Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

(1979) observed that “the smaller the lead particle, the higher blood lead level.” Similar
observations were made by Healy et al. (1992) using and an in vitro dissolution technique.
Drexler (1997) presented in vitro results on numerous Pb-bearing phases ranging in particle size
from 35 to 250 µm. While all phases studied showed increased bioavailability with decreasing
particle size, more significantly, not all forms showed the same degree or magnitude of change
(Figure AX7-1.1.2). Atmospheric particles are generally found to occur in bimodal populations:
fine; 0.1 to 2.5 µm and coarse; 2.5 to 15 µm. This distribution is both a function of there
transport mechanism and emission source. Although the upper size limit for particles that can be
suspended in air is about 75 µm (Cowherd et al., 1974), other means of mechanical entrainment
(saltation, and creep) can transport particles as large as 1000 µm, supporting the importance of
fugative emissions on media contamination. In addition, particle size can change post
depositional, as soluble forms re-precipitate or sorb onto other surfaces. Limited data are
available on the particle-size of discrete Pb phases from multimedia environments. One example
is the study by Drexler, 2004 at Herculaneum, Missouri. At this site, galena (PbS) was the
dominant Pb species with mean particle-size distributions of 4, 6, and 14 µm in PM10 filters,
house dust, and soils, respectively. These findings support the conclusion that aerial transport
was the primary mechanism for Pb deposition in residential yards. Finally, such laboratory data
have been supported by extensive epidemiologic evidence, enforcing the importance of particle
size (Bornschein et al., 1987; Brunekreef et al., 1983; Angle et al., 1984).
Particle Lability
The impact on bioavailability of a metal particle’s lability (its associations within the
medium matrix) is not well documented, but it follows the premise put forth by many of the
developing treatment technologies regarding its being bound or isolated from its environment.
Data from several EPA Superfund sites and the Region VIII swine study (U.S. Environmental
Protection Agency, 2004a) suggest that matrix associations, such as liberated versus enclosed,
can play an important part in bioavailability. As illustrated in Figure AX7-1.1.3, two different
media with similar total Pb concentrations and Pb forms (slag, Pb-oxide, and Pb-arsenate)
exhibit significantly different bioavailabilities. In the Murray, UT sample (bioaccumulation
factor [BAF] = 53%), a greater fraction of the more bioavailable Pb-oxides are liberated and
not enclosed in the less-soluble glass-like slag as observed in the Leadville, CO sample
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