Correctly assessing the Vapor Intrusion (VI) exposure pathway is a complex process, which is rarely straight forward. When VI does occur at or near hazardous material cleanup sites, it rarely results in a constant stream of contaminants into the breathing zone. Rather, there are many variables that affect the way vapors move from the subsurface into a structure. Daily and seasonal temperature fluctuations result in variable pressures within and beneath buildings that change VI conditions. The amount of precipitation that falls also affects vapor intrusion rates, and can at times temporarily stop it altogether. Building foundation and construction details can affect the pathway that intruding vapors may take once beneath or inside a structure, thereby resulting in a distribution of indoor air contaminants that aren’t intuitively consistent with predictive VI rules of thumb. Complicating things even more is the fact that numerous household products contain the same chemicals that we are trying to assess from below ground contamination. Taking into account all of these variables to determine if a human inhalation exposure is in existence, or may be in existence at some time during the year, is a highly detailed and specialized process.
Fortunately, the VI team at EnviroForensics maintains a line of highly specialized, field-based, analytical instrumentation that can be used to tease out the necessary details during VI assessments. One of the most highly developed instruments in our arsenal is a portable Gas Chromatograph/Mass Spectrometer, named HAPSITE. The HAPSITE was originally designed by its manufacturer for the U.S. Military and is designed to deliver real time qualitative and quantitative, laboratory-quality results for air samples in the field. The HAPSITE has the capability of identifying and measuring specific volatile organic compounds as low as the parts per trillion (ppt) range. The HAPSITE instrument can be calibrated to measure the ions specific to the contaminants of concern (COCs) at the Site.
The Enviroforensics VI team consists of a certified HAPSITE operator. When operated in ‘survey’ mode, the HAPSITE instrument provides real-time data, depicted as an on-going line graph on the front panel display of the instrument. We use this survey mode during VI assessments to find specific points in a structure where vapors may be entering the indoor air. We have found that cracks in floors and walls, floor drains, pipe chases, interior wall spaces and attics are all important features to explore. Background sources, such as those common household products mentioned earlier, can also be found hiding in closets, drawers, basement corners, etc. When an area of concern is located, the HAPSITE is switched to ‘run’ mode. This sampling mode collects an air sample that is analyzed by the instrument with laboratory quality, in a short period of time. This type of real-time analysis allows for adjustments to be made in the field and provides comprehensive conclusions with fewer mobilizations and sampling events.
During any VI assessment, the main line of regulatory thought is that if VI is occurring at any time during the day or year, that worst case exposure condition is used to calculate the occupant’s potential risk. By using highly-specialized instrumentation, such as the HAPSITE, the EnviroForensics VI team greatly improves the odds that we can interpret all of the variables that are part of VI assessments and properly report true exposure conditions.
