In The Air

VAPOR INTRUSIONThe first step to solving a vapor intrusion problem is recognizing it.   

By Wesley Robb

Vapor intrusion (VI) occurs when petroleum products or volatile chemicals contaminate groundwater, soil or a vapor plume then migrate into the breathable air of a building. It is a concern that was first brought to the public’s attention in the 1980s, but only recently grew awareness to the point where building developers are routinely exploring solutions for mitigating the risks associated with VI. 

Fortunately, knowledge of this threat continues to permeate among contractors, building developers and architects. This knowledge is critical, as the first step to successfully solving a potentially harmful VI problem is to recognize that it exists. 

Much of the awareness continues to be driven by the United States Environmental Protection Agency (EPA), as vapor intrusion was added to the Superfund Program as a potential pathway. In December 2016, the EPA included a subsurface intrusion component to its Hazard Ranking System (HRS). This is the principal mechanism the EPA uses to place sites on the National Priorities List. The EPA defines subsurface intrusion, including VI, as “the migration of hazardous substances and pollutants from the unsaturated zone and/or the surficial ground water into overlying structures.”

Why is Awareness of VI So Important?

If a building sits on soil and/or groundwater that is contaminated, there is a potential that the breathable air in its habitable zone is affected. Furthermore, if the air does become contaminated, all the individuals that occupy the building, in addition to building occupants surrounding the site, could suffer consequences. 

Exposure to air that has been contaminated by VI has been known to create potential health issues, not only for the people in the building, but also for future offspring. Additionally, there is a financial and/or legal risk that property managers and building owners face if they remain uninformed or unresponsive to the hazards associated with VI. There are documented cases where lawsuits have been brought about to organizations for not properly mitigating the risk of VI. Both public health and potential litigation make understanding the risks associated with VI crucial.

High Risk Areas

Anywhere the soil, groundwater or soil vapor has been contaminated by volatile organic compounds (such as dry-cleaning solvents or industrial degreasers) or semi-volatile organic chemicals (such as naphthalene) there is the risk of VI. From a practical standpoint, sites that formerly or currently occupy or are near businesses like dry cleaners, printers, auto repair facilities and manufacturers tend to have some of the greatest potential for VI. Unfortunately, there is no way to detect the presence of dangerous chemical vapors by sight or smell. Without these clear and evident warning signs, being cognizant of the potential for VI becomes even more critical.  

Before breaking ground on a new development, it is important to contact a specialist that can assess the vapor concentrations below a potential future building to determine if there is a VI risk in that particular area.  


So, what can be done in an area where the risk of VI is present? Is there a way to mitigate the risk without abandoning the site altogether? The simple answer is yes. There are vapor mitigation systems complete with chemical vapor barriers that can be specified before construction of a new building. 

The biggest hurdle that exists, once again, is awareness of the issue. Often, a contractor will receive the specifications for a new building and mistake a chemical vapor barrier for a moisture barrier. The goal of a moisture barrier, often misleadingly referred to as a vapor barrier, is to keep moisture out of the building. A water vapor barrier reduces the potential for mold to make its way into a building, which is a different kind of contamination. A chemical vapor barrier’s goal, however, is to mitigate the risk of VI. Most importantly, a contractor needs to understand exactly what is being specified.  

If a plan does in fact call for a chemical vapor barrier, it is likely that the sampling of the soil, groundwater, and/or soil vapor led to this specification. Effective practices to help mitigate the risk of VI generally consist of both a chemical vapor channel installed within the footprint of the building, and a coated physical barrier that gets laid down before the foundation. 

The barrier itself is designed to be chemically resistant and not allow the hazardous vapors from making their way into the building. Once these chemical vapors are held back from entering through the foundation, they are emitted out of the sub-slab of the building through a ventilation pipe.  

When a contractor receives specifications for a chemical vapor barrier, they can hire certified installers to put this barrier in before the foundation is laid. Although retrofitting options are available for buildings that have already been constructed, it has been most effective to establish a vapor mitigation system before the foundation is laid.

These chemical vapor mitigation systems can only be specified and followed if those responsible for doing so understand the problem and are aware of the solutions available.

Wesley Robb is the director of technical strategies and applications of Vapor Mitigation Strategies and has more than 29 years of environmental field and laboratory experience including several years of soil vapor sampling and analyses. Having joined Wellington Environmental Consulting and Construction in 2004, he has managed on-site activities of various kinds: underground storage tank removals; soil remediation; Phase I investigations; vapor, soil, and groundwater sampling; and specialty sampling including industrial hygiene sampling. To learn more, visit


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