Although there is considerable spread
Mass-Loading Approach
method is based upon measured or
in these data, there is no indication of a
239, 2405,
preferential association of
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
material is derived from the contaminated
aerosol, the specific activity is lower
soil,
than that of the soil.
The other approximate prediction
For fresh deposits this approach is
If we assume that this is generally
not valid because the freshly deposited
debris is much more likely to be resus~
true, a general and conservative. method
pended than the remainder of the
of predicting resuspended air concentra-
weathered soil surface,
After many
trons 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 for-the
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 maybe highly
239, 2405, were preferentially associated
correlated with the source in the sense
with the smaller particle sizes more
that children playing in sand, adults cul-
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!* and by the
more mass than would be recorded by a
high-volume cascade impactor study.
remote stationary sampler,
The latter study found an AMAD of 3.0 um
for 239, 240
Pu, whereas the total mass
The lower and upper bounds of ambient
air mass loading can be fixed rather
median aerodynamic diameter was 1,7 um.
easily for any site.
There has been con-
The specific activity of the material col-
siderable interest in establishing a
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 ug/m 3 (15)
Average data from all
established in a reasonable wav by the
five samplers are:
size, um
levels found in mine atmospheres which
239, 24054, dpm/g
>7
930
3.3 to 7
700
2.0 to 3.3
1030
1,1 to 2.0
1300
0.01 to 1.1
460
All stages
830
(Soil)
The upper bound can be
have led to a considerable prevalence of
pneumoconicsis in the affected ¥corkers >
Examination ci these data indicate that
current stundards for occupational dust
exposure {(~ 1-i0 me! im) have avery
Small, or perhaps no margin of safety,
Such that a reasonable upper bound can
17
(2700)
be taken as 1 mg/ m*,
tty
British data