Vapor Intrusion Emerges as a Major Cleanup Driver

When I started work as an environmental professional in 1986, I do not recall thinking about or even being aware of the risks posed by vapor intrusion. Vapor intrusion (sometimes referred to as “VI”) occurs when a volatile solvent or the volatile portion of a petroleum hydrocarbon from a release under or adjacent to a building migrates into the air inside a building.  Once in the indoor air, these volatile constituents can be readily inhaled by the building occupants, which is important because the lungs are the most efficient mechanisms by which chemicals enter the body.

How the VI Pathway Works

Volatile compounds are generally considered to include those that have boiling points less than that of water or vapor pressures greater than that of water, such as benzene from gasoline, industrial degreasers such as trichloroethylene (TCE), or perchloroethylene (PCE) dry-cleaning solvent.  These volatile compounds are the ones most likely to cause VI problems.   Following a release, these compounds are present in the ground partially in a liquid or solid phase, but they are also present partially in the vapor or gaseous phase.  As a gas or vapor, they can move readily through the soil to nearby buildings.

This mechanism is often intensified during winter heating conditions, when buildings are closed up, and heating systems vent to the outside, creating negative relative pressures inside buildings, sometimes called the ‘chimney effect.’  Even a relatively small relative negative pressure in a building significantly enhances the rate at which vapor intrusion occurs.

Comparing Exposures from VI to those from Soil and Groundwater

In the early days of waste site cleanup, we were focused on exposures and risks arising from soil and groundwater contamination.  While we are still concerned with the potential exposures to these media, limiting them is usually relatively simple (i.e. don’t eat or play in the soil and don’t drink the groundwater). In contrast, limiting exposures to indoor air containing volatile compounds is more problematic because everybody has to breathe continuously.  With experience has come the awareness that for volatile chemicals, VI is probably the most important exposure pathway to control for reducing risk when it is present.

Developing the Right Measurement Tools

While the Massachusetts Department of Environmental Protection became aware of VI issues in the late 1980’s, it wasn’t until they issued a  guidance document in 2002 that we had some state level agency guidance on how to proceed.  The guidance called for an initial field screening to evaluate whether soil gas was beneath a building which might result in vapor intrusion. With hindsight, has come the realization that the field screening methods of the day were not nearly sensitive enough to rule out vapor intrusion.

Vapor intrusion studies now usually rely on collection of soil gas samples in the field that are subsequently analyzed in a laboratory setting with highly sensitive instrumentation.  Alternately, portable gas chromatograph/mass spectrometers (GC/MS) can also be used.  In 2016, the Massachusetts Department of Environmental Protection issued final guidance, which describes the  state of the practice for “investigating, assessing, understanding, and mitigating vapor intrusion” in Massachusetts.

Remediating VI Conditions

The preferred remedial method for VI sites has become the installation of sub slab depressurization system or SSDS.  Borrowing from the radon remediation industry, regulators and environmental consultants realized that the mechanism by which radon gas enters buildings is almost identical to VI, and thus has a nearly identical solution. By capturing the vapor within the soil beneath a building, and venting it to the outside air before it can migrate into a building, we can almost eliminate the inhalation exposure that would otherwise occur in the building. This diagram presents a generalized depiction of an SSDS.


BHN blog figure

In designing an SSDS, environmental practitioners need to consider a number of site-specific factors such as how large an area the SSDS needs to cover, the permeability of the soil below a building, and the characteristics of the vacuum fan required.  A building underlain by a coarse sand or gravel might need only one vapor extraction point to cover the entire footprint, while a building underlain by finer grained soils might need more extraction points or horizontally laid slotted pipe to get the desired coverage.

At OTO, we typically perform an initial evaluation to help design an SSDS with an appropriately sized fan and geometry (i.e., a single extraction point or horizontal pipe). A small system for a residential fuel oil release under a portion of the basement might need a single extraction point with a small fan.  A system like this uses about the same amount of electricity as a standard light bulb.  In contrast, a large industrial property may need a larger system with multiple zones and stronger fans, and will result in a significantly higher electrical bill.

My first experience installing and operating an SSDS came in 2005, as a response to a relatively small residential fuel oil release. Since then, I have had a number of projects including SSDS’s ranging from relatively small and simple to quite complex.  To me, no other innovation in our industry has had a greater impact on our ability to reach acceptable endpoints for vapor intrusion than sub-slab depressurization systems.

As an environmental professional, many of the problems I have worked on have been difficult to solve. However, because of their relatively low cost, ease of installation and ability to improve conditions rapidly, solving vapor intrusion through installation of an SSDS has been a genuine bright spot in my career.




This past October I attended the Soils Conference at the University of Massachusetts and was fortunate enough to sit in on a number of excellent presentations. Despite this positive experience, I was planning to leave early on the last day of the conference, but as I thought about my cluttered desk and pile of unanswered messages back at the office, I decided to stay a little longer and hear three presentations on vapor intrusion assessment.  This proved to be a very good decision as each of them was outstanding.

New VI Assessment Method

The first of these presentations was given by Ms. Lila Beckley of GSI Environmental in Austin Texas; her title was “Vapor Intrusion (or Not): Sniffing out the Source of VOCs in Indoor Air”.   She described a new method for assessing vapor intrusion (new to me anyway) and gave an eye opening description of their techniques.  Her firm has developed a vapor intrusion assessment method that uses real time sampling and analysis of indoor air while the pressure within the building is adjusted from positive to negative.  Based on the pattern of change observed in VOC air concentrations as the pressure is reduced, it was possible to determine whether the VOCs were truly from vapor intrusion or whether they actually originated from a source within the building. 

This technique represents a significant improvement over the methods previously used to assess vapor intrusion. 

A Specialized Application of the Same Technique

Following a short break, there was a presentation on vapor intrusion assessment by Mr. David Shea of Sanborn, Head & Associates that used the same general technique described by Beckley, but with the additional objective of identifying specifically how and where vapor intrusion was taking place in a large building located over a known groundwater contaminant plume.  The example used in Shea’s presentation nicely complemented and expanded on the methods illustrated in the previous talk. 

In summary, both presenters described a highly credible method that appears to quickly and accurately assess vapor intrusion.  The method requires only a 1-2 day assessment, but does need to be conducted with sensitive specialized field instruments.  The method seems capable of differentiating between indoor sources of VOCs in air and those originating from a sub-slab source.

How Good is the Correlation between Sub-Slab VOC Concentrations and Vapor Intrusion?

The last of these three talks was given by Mr. Ben Matrich of Geosyntec Consultants’ Anchorage Alaska office.  His title was “The Case for Less Emphasis on Sub-Slab Data in Decision Making for the Vapor Intrusion Pathway”.  The thrust of his presentation was that investigators have come to place too much emphasis on sub-slab soil gas measurements without adequately understanding their limitations.  The scientific basis for his talk comes from the comprehensive vapor intrusion research the USEPA has sponsored at Sun Devil Manor, a house purchased for this purpose in Layton, Utah.  Investigators are using the Sun Devil Manor (which is located over a contaminated aquifer) to study the detailed mechanisms of how vapor intrusion takes place; the results have upset much of the old thinking about vapor intrusion.

Details of the findings go beyond what I have space for here, but here are some examples.  EPA found that at Sun Devil Manor sub-slab soil gas concentrations can vary more than expected over time, as can the indoor air concentrations caused by vapor intrusion.  Temperature, barometric pressure and wind speed can all strongly affect vapor intrusion rates as well as the resulting indoor air VOC concentrations. Of course these findings all come from a single location and may not be generally applicable.


The simple vapor intrusion models used to develop the early regulatory efforts to limit the vapor intrusion pathway are being revised based on new studies and assessment techniques.  It is reasonable to expect that the approach to vapor intrusion will continue to evolve as additional research is published.

What are USEPA’s New PCE Toxicity Values About?

It has been a year since the USEPA issued its new toxicological profile for tetrachloroethylene (PCE).  The new profile resulted in the revision of PCE’s toxicity values in EPA’s Integrated Risk Information System (IRIS).  Despite their obscurity, IRIS toxicity values carry great importance because they are at the heart of the risk assessment process and thus play a central role in determining the extent of waste site cleanups.

What was unusual about the PCE toxicity value change is that the new values indicate PCE is less toxic than previously thought; this is a rare occurrence because most IRIS values changes have gone the other way.  EPA did not come up with the idea of lowering its estimate of PCE toxicity by itself; it received “encouragement” from a National Research Council (NRC) expert advisory committee.  To EPA’s credit, they solicited the input from NRC, even if not all at the agency were happy with the recommendations they received.  It turned out that NRC placed greater emphasis on higher quality scientific studies (those with more controls and less ambiguous toxicity endpoints) and urged EPA to discount studies of lesser scientific quality.  The higher quality studies indicated that PCE was in fact less toxic than previously thought

Very Interesting, but why is this Important?

