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OCR for page 261
Appendix D
CONCEPTUAL MODEL OF FIBER EXPOSURE
- I,
Because measurement of exposures to all potentially hazardous
asbestiform fibers is technically infeasible in some cases and
prohibitively.expensive in others, indirect methods of estimating
exposure must augment the direct measurements. Although the committee
did not develop a comprehensive mathematical model of fiber exposure, it
did develop a conceptual model of the computations that would be
necessary for a full mathematical model of the exposure process. This
conceptual model enabled the committee to identify the factors that could
be important in determining exposure so that it could seek information in
an organized way and attempt to relate the info,.~ation about one fiber
type to that for other fibers to facilitate an analysis of comparative
exposure potentials.
Figure D-1 provides a conceptual overview of the calculations that
would be necessary to estimate nonoccupational exposures of humans in the
absence of direct measurements. The scheme shows four types of
information: quantities, factors, units, and operators. A quantity is a
calculated numeric value that represents some physical aspect of exposure
to asbestiform fibers. A factor is an exogenous (external) input to the
calculation, which can be measured or itself calculated outside the
system. Without factor inputs, the quantities cannot be calculated.
Units are the physical units of measurement for the quantities and
factors. An operator is a mathematical manipulation that derives a new
quantity from one or more factors and other quantities. For example, the
quantity "human intake rate (by inhalation)" is calculated by multiplying
(using the multiplication operator on) the quantity "ambient
concentration in air" by the factor ''breathing rate. " In each step, the
input quantity is a result of all the previous steps; the factor
represents a new, physically important parameter not a result of the
previous steps; and the output quant ity serves as the input quantity for
the next step. In every case, the unit s of the input quant it ie s and
factors must combine correctly under the operator to yield the units of
the output quantity.
In this conceptual-level scheme it is not necessary to be able to
measure each factor physically, but each must describe a phenomenon of
interest and be, at least in principle, estimatable from physically
261
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262
Conmerc ial Flow
[Occurrence a| _
_ _ _ _ _ _
(in millions of aetric tons, e.g., in U.S.)
x ~Exploitation factor a~ (%/year)
_ _ _ _ _ _ _ _
Environmental Flow
x |Weathering factors1 (2/yr)
_ _ _ _ _ _ _ _ _
.
- iEnvironn~ental f lwces ( thou~ands
of meeric tons/year)
-lGro~s production ~(millions of m~tric tons/year)-9 xiDischarge factor ~ (~)
-iEnvironmental fluxes (thousands
-
of metric tons/year)
~iImPort ~ (millions of metric tons/year)
-`Export ~ (millions of metric tons/year)
_ _ _ _ _ _ _ _ _ _
~ IRecycled material~l (million~ of metric tons/year)
_ _ _ _ _ _ _ _ _
+ jChan~es in inventory (millions of
_ _ _ _ _ _ _ _ _ _ _
met ric tons/year)
"Consumption ~ (millions of metric tons/year) ~ ~ x ~ischarge factors~(%)
_ _ _ _ _ _ _ _
x IPe rcenea~e utilization: (X)
_ _ _ _ _ _ _ _ _ _ _
-1Env~ronmental fluxes ~(thousands
of metric tone/year)
-~Use by category ~ (thousants of metric tons/year)-} x1Di~charze factor ~ (%)
_ _ _ _ _ _ _ _ _ _
-iDisposal by cate~ory~ (millions of >
_ _ _ _ _ _ _ _
metric eons/year)
INet increase in use ~ (millions of
metric tons/year)
It=~Total in usel(thousants of
metric tons/year)
Ix Re lease factors ( % /Year)
-~Local fluxes] (metric tonsfyear)
,,
aNatural materials only.
-[Environmental fluxes ~(thou~ands
of metric tons/year)
_ _ _ _ _ _ _
-1Disa~pearanc ~ (thousants of
metric tons/year)
-iNet increase of disposed material~
-
~a~;~
-~Total disposed ~ (thousands
t
of metr~c tons)
- - - - - - - - -
x ~ischarge factore (%/year)
- IEnvironmental fluxes 1 (metric
tone/year)
~Total fluxes~ ('Detric eons/yr)
FIGURE D~1. General flow of computational logic for estimating
exposures to f ibers.
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263
FIGURE D~1 (Cont.)
-
ITotal fluxes (metric tonetyear)
- ~kposition j(metric tone/year)
. ~ilution factors ~ (m3|yr; liters/year)
-[Ambient concentrations ~ (pg/m3
, ug/liter)
x IConversion faceor~] (fibers/cmd per ug/m~;
fibere/ml per ug/liter)
-tFiber concentration. }(fibers/cm3; fibers/ml)
x ~reathin~, drinkin~ rate ~ (m3/tay, liters/day)
-~.Human intake ratesl (fibers/day)
x iBiodis~osition factors~ (~/organ)
_ _ _ _ _ _ _ _ _ _
-':Tis~ue fluxes ~ (fibers/day)
~i~a~pearanc ~ (fibers/day)
~ ~ (fibers/day)
f ~ ~Tissue burden~ (fibers)
t
_ _ _ _ _ _ _ _ - - _
xiTissue clearance rat,3 (%Itay)
l
~ Disappearance |
Key: j j ~ quantity
factor
) ~ unit
~ link from commercial to environmental flow
t ~ tzme
Metric ton ~ 2,205 pounds.
