Although there is considerable spread
Mass- Loading Approach
The other approximate prediction
in these data, there is no indication of a
method is based upon measured or
preferential associaticn of 239, 2405,
assumed levels of particulate matter in
with a particular particle size; as would
ambient air with the assumption that this
be expected as a result of dilution by inert
materia! is derived from the contaminated
aerosol, the specific activity is lower
soil,
than that of the soil.
For fresh deposits this approach is
not valid because the freshly deposited
If we assume that this is generally
debris is much more iikely to be resus-
true, a generai and conservative.method
pended than the remainder of the
of predicting resuspended air concentra-
weathered soii surface,
After many
tions of contaminants would be to simply
years of weathering since the initial
multiply the ambient air mass loading by
deposition, however, the contaminating
the contaminant concentration in soil,
material should be reasonably well mixed
factor of some uncertainty for a specific
with a centimeter or two of soil, such
calculation is what value to use forthe
that the contaminant activity per gram of
ambient air mass loading in the absence
airborne particulate should approximate
of specific data,
that in the upper soil.
more uncertain because of the possibility
However, a major
A
This becomes even
difficulty could arise if, for example,
that the people involved may be highly
239, 2405, were preferentially associated
correlated with the source in the sense
with the smaller narticte sizes mare
that children plaving in sand. adultes enl-
likely to become airborne,
tivating crops, etc,, may generate their
For the
Nevada Test Site, such is not the case as
own "ambient air" which contains much
determined by soil analyses! 4 and by the
more mass than would be recorded bya
high-volume cascade impactor study.
remote stationary sampler,
The lower and upper bounds of ambient
The latter study found an AMAD of 3.0 wm
for 239, 240
air mass loading can be fixed rather
median aerodynamic diameter was 1.7 um,
easily for any site.
The specific activity of the material col-
siderable interest in establishing a
Pu, whereas the total mass
There has been con-
lected on each stage can also be examined
"background level" of mass loading, and
for a preferential association of plutonium
this is generally believed to be about
with particle size,
10 we/m 3 (5)
Average data from all
established in a reasonable way bythe
five samplers are:
239, 240 Pu,
Size, um
>7
950
3,3 to 7
700
2.0 to 3.3
1030
1,1 to 2.0
1300
0.01 to 1.1
480
All stages
(Soil)
|
The upper bound can be
890
(2700)
dpm/g
levels found in mine atmospheres which
have led to a considerable prevalence of
pneumoconiosis in the affected workers ©,
Examination of these data indicate that
current standards for occupationa! dust
exposure (~ 1-10 mg/m?) have a very
small, or perhaps no margin of safety,
such that a reasonable upper bound can
be taken as 1 mg/m°,
British data’
i11-24