August Mack Newsletter | October, 2018

Vapor Intrusion: Is Your Data Telling You The Truth?
by Pilar Cuadra

Vapor Intrusion occurs when there is a migration of vapor-forming chemicals from any subsurface source into an overlying building. Over the last several years, Vapor Intrusion exposure pathways has become a key factor guiding the regulatory closure process, thus it’s important to fully understand and be fully engaged in the vapor intrusion data collection process.  The Indiana Department of Environmental Management (IDEM) and many other state agencies offer guidance that include  a series of steps to follow along with your data collection that compliment and assist in the data interpretation process.  Without the information gathered during those complimentary steps, your analytical results are, at best, incomplete and have the potential to generate data that does not truly represent the concentration of contaminants coming into the indoor air through the subsurface soil.  The following paragraphs provide an overview of a few key issues to consider when collecting vapor intrusion data and interpreting those results.

When to Sample? Winter and Summer Worst-case Scenarios

Most regulatory agencies require vapor intrusion sampling (particularly indoor air samples) during the hottest and coldest seasons of the year.  The rationale behind this approach is not arbitrary, and is not a sadistic request by the regulatory agency to make sure our field crews are outside during the hottest and coldest days of the year. Rather, these seasons are thought to represent the most conservative conditions. During the winter worst-case scenario (indoor temperature typically 10 degrees higher than the outdoor temperature), it is assumed that the home or commercial building heating has been running consistently for a few weeks and that windows have not been opened for a long period of time. This represents a worst-case exposure scenario for any potential receptors living or working within the structure and the indoor air sampling results should be a good indicator of the highest concentrations of contaminants of concern that could make into the building. A similar scenario occurs during the warmest summer months, when the temperature inside the structure is about 10 degrees cooler than the outside temperature (as a result of air conditioner operation).  During this cooling season, the windows and doors of the home or structure are generally closed for long-periods of time.  Much like the winter worst-case scenario, sampling during the hottest times of the year captures the conditions that represent the potential highest exposure to the contaminants. If samples collected under worst-case conditions do not exceed action levels, then it can be assumed that unsafe conditions will not occur unless contaminant concentrations in the source area increase, conversely, if indoor air sampling does not occur during worst case conditions, an additional set of indoor air samples collected under worst case conditions might be necessary to confirm the initial results.   

We Need the Info - The Building Checklist

Prior to any indoor air sampling event, standard practice is to submit a “Building Checklist” to the property owner to complete.  The underlying purpose of that checklist is two-fold: 1) to determine the best sampling locations, and 2) understand the potential for “background” condition within the structure. It is important the homeowner be as accurate and complete as possible when completing this checklist.   

The overall structure of the building (presence of crawl space, basement, slab) and the conditions of the structure (cracks in the floor, types of windows, age of the structure, etc.) must be understood in order to develop an optimal sampling plan for the building.    The checklist also helps document the habits of the current residents/workers and if they could be contributing to the presence of any of the contaminants of concern in the indoor air. For example, if one of the residents/workers routinely smokes cigarettes inside the structure or smoked cigarettes inside the structure within the 24 hours prior to the sampling event, it would not be surprising to have detectable amounts of benzene and combustion by-products in the analytical results. A similar concern is raised when chlorinated solvents are the contaminants of concern and the residents are regular users of dry cleaning services, or have cleaning solvents in the home.  It is important to document the last time that freshly dry cleaned clothes were brought into the home (if recent) because in that case the sampling method could detect perchloroethylene (PCE), a common dry cleaning product. Failing to properly document the required information in the checklist could result in the misinterpretation of an actual VI condition.

What About All Those Paint Cans and Cleaning Supplies

The building checklist aids in identifying all the potential sources of contaminants of concern that can be present in your indoor air.  These sources of indoor air contamination (non-VI) include many household cleaning products, paints, varnishes, paint thinners, tool cleaners, adhesives, and other commercial products that potentially contain some of the same chemicals tested for during vapor intrusion sampling. These household products need to be removed from the structure 24-hours prior to the beginning of the sampling event and an inventory of the products should be completed. Their presence inside the house during the sampling event can bias the sampling results high. Yes, the contaminant is in your indoor air but, in that case, it could be because of vapors from the household products and not from vapor intrusion from the subsurface.  

Watch Out For Rain!

It is important to follow the guidelines and refrain from conducting vapor intrusion sampling for 72 hours if a significant rain event has affected the area. The soil moisture content affects the migration of contaminants through the subsurface and will affect your results. The amount of rain considered significant would vary depending on the amount of moisture already present in the soil from past events but a good rule of thumb is to delay the sampling event if over one inch of rain is recorded in the area in the 24-48 hours prior to the sampling event. 

As explained above, there are many factors that can affect the results from a vapor intrusion sampling event. The data needs to be analyzed and interpreted as a whole, taking into consideration the many additional factors that can affect it. The potential effect of these factors should be minimized as much as possible before sampling, but this is not always possible. It is important to have the right team of professionals planning and conducting your sampling events and evaluating your data. If you have additional questions about Vapor Intrusion sampling, do not hesitate to contact us, we are happy to assist you!

To learn more about this topic, register to attend our free webinar, Vapor Intrusion: Is Your Data Telling You The Truth?, on January 16, 2019 @ 3:00pm.


Pilar Cuadra has over eleven years of experience in environmental consulting and is a Project Manager with August Mack Environmental, Inc. in its Indianapolis, Indiana office. . She is a Licensed Professional Geologist in the State of Indiana. Her professional experience and technical expertise include geophysical, geological and hydrogeological investigations; soil and groundwater sampling and data evaluation; remedial evaluation selection, design, and construction. She has performed a wide range of project management related work on hazardous and non-hazardous sites including commercial gas stations, dry cleaning facilities, machine shops, former wood treatment facilities and former manufactured gas plants. Ms. Cuadra is familiar with the State of Indiana’s Voluntary Remediation Program (VRP), the State Cleanup Program (SCP), the Underground Storage Tank (UST) program, the Excess Liability Trust Fund (ELTF), the Indiana Department of Environmental Management (IDEM) Risk Integrated System of Closures (RISC) and the IDEM Remediation Closure Guide (RCG). Pilar can be reached at

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