Contaminant Risk Factors - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 366

posed by specific contaminants to air, termed the Air Contaminant Risk Factor (CRF AIR ),
can be computed by modifying the equations of the CRF GW and CRF SOIL (Kaufman et al.
2005; Rogers et al. 2007b).
The CRF AIR is expressed in general terms as Equation 10.7 (Kaufman et al. 2005; Rogers
et al. 2007a).

CRF = Toxicity Mobility × [( × Persistence ) + ( Mobility × Persistence )]

AIR gas gas par ticulate particulate

(10.7)

Toxicity values are obtained from the USEPA IRIS (2009a). The values selected should
be the most conservative for each exposure pathway—ingestion, dermal adsorption, and
inhalation. Using the most conservative value is appropriate since exposure to contami-
nated air can occur in each of these pathways. For instance, dermal adsorption of soil with
sorbed contaminants can occur during any outdoor activity, ingestion can occur through
hand to mouth contact and from swallowing, and inhalation can occur through inhala-
tion of contaminants in the gas or particulate matter, especially if the particulate matter
is very fine.
Mobility for air is represented separately for contaminants in the gas phase and for solid
particulate matter, and is combined with persistence. This separation is necessary because
the half-life of many contaminants is significantly different depending on whether the
contaminant is in the gas phase or solid particulate matter. For instance, xylene has a half-
life of 6 days in the vapor phase, but its half-life is 1 year when present in the particulate
phase.
To evaluate mobility in the gas phase, Henry's law constant alone is sufficient because
a retardation factor is not necessary. The mobility of atmospheric contaminants in the gas
phase can be expressed as in Equation 10.8.

M H = ( )

(10.8)

where
M is the mobility
H is the Henry's law constant

As with the CRF GW and CRF SOIL , Henry's law constants are derived from Equation 10.2,
or from the literature including USEPA (1996a); Sander (1999); Wiedemeier (1999); Suthersan
and Payne (2005); Payne et al. (2008).
The mobility of a contaminant sorbed to particulate matter is represented by
Equation 10.9.

1

M

) =

× Koc

(10.9)

( particulate

SpG

where
M (particulate) is the mobility of a contaminant sorbed to particulate matter
SpG is the specific gravity
Koc is the partitioning coefficient

Specific gravity is an important determinant of mobility because as the specific gravity
of a contaminant increases, its buoyancy in the atmosphere decreases. This relationship