testing, extraction well design, groundwater quality evaluation and treatment, vapor intrusion health risk assessment and mitigation, predictive modeling, and contaminated soil and groundwater remediation design.

He is responsible for designing analytical, geotechnical, and hydrogeological data collection programs to complete subsurface assessment and remediation. He has prepared subsurface assessment documents, property mitigation plans, vapor intrusion risk assessment documents, soil management plans, aquifer characterization documents, conceptual site models, and groundwater remedial design and implementation documents.






Posted by Diane Samuels at 6:00 am

February 18, 2020

The Department of Toxic Substances Control (DTSC), State Water Resources Control Board, and San Francisco Bay Regional Water Quality Control Board have developed supplemental vapor intrusion guidance for conducting vapor intrusion evaluations in California. The Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion is available for review and public comment until 12:00 noon, April 30, 2020. Click here for available public meetings and comments.

Take the time to read the draft supplemental guidance and send comments so that the resulting policy and guidance are rational and science-based.


According to the published Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion Executive Summary

Toxic vapors can move from contaminated groundwater and soil to indoor air. This process is called vapor intrusion. Vapors inside buildings can threaten human health. The science behind vapor intrusion has been evolving quickly. To protect the health of Californians, the DTSC and the California Water Boards drafted a supplement to the existing vapor intrusion guidance. This is important information to collect for environmental protection. This document is called the “Draft Supplemental Guidance: Screening and Evaluating Vapor Intrusion” (Draft Guidance). This Draft Guidance contains recommended improvements for vapor intrusion investigations and promotes consistency throughout the state. It also offers suggestions on the following topics:

  • Which buildings to sample first
  • How to screen buildings for vapor intrusion
  • Where to sample
  • When additional steps are necessary
  • When sewers may contribute to vapor intrusion
  • What information we use to refine our approach

The Draft Guidance is intended to be used with existing State guidance – DTSC 2011 Vapor Intrusion Guidance and San Francisco Bay Regional Water Board 2014 Interim Framework 1 – when there is a spill or disposal of vapor-forming chemicals. This guidance does not apply to any leaking petroleum underground storage tanks (USTs) since they are governed under the State Water Resources Control Board’s Low-Threat UST Case Closure Policy.

Four Steps to Evaluate Vapor Intrusion
The Draft Guidance describes four recommended steps to decide if there is vapor intrusion that could pose a risk to the health of people inside buildings. These actions are meant to protect public health and should be carried out under the oversight of the lead regulatory agency.

Step 1 – Decide which buildings should be tested first and how.
When there are several buildings, start with those that are occupied and closest to the contamination. If a building is directly above or very close to the spill, or if it is likely that the sewer could bring toxic vapors inside, skip Step 2 and go directly to Step 3.

Step 2 – Screen buildings from outside.
Measure vapor-forming chemicals underground at these locations:

  • Between the spill and the building
  • Just outside the building
  • From at least two depths at the same location(s)
  • Sample in at least two different seasons and use the data to assess whether people are likely to be affected.

Step 3 – Test indoor air.
Measure vapor-forming chemicals in indoor air, beneath the building’s foundation, and outdoor air at the same time:

  • Test the air in at least three rooms
  • Test below the foundation near where the indoor air was tested to check if vapor-forming chemicals are coming from under the building
  • Test the outdoor air to check if the vapor-forming chemicals are coming from outside
  • Repeat the sampling in different seasons, and
  • Test with the heater or air conditioner (on and off) to see if that changes the results
    Use these test results to estimate if people are likely to be affected.

Step 4 – Act to protect public health.

  • To protect current occupants, take action based on the amount of vapor-forming chemicals in the indoor air
  • To protect future occupants, take action based on the amount of vapor-forming chemicals underground because the building characteristics can change over time
  • The best response is to clean up the contamination at the spill (remediation)
  • Use protective technologies when remediation is not feasible or until the spill is cleaned up
  • In extreme cases, occupants may need to be temporarily relocated
    The overall cleanup should be designed when the contamination is fully understood and should consider the characteristics of each site.

Toxic Vapors Can Travel Through Sewer Pipes
Vapor-forming chemicals can enter sewer pipes that run through contaminated soil or groundwater. Once inside a sewer, vapors can move through the pipes and escape through cracks or openings, under or inside a building. Some of the traditional ways to test for vapor intrusion could potentially miss vapor-forming chemicals moving through sewer pipes. This Draft Guidance recommends evaluating whether the sewer could bring toxic vapors inside.

