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Vapor intrusion occurs when volatile chemicals migrate from contaminated groundwater or soil into a building. Volatile chemicals can emit vapors that may migrate through the subsurface and into indoor air spaces of overlying and nearby buildings in ways similar to that of radon gas seeping into homes (see text box). Most volatile chemicals are volatile organic compounds (VOCs), but some semivolatile organic compounds (SVOCs), such as petroleum products, and inorganic constituents, such as elemental mercury and radon, can emit vapors leading to vapor intrusion. (See also EPA's Vapor Intrusion Webpage)
Radon, a colorless, odorless gas, is formed from the decay of radium, a radioactive element that occurs naturally in the bedrock and soil. Radon poses a threat to the health of building occupants once the gas migrates at high enough levels from soil and rock into homes and the work place. Vapor intrusion is similar to the behavior of radon. As a result, the mitigation approaches developed for vapor intrusion are often similar to those for radon. For more information on health risks, as well as detecting and mitigating radon in homes and businesses, visit EPA’s radon page.
In extreme cases, the vapors may accumulate in homes and other occupied buildings to levels that may pose near-term safety hazards (e.g., explosion), acute health effects, or odor problems. Typically, however, the chemical concentration levels with vapor intrusion are low, and the odor unnoticeable. Depending on site-specific conditions, the vapors may not be present at concentrations detectable by analytical instruments. In buildings with low concentrations of volatile chemicals, the main concern is whether or not the chemicals pose an unacceptable risk of chronic health effects due to long-term exposure to these low levels. A complicating factor in evaluating the potential risk from chemical exposure due to vapor intrusion is the common presence of some of the same chemicals from sources with the building (e.g., household solvents and paints, gasoline, drycleaned clothing, and cleaning agents) that may pose, separately or in combination with vapor intrusion, a significant human health risk.
Figure 1 illustrates a simple conceptual model of the vapor intrusion pathway the upward movement of vapors from a soil and groundwater source toward and into buildings.
Vapor intrusion is a standard consideration during investigations at hazardous waste sites, especially those related to the Resource Conservation and Recovery Act (RCRA); underground storage tanks (USTs); and the Comprehensive Environmental Response, Compensation and Liability Act (CERCLA or “Superfund”).
Review of Recent Research on Vapor Intrusion
Fred D. Tillman and James Weaver, U.S. EPA National Exposure Research Laboratory
EPA/600/R-05/106, 47 pp, September 2005.
Reviews research in the area of vapor intrusion of organic compounds into residential buildings including challenges in evaluating the subsurface-to-indoor air pathway, fate and transport mechanisms affecting vapors, Federal regulations and proposed guidance, site studies published in scientific literature, and published approaches to modeling.
Petroleum Vapor Intrusion Compendium
U.S. EPA, Office of Underground Storage Tanks website, 2011
U.S. Environmental Protection Agency, Office of Underground Storage Tanks (2011) has prepared this online resource with vapor-intrusion literature lists related to petroleum releases and cleanup.
Superfund Vapor Intrusion FAQs
56 pp, February 2012.
A team of Headquarters and Regional EPA staff developed answers to a collection of Frequently Asked Questions (FAQ) about VI at Superfund sites. These FAQs provide information and recommendations based on experiences garnered over the last few years in addressing VI at Superfund remedial and removal sites and cleanups undertaken using other statutory authorities. The primary purpose of this document is to help Superfund site managers understand some of the key concepts related to assessment and mitigation of the VI pathway and to help foster appropriate consistency in technical approach. Other OSWER cleanup programs may find it helpful to consider parts of the FAQs for their own specific needs.
Petroleum Hydrocarbons And Chlorinated Hydrocarbons Differ In Their Potential For Vapor Intrusion
U.S. Environmental Protection Agency, 13 pp, September 2011.
Discusses and compares petroleum vapor intrusion (PVI) and chlorinated solvent vapor intrusion with respect to processes that influence whether and how vapors can migrate through vadose zone materials into buildings and other confined spaces as well as some implications for addressing PVI.