Vapor Intrusion
Introduction
Vapor intrusion (VI) caused by releases of volatile chemicals from contaminated soil and/or ground- water is rapidly emerging as a serious concern with potentially significant impact on thousands of properties across the nation. A vapor intrusion condition (VIC) occurs when these rapidly evaporating chemicals make their way into indoor air. Volatile chemicals such as trichloroethylene, perchlo- roethylene, and benzene may be released from contaminated soil and/or groundwater at properties such as existing or former gas stations, dry cleaners, and industrial facilities. These volatile chemicals may migrate through subsurface soils and into indoor air spaces of overlying structures similar to the way radon gas can seep into homes. The driving force can be convective (bulk flow) or diffusive (concentration) driving force and commonly enters via cracks or openings in the building floors or walls. These vapors can also migrate as a result of pressure differences between the building’s interior (lower pressure) and exterior. This condition can create a negative pressure within the structure that effectively draws the vapors into the building. Thus, contaminated groundwater can pose a potential VI threat to inhabitants of nearby buildings and/or structures.
The concern is widespread. According to the U.S. General Accounting Office (GAO), an estimated 200,000 underground storage tanks currently in operation may be leaking. According to the Environmental Protection Agency (EPA), there are more than 36,000 active dry cleaning facilities in operation in the United States, w'ith more than 75% of them estimated to be contaminated with volatile chemical solvents. Tens of thousands of current and former industrial sites across the United States are contaminated with volatile chemicals. An earlier ЕРА VI guidance document references a total of 374,000 contaminated sites, the National Research Council reports that the number may be as high as 439,000, and an often cited total in brownfields redevelopment literature is 500,000 sites. The fraction of these contaminated sites with conditions favorable for VI also is not known with certainty, but will depend, in part, on the number of sites that contain volatile organic compounds (VOCs). Volatile contaminants have been reported at approximately one-half of all Superfund and similar cleanup sites. Preliminary estimates suggest that approximately one-half of volatile-contaminated sites have conditions that could be favorable for VI. This suggests, therefore, that VI may be an issue at one-quarter of the total number of contaminated sites in the United States [I]. Furthermore, property owners who hold the deed to a property identified as the cause of a VI issue may not only face property value losses and potentially hefty cleanup costs, they may also be the target of class action lawsuits from nearby residents, seeking claims for property devaluation, personal injury, or other types of damages.
Health Concerns
The health effects from chemical vapor exposures vary based on the individual exposed, the chemical involved, and the exposure dose and time [2]. Impacts may include eye and respiratory irritation, headache, and/or nausea. Low-level exposure over long periods of time may also raise a person’s lifetime risk for developing cancer.
The three most commonly considered routes for environmental contaminants to enter the human body are ingestion, dermal contact, and inhalation. The VI route of exposure is inhalation.
VI exposures occur indoors and people in the United States spend much of their lives indoors. People who are unhealthy or who are relatively more susceptible to the effects of toxicants—for example, people who are elderly, ill, or immobile; pregnant women and their developing fetuses; newborns, infants, and toddlers—also spend much of their time indoors.
Children are at a higher risk than adults for both physiological and logistical reasons. Physiology-based studies indicate that there is a twofold greater inhalation dose in children than adults. Also, very young children spend substantial amounts of time at floor level, potentially closer to the location of intruding vapors. They presumably could be exposed to higher concentrations than adults since the molecular weight of the vapors of concern is greater than that of air. Their greater density is more likely to produce higher concentrations at ground (floor) level.
Inhalation is not voluntary. A typical adult is assumed to inhale approximately 20,000 L/day of air and consume approximately 2 L/day of drinking water. Individuals may forego drinking tap water and use alternative sources, but they cannot forgo inhaling air. Obviously, the concentration of contaminants in breathing air is important, and measuring or predicting this concentration is a focus of some VI studies.
As expected, the inhalation route of exposure has been observed to lead to higher toxicities than exposures via the oral route or entry. The higher toxicities for inhalation may reflect the fact that the barrier between contaminated air and the human blood system is as small as a single cell and that these cells are membranes whose purpose is the exchange of inhaled gases w'ith the blood.
Pollutant concentrations of 100 pg/m3 or greater have been observed in indoor air due to VI. Assuming a 24-hour exposure and 20mVday of respiration, the expected adult applied does of 2000 pg/day (2 mg/day) of these toxicants could be significant to the health of some individuals. Even higher levels of exposure are possible. Concentrations for a single VOC of 790 pg/m3 or even 1700 pg/m3 have been observed in indoor air due to VI.
