Vapor Intrusion Fundamentals – Recognizing Background Vapor Sources


Vapor Intrusion Fundamentals 

Recognizing Background Vapor Sources

One of the things that makes vapor intrusion tricky is figuring out which constituents in indoor air are not from vapor intrusion. Indoor air samples nearly always contain vapors from outdoor ambient air, and from myriad substances within the building, which we refer to as “background”. In this issue of Focus on the Environment, we’ll discuss how we distinguish between vapor intrusion and background.


If you’ve done environmental assessments of soil or groundwater, you’re probably already familiar with the background, i.e., the contamination that’s already present. For example, if groundwater is contaminated upgradient of a site, its background, and the property owner is typically only responsible for groundwater contaminants added by their site. Similarly, soil often contains arsenic, lead, or other toxic metals, but if one can prove that they’re present at naturally occurring concentrations, they’re written off as background.


In vapor-intrusion investigations, the background is especially complicated, for several reasons:


  • Some background constituents, notably benzene, are present in over 90% of residences
  • Background concentrations can vary by orders of magnitude over the course of a day
  • Commercial/industrial (CI) air limits under the Occupational Safety and Health Administration (OSHA) may be thousands of times higher than vapor-intrusion limits, making indoor sources potentially huge in occupational settings


The graph below is from the US EPA’s Background Indoor Air Concentrations (2011), which compiled data from a number of investigations that measured indoor air in residences with no known or suspected vapor intrusion.


You can see that the BTEX compounds (benzene, toluene, ethylbenzene, and toluene), which are associated with petroleum hydrocarbons, are ubiquitous in the background. The three main risk drivers, in my experience, are benzene, chloroform, and trichloroethene (TCE), all of which are present as background in over 40% of residences. Using Ohio standards of 1 in 100,000 Excess Lifetime Cancer Risk for carcinogens, and a Hazard Index of 1.0 for non-carcinogens, benzene, chloroform, and TCE have residential indoor Vapor Intrusion Screening Levels (VISLs) of, 3.6, 1.2, and 2.1 micrograms per cubic meter (ug/m3), respectively. Thus, residential screening levels for chlorine and TCE are within the range of background concentrations, according to EPA’s report (parenthetical values in the graph above).


Vapor concentrations can fluctuate dramatically in indoor air, but background vapors are especially prone to temporal variation, due to their abrupt release from product use, such as when spraying from an aerosol can. And as we’ve discussed in the November 2014 issue of Focus on the Environment, Permissible Exposure Limits (PELs) under OSHA are often vastly higher than VISLs, but some states, including Ohio, don’t allow you to exclude compounds from consideration, even when they’re used in the workplace. You’re expected instead, to figure out how much of a compound came from products, and how much came from vapor intrusion.


So, how do we know the difference between vapor intrusion and background? We have to answer the following questions:


  • Are indoor vapors attributable to indoor product use? Indoor-air sampling is normally preceded by an occupant survey and site walkover for indoor sources. Unfortunately, these can be time-consuming in CI settings, and uncomfortable in residential settings. No one likes discussing their antiperspirant spray or gun-cleaning fluid. Building occupants are asked to refrain from using vapor-generating substances prior to sampling, but even if they do, residual vapors will continue to outgas from sorbent materials at some level.
  • Are indoor vapors present in outdoor air? To answer this question, outdoor ambient air should always be collected with indoor air, analogous to collecting upgradient groundwater. If for some reason ambient air was not collected with indoor air, it may be useful to compare the results to regional data, such as Ohio EPA’s Air Toxics Report (2010), which provides ambient vapor concentrations for various counties.
  • Are indoor vapors among the common background constituents? As shown in the above graph, a number of compounds are present in more than half of the tested residences.
  • Are indoor-air detections common laboratory contaminants? Some chemicals, especially methylene chloride, but also acetone, 2-butanone (MEK), and cyclohexane are prone to show up in samples, and may be detected in some samples but not others.
  • Are contaminants from the air containers? “Summa” cans sometimes contain residual contamination from previous sampling events. For an additional charge of, typically, $30 to $50, the lab will clean-certify individual canisters, which consists of filling them with helium or other carrier gas and testing it, prior to field use. Any detections are treated as blank values when validating samples from the corresponding canister.
  • Is the list of indoor-air detections consistent with those found in soil gas? If contaminants are present in indoor air, but not soil gas, they probably have an indoor source. This is why vapor-intrusion assessments typically consist of sampling soil gas first and restricting the list of analytes during indoor sampling to those observed in soil gas. Obviously, this increases costs and lengthens the timeframe needed for evaluation.
  • Are the ratios of indoor-air/sub-slab soil-gas concentrations consistent with each other? The only thing that sub-slab soil gas undergoes when it enters a building is dilution, and the amount of dilution is the same for all compounds. Consequently, vapors with relatively higher indoor-air concentrations are suspected of having indoor sources.
  • Are indoor/sub-slab ratios consistent with the conceptual site model (CSM)? EPA’s vapor intrusion database (2012) shows a median ratio (a.k.a. Attenuation Factor) for indoor air to sub-slab soil gas in residential settings of 0.003. If the Attenuation Factor is higher than 0.03, the indoor air is probably the result of the background. Soil gas is usually diluted more in CI settings, due to their higher ceilings and greater air-exchange rate, so the ratio of indoor-to-sub-slab concentrations should be closer to 0.0004 or 0.0005. As such, indoor-air concentrations more than 1% higher than soil-gas concentrations are probably background.


In the end, we try to explain away indoor contaminants suspected of being background, but we generally can’t adjust the concentrations or strike suspected background constituents from indoor-air data. Nevertheless, with proper sampling, analysis, and data interpretation, the effect of background on vapor-intrusion data can be minimized.