Introduction

During the past few years, it has become apparent that intrusion of toxic vapors into occupied buildings is a serious and potentially widespread problem associated with contaminated sites—specifically, sites at which soil and groundwater are contaminated with volatile organic compounds (“VOCs”). The phenomenon has long been understood, but it was generally and erroneously believed that it was rare for vapor intrusion in buildings to reach levels that presented health concerns, especially at sites where groundwater contamination had been satisfactorily addressed. We are now learning that the problem is more common, more persistent, and more severe than had been imagined as recently as three or four years ago.

Why Does Vapor Intrusion Occur?

When there is lower pressure inside than outside, soil vapor can enter buildings through cracks and openings in slab foundations or basement floors and walls. This intrusion is very similar to how radon gas seeps into buildings...

The chemicals associated with vapor intrusion problems are VOCs. Among these, some of the most common and the most dangerous are chlorinated compounds such as trichloroethylene (“TCE”), trichloroethane (“TCA”), and tetrachloroethylene (also known as perchloroethylene, “PCE” or “Perc”)...

When VOCs contaminate groundwater aquifers, they can readily volatilize near the water table and pass into the soil pores above. Pressure gradients drive vapors up through the soil toward the surface. [V]apors may accumulate, or move laterally, or continue to move upward through cracks or openings in the obstacle. When the obstacle is a building foundation, and the vapors find a pathway through it, the result is the intrusion of vapors into the interior living or working space.

As noted, it is a difference in pressure above and below foundations that can drive vapors indoors. Such differences can be induced by a variety of common phenomena, including...

Why Is Vapor Intrusion A Concern?

Many common groundwater contaminants are volatile. In their vapor phase they can move readily through soil and into structures. These vapors can be hazardous, even in low concentrations. Chlorinated VOCs like TCE and PCE, when inhaled, are carcinogenic and can cause a variety of other illnesses...

What’s New About This?

Vapor intrusion is not a new phenomenon. We have been concerned about radon gas intrusion for decades, and we have long understood that toxic chemicals volatilizing off contaminated groundwater can similarly migrate up through soil and into buildings...

However, our understanding of the vapor intrusion process has evolved rapidly during the past few years...we have begun to recognize that many old assumptions are invalid or must be modified substantially.

Groundwater As A Source

Old Assumption: Very high concentrations of volatile chemicals would need to be present in groundwater for there to be a potential for indoor air problems.

New Findings: Indoor air problems may occur even when levels of groundwater contamination are quite low. Moreover, the model most commonly used to make predictions about vapor intrusion, the “Johnson-Ettinger” Model, has been found to significantly under-predict indoor air vapor levels in a variety of situations...

Soil Vapor Levels As Predictor

Old Assumption: Soil vapor samples collected near a building, at foundation depth, are representative of soil vapor conditions beneath the building’s foundation, and can therefore be used for screening.

New Findings: At Endicott and elsewhere, it has been found that vapor levels beneath the foundation may be more than a hundred times greater than in samples collected near the building. The foundation can act as “confining layer” beneath which vapors accumulate and concentrate.

Attenuation (Or Dilution) Factor

Old Assumption: The ratio of vapor levels in soil to those inside a building is high... The foundation was assumed to be effective at blocking most of the vapors from entering the building, and those vapors that do get in become diluted in the ambient indoor air. Thus, indoor vapor levels are assumed to be many times lower than the soil vapor levels outside...

New Findings: Attenuation factors or ratios actually vary widely, and are often much lower than previously expected...

Implications Of The New Findings

One of the important implications of the new findings superseding the old assumptions is that groundwater VOC contamination sites which were considered to have been adequately addressed may require further investigation and remedial work...

The major implication of the new findings is, of course, that human exposures at potentially dangerous levels may have occurred for years or decades, even after a site was recognized and (as we thought), satisfactorily addressed. We may presume that our relative ignorance in this arena will unfortunately have contributed to some number of additional cancers or other illnesses that could have been prevented.

A related implication is that there is the potential for significant toxic tort liability for responsible parties...

Human Exposures

A growing body of evidence suggests, however, that a thorough risk assessment should include sampling of the vapors that may have accumulated immediately below the foundation or concrete slab of the building (sub-slab sampling). These levels are likely to be higher than current indoor levels, but nevertheless represent the potential (or possible future) exposure to the occupants...

Sampling For Vapor Intrusion

Based on the new findings replacing the old assumptions, best professional judgment may suggest that sampling for vapor intrusion should be performed in many more situations than would formerly have been thought necessary. Sampling may well be indicated where groundwater is contaminated with even comparatively low levels of volatile hazardous chemicals, the contaminated aquifer is located under or near occupied buildings, and the local geology is conducive to soil vapor movement...

[C]urrent evidence suggests that less reliance should be placed on outside soil vapor sampling; instead, sampling should more frequently proceed to indoor and sub-slab sampling, to assess both current and potential future risks...

Vapor Intrusion In The Workplace

There are existing federal regulatory standards governing occupational exposures— the Permissible Exposure Limits (“PELs”) set by the Occupational Safety and Health Administration (“OSHA”). The PELs for some of the common contaminants of concern are, however, seriously outdated, and are almost without question not protective. For example, the PEL for TCE, established in 1967, is 537,000 micrograms per cubic meter, an incredible seven orders of magnitude less protective than the lowest figures in use today...

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