Vapor Intrusion Attenuation Factors
Attenuation factors are used to estimate how much of the vapors underground or in groundwater end up in the indoor air. This Draft Guidance uses attenuation factors recommended by the U.S. Environmental Protection Agency. These were calculated from a large study of buildings at contaminated sites around the nation, including California.

California Vapor Intrusion Database
Data from sites evaluated using the process described in the Draft Guidance will be entered into a database that will be publicly available. The State Water Resources Control Board (State Water Board) added capabilities to the GeoTracker database including building-specific information for a cleanup case and the ability to differentiate Field Points for collecting samples. The State will analyze the information in the database and learn how to better protect the people of California from vapor intrusion.

Where to Find the Draft Guidance









Posted by Diane Samuels at 6:03 am

January 16, 2017

Did you know that vapor intrusion is an environmental issue impacting property development? The U.S. Environmental Protection Agency (USEPA) and related state agencies continue to develop and refine vapor intrusion procedures. How and when you address vapor intrusion can have a significant impact on your bottom line. These tips will help minimize the risk:

Invest in Early, Accurate Detection
Vapor intrusion happens when volatile chemicals from soil or groundwater contamination migrate into nearby buildings. These vapors can come from places like dry cleaners, gas stations, or industrial facilities, and like radon can affect people who live or work in impacted buildings. Prioritize due diligence because waiting can drive up costs. In one recent case in the news, vapor mitigation systems had to be installed in more than 70 homes near a corporation in Wisconsin to mitigate vapor intrusion issues. Even a small error in the sampling technique can dramatically affect the results. So early, accurate detection is key before you buy or build.

Get to Know Your Options
Once assessed, developers have multiple options for saving time and money. For example, it is far more cost effective to incorporate vapor barriers or HVAC controls into building construction rather than to fix the problem after the fact.

Find a Partner Who Knows the Rules
In 2017, some state agencies will update their vapor intrusion guidance documents to include more details on requirements for remediation and redevelopment sites. These regulations are actively evolving, so it’s important to work with someone who knows the changes to regulatory requirements and how they apply to your specific property.

If you are buying or selling a property, it’s time to think about vapor intrusion. Even if a property doesn’t have a history of contamination, it may still be affected if a nearby property has soil or groundwater contamination.


“Vapor Encroachment and Intrusion, What are the real estate risks?”
Journal of Property Management

Is Vapor Intrusion Threatening Your Site?
Public Works Magazine

Vapor Intrusion Mitigation Systems

Environmental Due Diligence and All Appropriate Inquiries

Brownfields and Voluntary Remediation

Contact your local SCS Office to ask our professionals for guidance in your jurisdiction.

Posted by Diane Samuels at 1:24 pm

May 2, 2016


No matter where the real property is located you may need a vapor intrusion pathway screening.


If you are buying or selling a property with a history of soil or groundwater contamination in Wisconsin, the state’s Department of Natural Resources requires vapor intrusion pathway screening. The screening is also necessary when buying or selling a property adjacent to a property with soil or groundwater contamination. Is this analysis necessary?

The screening is essential; vapors from contaminated soil or groundwater may transfer to indoor air, causing health risks. The vapors may or may not have an odor. Screen testing and analysis will determine their existence and the level of concentration. Most commonly levels are low, and no additional action is necessary. If beyond the threshold determined safe by your state, some mitigation may be required before purchase.

It is not a simple matter to apply an individual state’s current regulatory guidance to determine the need for vapor intrusion mitigation. The actual intrusion, or expected intrusion in the case of new buildings, is often overstated, and some regulatory agencies use screening values for indoor air chemical concentrations that are at or below levels commonly found in buildings. The slightest error in sampling technique can dramatically affect the resulting data.

SCS offers the full array of vapor intrusion services for residential, commercial, and industrial properties, and for developers, municipalities, lenders, attorneys, industrial facilities, tenants, landlords, and buyers and sellers of real property.

Contact SCS at 1-800-767-4727 or email us at .

Question about  this blog, please email one of the authors  Robert Langdon and Thomas Karwoski.

SCS has offices nationwide to serve our customers. To learn more about vapor mitigation, please visit the SCS website here:

Posted by Diane Samuels at 6:00 am