One Regulatory Perspective on the Vapor IntrusionPathwayby John Fitzgerald

Environmental regulations should integrate and balancescientific principles and uncertainties with stakeholder andpublic policy interests. They should be clear, defensible, fair,and sufficiently protective. The vapor intrusion pathway isa vexing challenge in this regard for regulators; it is a ‘‘perfectstorm’’ of technical, logistical, and risk management issues.

This pathway is finally getting the attention it deserves.For many years, regulations, decisions, and actions at con-taminated sites focused almost exclusively on real orpotential impacts to drinking water supplies, often in sub-urban and rural areas. Although this was and remainsa concern, we now understand that significant, real expo-sures can occur in urban areas where the ground water isnot used for potable purposes, including inner city andEnvironmental Justice areas.

In hindsight, the concern is obvious. On average, weconsume roughly 2 L of drinking water per day. Duringthat same 24-hour period, we inhale about 20,000 L of air.Thus, concentrations of concern (those producing equiva-lent exposures) are three orders of magnitude smaller forindoor air than for ground water (when expressed in com-parable mass/volume units). Although there are importantexposure duration and absorption factors to consider, thesheer magnitude of that ratio is compelling.

Why did it take so long to recognize the significanceof the vapor intrusion pathway? Why are many regulatorsstill struggling with this issue? Like the pathway itself, thereasons are varied and complex.

Technical ChallengesThe technical complexities of the vapor intrusion path-

way are by now well recognized, if not completely under-stood. These complexities are in large part due to thevagaries of the subsurface world, and the interactionsbetween it and the world above.

Regulators are now accustomed to the notion of a het-erogeneous subsurface, from waste and contaminated filldeposits to complex hydrogeologic systems and plumes.

However, vapor intrusion superimposes on this the spatialand temporal variability of subsurface vapor transport andthe interactions among building construction, building use,building system operations, and subsurface chemicalmigration processes, which makes understanding andanticipating impacts from this new pathway difficult.

The vadose zone is also new territory for many regula-tors, who are often more familiar and comfortable withcontaminant transport in the saturated zone. There are newconcepts, equations, site investigative tools, and approachesto learn. For example, although ground water flow in anoverburden aquifer can usually be evaluated and discernedon a macroscale, with reliance on potentiometric data andthe sure, comforting force of gravity, the advective transportof subsurface vapors can seem to be a fickle, microscalephenomenon, subject to diverse and varying barometric andanthropogenic pressure gradients.

Unfortunately, increased complexity dictates the needfor robust data sets, which seems to be the conclusionfrom recent research efforts (Dawson 2008). This thenleads to the ancillary challenge of budgetary constraints(more data generally equals more cost), further strainingthe dynamic balance between a regulator’s need for dataand the private (and public) sector’s ability and willingnessto pay for that data.

Logistical ChallengesLogistical issues are unusually problematic in vapor

intrusion cases. As every regulator knows, obtaining envi-ronmental data ‘‘off property’’ can be a difficult proposi-tion. Historically, such efforts have focused on groundwater characterization and drinking water impacts, with in-vestigations that use public roadways and area-wide dataextrapolations. Although generally sufficient for thatobjective, such an approach is only a starting point forvapor intrusion investigations, which usually requirea building-by-building evaluation.

This approach can be especially traumatic, not only forthe occupants of the building but also the responsibleparty, who must inform these occupants of this concernand obtain an access agreement to conduct needed investi-gations. And unlike simply obtaining a tap water sample,

Copyright ª 2009 The Author(s)Journal compilationª 2009National GroundWater Association.

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vapor intrusion investigations can be highly invasive (e.g.,subslab soil gas vapor probes) or long term (e.g., indoorair monitoring) or both. They may also involve requests toempty the structure of VOC-containing products or closingwindows and terminating HVAC operations for severaldays (the latter being somewhat controversial in light ofthe increased exposure potential).

Risk Management ChallengesLastly, and perhaps most importantly, the vapor intru-

sion pathway presents unique exposure, risk communica-tion, and risk management issues.

Regulators often deal with theoretical or transientexposure concerns. This is typically not the case at vaporintrusion sites, where exposures are very real, direct, andchronic. Beyond the mechanism of exposure is the natureof contaminant receptors, which are often sensitive popula-tions such as pregnant women and young children, whomay spend a good deal of time within an impacted build-ing (e.g., school or home).

