For environmental investigations, the term "background" generally refers to levels of contaminants that are either present due to naturally-occurring phenomena or those otherwise present in the absence of a release of that substance to environmental media (USEPA 2002c). For this appendix, “background” refers to indoor air levels of contaminants that are present in the absence of a release to the environment or in the absence of a complete PVI pathway. These VOCs may be present as a result of consumer products; hobbies; indoor or adjacent storage of VOC sources (USEPA 1992); residential heating fuels; household or building materials (Otto et al. 1990; USEPA 1998); and other sources (USEPA 1987; Brown et al. 1994).
Since background levels in indoor air may confound vapor intrusion investigations, the role of indoor air background has received attention from regulators and the regulated community and requires careful consideration (EPRI 2005; USEPA 2011). VOCs such as BTEX require particular attention, since these compounds are common to personal indoor air (Gordon et al. 1999; Clayton et al. 1999; Kinney et al. 2002;Sexton et al. 2004) and outdoor ambient air sources (USEPA 1988b; USEPA 2000).
BTEX levels have been identified in new (finished but unoccupied) prefabricated and site-built houses (Hodgson et al. 2000), and higher background levels of VOCs have been observed in homes with attached garages (Kurtz and Folkes 2004; Graham et al. 2004). Such VOCs are also commonly found in commercial buildings (Daisey et al. 1994; Girman et al. 1999). Indoor air background levels of BTEX VOCs have been identified at statistically higher levels than those collected at soil vapor intrusion investigations at MGP sites (EPRI 2007). Accordingly, investigators should pay careful attention to indoor air background sources at PVI sites. In the event that indoor air background levels have confounded a PVI investigation, forensic analysis may be required to assess the relative contribution from subsurface and background sources, possibly requiring multiple lines of evidence and multiple methods of interpretation (Plantz et al. 2008). Other site investigation methods detailed in Chapter 4 may be used prior or in addition to indoor air sampling if there is a concern that indoor air background sources will confound a PVI investigation.
For a general discussion of background sources, see Section 1.6.1 of the ITRC VI guidance document (ITRC 2007). For a more detailed discussion of indoor air sampling and background issues (including other sources) seeSection 3.5.4 and Section 3.7.3 of the ITRC VI guidance document.
In addition to the references included in Section 5of the ITRC VI guidance and those named above, the following studies have been performed regarding background concentrations: