Environmental Engineering Reference
In-Depth Information
products may be identified through the National Institute of Health Database
( http://hpd.nlm.nih.gov/index.htm ).
Many of the contaminants considered in the vapor intrusion pathway evalua-
tion are also present in outdoor air (Berry-Spark et al. 2004 ; McDonald and Wertz
2007 ), particularly in urban areas. Potential sources of outdoor air impacts include
automobile emissions, manufacturing sites, and locations with significant contami-
nant use (e.g., dry cleaners). In some cases, these outdoor air concentrations may be
greater than the target risk-based indoor air concentrations. Outdoor air quality may
contribute vapors to indoor air at concentrations above target levels. Therefore it is
advisable to include collection and analysis of outdoor air samples in the scope of
work conducted for any indoor air quality survey. It may also be informative to col-
lect outdoor air samples when conducting a soil gas survey in urban areas, because
ambient air can lead to detectable concentrations of VOCs in soil gas when detec-
tion limits are as low as those required to meet soil gas target levels or screening
levels.
In the US, several studies of background concentration of contaminants in indoor
air have recently been compiled (Dawson 2008 ; Dawson and McAlary 2008 ). This
compilation identified several contaminants (benzene, tetrachloroethene, chloro-
form, carbon tetrachloride, and methyl tert-butyl ether) whose average concentra-
tions are similar to the range of common risk-based quality standards. Therefore,
there will very likely be issues to resolve regarding the relative contribution of
subsurface and indoor sources for vapor intrusion investigations at sites where
any of these contaminants are present in the subsurface. Occupant's habits and
commercial products vary considerably from house-to-house, so control properties
may not provide relevant background data. Therefore, it is generally not recom-
mended to include indoor air quality monitoring in homes outside of the study
area in order to assess background concentrations. Alternate approaches have been
used to assess indoor air background concentrations. These alternate approaches
include:
Qualitative comparison to published data for background (indoor) and ambient
(outdoor) air quality.
Use of a tracer compounds to select homes with no (or insignificant) subsurface
impacts. Tracer compounds are compounds that are present in the subsurface
and indoor air at concentrations clearly higher than background levels. This may
include compounds that are not common in consumer products and building
materials (e.g., 1,1-dichloroethene, cis-1,2-dichloroethene), naturally occurring
radon, or other contaminants.
Evaluation of indoor air concentrations prior to and after operation of a sub-
slab venting system (e.g., Folkes 2000 ), or under conditions where the building
pressure is manipulated to be greater than and less than soil gas pressures (Berry-
Spark et al. 2005 ; McHugh et al. 2006 ).
Evaluation of the ratio of indoor air to subsurface concentrations for a large data
set including a wide range of subsurface concentrations (Ettinger 2003 ; Johnson
et al. 2002 ).
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