EPA’s old PCE toxicity values suggested that PCE was so toxic that even concentrations in air that were too low to measure could pose a serious health risk.  As a result, PCE became a significant driver of cleanup actions at many waste sites where vapor intrusion was a pathway of concern.

As you likely know, vapor intrusion is an exposure pathway whose significance many environmental scientists and regulators consider to have been underestimated in the past. Remedial actions to address vapor intrusion have thus become more common, even in situations previously thought to have been satisfactorily closed-out.  In many of these vapor intrusion situations it has been the presence of PCE in air that drives remedial actions.  With PCE now recognized as being less toxic, some of these remedial actions may not be necessary.

Have State Agencies Adopted the New PCE Toxicity Values?

Much of the waste site cleanup work in the US takes place at the direction of state governments.  Most states and political bodies with waste site cleanup laws specifically cite EPA’s IRIS database as the first choice for all risk assessment toxicity values.  However, some states take an à la carte approach with IRIS; reserving their right to use their own toxicity values when they see fit.  Massachusetts is just such a state and its PCE toxicity values date back to the early 1980s (and have evolved since then), a time when there were no federal standards for PCE in drinking water.  MassDEP (then DEQE for the nostalgic) was responding to a big PCE problem in drinking water pipes and in the absence of federal criteria, it took a commendable DIY approach.

What about New Jersey?

But, this post is not about Massachusetts, it’s about New Jersey and its January, 2013 adoption of EPA’s new toxicity values for PCE. Like Connecticut, New Jersey tried to adopt a semi-privatized waste site cleanup law (modeled on the Massachusetts Contingency Plan), but neither state had the much success with their program..  Some place the blame for this lack of success on the inflexibility of NJ DEP and CT DEEP; I am not quite close enough to either situation to comment.

Now the New Jersey DEP seems intent on getting its privatized waste site cleanup program back on track.  It is breathing new life into its LRSP program and in January of this year it issued final guidance to address vapor intrusion sites.   As part of its vapor intrusion guidance, NJDEP has adopted the new EPA IRIS toxicity values for PCE.  By adopting the EPA values, New Jersey raises the threshold at which remedial action is required at sites with PCE.

Among the states, New Jersey is generally perceived to err on the side of environmental cautiousness and its adoption of the new EPA PCE toxicity factors can only add to the momentum in favor of  nation-wide adoption.   New Jersey is off to a good fresh start with its privatized cleanup program.

Impacts to indoor air quality from volatile organic compounds (VOCs) have been receiving greater attention recently due to a growing awareness of vapor intrusion (VI) from underground oil and chemicals.   VI occurs when chemicals spilled on the ground migrate under structures and then volatilize up into indoor air.  After a recent residential basement oil spill I was called in to provide a second opinion on why high indoor air VOC concentrations persisted in the home after the cleanup had been completed.  Some of the results were very surprising.

Locating VOCs in the Basement

Following the release, a well qualified response contractor had conducted a thorough cleanup.  The remediation included removing portions of the floor slab, wall board, wood framing and most other building materials that had been contacted by the oil.  Despite the cleanup, indoor air concentrations in the basement and first floor of the house exceeded the Massachusetts Department of Environmental Protection criteria.

The contractor suspected the problem was the first course of concrete chimney blocks, which had likely absorbed oil in the aftermath of the spill.  The oil in the blocks was now likely volatilizing into the air.  Removing and replacing the contaminated  blocks presented an obvious structural challenge so initially an epoxy sealant was applied to the entire chimney to prevent further oil volatilization.  However, indoor air testing conducted after the epoxy had cured showed that indoor air concentrations remained stubbornly high.

To assess the cause of the indoor air levels, I visited the subject home with a ppbRAE to see if it would help me locate the source of the organic vapors.  Once in the basement, it did not take long to discover that the epoxy sealant was not preventing VOC migration out of the concrete chimney blocks; the blocks were still off-gassing VOCs to the basement air.  While there were also a few pieces of previously unidentified wood framing off-gassing VOCs, the concrete blocks looked to be the main culprit.

But What’s Going on Upstairs?

With the basement VOC source identified, I went upstairs to check on first floor; what I found there was completely unexpected.  While ambient basement air VOC readings had been just above zero (at some distance from the chimney), ambient levels on the first and second floors were about 200 ppb!  How could this be?  I walked through the house with the home owner trying to identifying potential VOC sources.  After an hour of looking I hadn’t been able to identify a source and almost everywhere in the occupied space I was measuring 200 +/- 40 ppb of VOCs in the breathing zone air; there were no odors.  Big mystery!

Finally, on a high book shelf in the living room I noticed two glass hurricane lamps; each containing several ounces of clear liquid lamp oil.  When I held the tip of the ppbRAE probe over the glass lamp chimneys the instrument’s numerical readout shot up; the mystery of the upstairs VOC source was seemingly solved!  And the source was completely unrelated to the basement oil spill.

What is in lamp oil that causes such a strong response on the ppbRAE?  From my limited on-line research, there does not appear to be a commonly accepted formula for lamp oil.  At one time kerosene was used, but this now seems less common except in outdoor settings.  Whale oil was also once used, a practice now thankfully in the past.  The oil in these lamps had no odor, but beyond that I do not have any information on what it was.  I did not collect a sample for lab testing, so I do not know specifically what the ppbRAE was responding to.

Lessons Learned

This experience was a good reminder of just how sensitive today’s air monitoring equipment has become.  Even very small contributions from sources that do not seem particularly volatile can have a dramatic impact on indoor air testing measurements.  It is important to keep a watchful eye out for unanticipated VOC sources when conducting indoor air testing.

As we all know, one of the hottest topics in the environmental industry right now is vapor intrusion.  In Massachusetts, vapor intrusion considerations have been around for more than 20 years, since the beginning of the Massachusetts Contingency Plan (MCP) program.  This is no surprise, as Massachusetts has been a technical and regulatory leader in the environmental field since the early days.  However, Massachusetts has reached a critical crossroads in the regulation of the vapor intrusion pathway, and stakeholders, especially those who are involved in brownfields redevelopment projects, are hoping that they choose a wise pathway moving forward.

As a rookie regulator at the start of my professional career in 1992, I first became aware of vapor intrusion as an exposure pathway when a colleague of mine was working on a project in Needham.  Apparently, an industrial facility was discharging wastewater containing chlorinated solvents into corroded subsurface drain lines, resulting in groundwater contamination.  The impacted groundwater migrated downgradient to a school and a residential subdivision.  An indoor air testing program detected the solvents in indoor air.  From that event, vapor intrusion considerations were propelled forward, and the concept of MassDEP’s GW-2 standards based on the Johnson & Ettinger Model was born.

MassDEP’s GW-2 standards were based on an early version of this model that had been around since the early 1990s.  However, not long after implementation in late 1993, MassDEP technical staff began questioning whether the model was portraying an accurate picture of what was being observed in the real world.  In the late 1990s, MassDEP began to see a growing body of data collected at sites that indicated the Johnson & Ettinger Model was not providing a consistent prediction of what was being observed.  MassDEP lowered GW-2 standards for many common chlorinated volatile organic compounds and continued its evaluation of the data collected during site assessment and clean-ups.

Fast-forward to where we are today:  MassDEP has issued updated Vapor Intrusion Guidance, currently in draft form dated December, 2010.  While MassDEP’s updated guidance may be the state of the science available at this time, it is a substantial departure from how vapor intrusion has been regulated in Commonwealth over the past 20 years.  As it is currently issued by MassDEP, the updated guidance appears to bring a substantial amount of additional uncertainty to the redevelopment of brownfields sites.  Many brownfields projects are undertaken due to the availability of transferrable tax credits which make these projects economically feasible.  Without the availability of these credits, many projects would fail to receive financing.  However, the availability (and the amount) of the tax credits is dependent on these brownfields projects reaching some type of Permanent Solution.  The updated guidance, as currently proposed, reduces the likelihood of Permanent Solutions at vapor intrusion sites.

While all of us who work on vapor intrusion sites appreciate the hard work that MassDEP has put into the updated guidance, we also hope that the final version will not make it more difficult to achieve Permanent Solutions, especially at brownfields projects.  While we can not predict what the final guidance will include, there are actions that can be done now to reduce the likelihood of regulatory risk on projects now in the pipeline.  These actions include designing projects to include vapor intrusion barriers and properly engineered subslab depressurization system piping in new construction and rehab work.  First and foremost, consideration of potential vapor intrusion issues, and appropriate early due diligence and evaluation, should be at the top of every brownfields project to-do list.