What is most unique about the vapor intrusion pathway,however, is the ‘‘inescapable’’ nature of exposure. Specifi-cally, one can choose to eliminate contact with contaminatedsoils by avoiding the areas of concern. One can choose toavoid ingesting contaminated drinking water by purchasingand consuming bottled water. Conversely, one cannot avoidbreathing within an impacted structure. In the occupant’smind, this exposure is omnipresent, constant, and toxic.

This is one of the factors that lead to a high ‘‘outrage’’condition at vapor intrusion sites, as that term is used inthe standard risk communication axiom of risk ¼ hazard +outrage (Sandman 1987). There are many others: Theexposure is involuntary. It is beyond an occupant’s control.It is unfair, especially when neighboring structures are notimpacted, or when viewed as one more insult in an histori-cally industrialized and degraded area. And it involvesexotic-sounding chemicals that are often proven or sus-pected human carcinogens, which in turn manifests a dread-ing of an incurable disease.

This outrage can complicate all facets of the investigationand remediation of the pathway, including obtaining accessagreements, conducting long-term monitoring programs,instituting remedial measures, and, most importantly, decid-ing ‘‘how clean is clean?’’ Nowhere is this more true than atelementary schools and daycare centers, where outraged pa-rents often demand immediate actions and zero exposure.

Which leads to the most subtle question and delicateissue of the vapor intrusion puzzle: What is zero exposure?

Routine indoor testing procedures (e.g., EPA MethodTO-15) can now readily achieve parts-per-trillion detectionand reporting limits for many common volatile organiccompounds. This leads to routine reporting of low levelsof analytes that may be attributable to ‘‘background’’ con-ditions unrelated to a subsurface vapor intrusion pathway(e.g., offgassing from building materials, emissions fromhousehold cleaning or cosmetic products, infiltration ofcontaminants from the outdoor air). Discerning the differ-ence is an investigatory challenge, most often involvingthe generation and evaluation of multiple lines of evidence

(e.g., ground water, soil-gas, subslab, and ambient airdata). Explaining the difference is a risk communicationchallenge: the levels of PCE from the dry-cleaned coat areacceptable, whereas the lower levels of PCE entering thebasement are not. Such explanations strain the sensibilitiesof many, who incorrectly assume that ‘‘safe’’ is an abso-lute, not relative regulatory concept.

Where risks are deemed unacceptable, risk mitigationmeasures must be considered and implemented.

The good news is that the vapor intrusion pathway canusually be adequately controlled or eliminated in a cost-effective manner by some combination of sealing cracks,venting sumps, modifying HVAC operations, and installingsubslab depressurization (SSD) systems. The bad news:these are often temporary fixes with potential long-termmonitoring requirements and, in SSD systems, invasive ac-tions subject to long-term operation and maintenance obli-gations. There is also some debate as to where theseactions fit into the regulatory continuum of site remedia-tion and closure.

Interestingly, and somewhat uniquely, the nature of thepathway is such that it is often more expedient and cost-effective to install an SSD system within a building than toundertake the investigatory actions needed to ascertainwhether the pathway is even present. This presents adilemma: while immediate, conservative, and health-protective remedial actions are standard regulatory dogma,installing and operating these systems can be a burden tothe owners of the impacted structures, and may precludefuture findings on whether a pathway of significance iseven present. This in turn raises questions on when and ifthe system can be shut down, and whether regulatorycleanup and closure of this portion of the site or contami-nated area have been achieved.

ConclusionsVapor intrusion represents a new, unique, and complex

regulatory challenge. With time and effort, however, ade-quate solutions will continue to develop and evolve.

Continued research and the publication and dissemina-tion of pertinent findings will lead to better regulation.Further thought and debate on the public policy and riskmanagement aspects of vapor intrusion will lead to moreconsistent and transparent positions and actions. Improvedrisk communication strategies will lead to more effectiveand acceptable resolutions.

The biggest challenge may well be to come to termswith site- and building-specific variations and uncertaintiesthat are likely to remain a prominent and defining elementof this phenomenon, and to seek and maintain the rightbalance between competing interests.

ReferencesDawson, H. 2008. EPA vapor intrusion database, preliminary analy-

sis of attenuation factors. Presented at the Vapor Intrusion Work-shop, AEHS Spring 2008, San Diego, California, March 13.

Sandman, P.M. 1987. Risk communication: Facing public outrage.EPA Journal 13, November: 21–22.

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