..
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264
measurable quantities. For example, the "deposition" of fibers on their
way from source to exposed humans may not be directly measurable, but the
principle can be demonstrated by measuring concentrations of fibers at
various distances from known and quantified sources, and then describing
the deposition an a function of distance through appropriate computations.
The model sketches in Figure D-l is intended to apply to virtually
any fiber type, but not all of the steps would apply to every type. For
example, occurrence (mil lions of metric tons of fibrous material known or
suspected) and weathering (relative rate of 1088 of such material) would
not apply to man-made fibers. Moreover, the commercial flows on the left
side of the scheme would be of dominant importance for some fibers,
whereas the environmental flows on the right side would predominate for
others. This commercial versus environmental flow distinction is
important, as explained for Figure 1-2, because of potential need for
controls of both types of flow.
In brief, the factors shown in Figure D-l take into account the
following phenomena:
Occurrence: Geologic occurrence in the United States. In princ iple,
this factor could be measured by the proven and indicated reserves of the
mineral, if commerc ial ly important, or by a re let ive abundance f igure for
others. It can be measured in millions of metric tons.
Weathering: The amount of material in place that might be released
into the environment (as either airborne or waterborne particulates) per
year. The natural weathering processes may occasionally be enhanced
through noncommerc ial disturbance by humans.
Exploitation: The amount deliberately extracted for use. Should
include amount used with and without further processing, for example, the
asbestos content of road surfacing aggregates.
Imports, exports: The flows of materials to and from foreign
countries. For example, on the basis of relative amounts, asbestos flows
from Canada -are greater than those resulting from extraction in the
United States.
Recycled materials: Fibers suitable for recycling after disposal
from first use. This practice does not seem to be very widespread in the
industry because of the low cost of original production.
Percentage Otil izee ion: Essentially synonymous with "use patterns.''
Cons ~ total consumption in the United States that
goes into each use. There may be a chain of uses. For example, asbestos
fiber may go into asbestos paper, which in turn is used in insulation for
electric appliances. In principle, opportunities for release of asbestos
occur both in the manufacture of the paper and in the manufacture of the
appliance as well as during use of the appliance.
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26S
Disposal: Disposal of fiber products after use. Virtually every
fiber product has a finite useful life. Afterward, most of the fibers
reach some form of landfill, but some enter air, or possibly water,
during demolit ion. The f ibers in landf i l l then pose a secondary source
of potential- exposure. Fibers 108t from such sites ("disappearance")
decrease the inventory there, thus decreasing the rate of accumulation.
Discharge factors : The potential for release into environmental air
or water for each process through which the fibers pass. The factors can
be expressed as a percentage of throughput (ices ~ metric tons released
per thousand metric tons processed, multiplied by 100) or as a percentage
rate of total inventory (i.e., metric tons discharged per year per metric
ton in place, multiplied by 100~. Generally, the release is called a
"discharge" when associated with a manufacturing process, but a "release
when associated with product use, e.g., when fibers are worn off vinyl
asbestos ti les .
Dilution factors: The net effect of all processes that disperse
fibers in air or weeer away from the source. If fibers are released
inside a building, the dilution factors are related to the number of air
changes per hour and the volume of air in the enclosed space. In ambient
air, the factors are used to convert the discharge rate to ambient
concentration as a function of distance \from the source, wind direction,
and other influences. In water, they are used to convert the discharge
rates to the concentrations in water supplies. In tap water, the actual
concentration may be lower than the calculated concentration because of
filtration and nettling. In each case, the result of applying a dilution
factor is to compute a concentration in a medium of exposure (generally
air or water) at a location where people are exposed to these
concent rat ions .
Conversion factors: Factors used to convert measurements to number
of fibers per unit volume. Concentration is often measured in terms of
mass per unit volume. Conversion factors are used to change these
measurements to fibers per unit volume to conform with the usual
measurements of dose in dose-response relationships. They are functions
of fiber type, releasing activity, distance from point of release, and
other considerations.
.'
id.
Breathing and drinking rates: Factors used to convert the exposure
dosei: 6~7~: 6~bi 76~i or dose rates. For example, if a worker
breathes air at a rate of ~ m3 per S-hour day, then one can calculate
the intakes of fibers per day, week, year, or working lifetime from the
average concentration in the air of the workplace. For nonoccupational
exposures, one must also account for such variations in rates as those
occurring between working and other activities (including sleep), between
ingestion of water or (in principle) food, between high and low exposure
areas, and between adults and children.
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266
Biodisposition factors: Factors used to convert intake rates to dose
rates for particular tissues. For example, if one estimates that 30X of
the inhaled dose is subsequently swallowed (National Research Council,
1983), then one can calculate the dose entering the gastrointestinal (GI)
tract (fibers/unit time) from the inhalation rate (fibers/unit time).
Disappearance: Removal of fibers from tissues. Fibers may disappear
from tissues through excretion or through various degradation processes.
For example , fibrous glass appears to gel (Klingholz and Steinkopf,
1981), whereas chrysotile separates into finer fibers and fibrils
(Jaurand et al., 1977) and shorter fibers may be removed from tissues by
m~crophages. These processes limit the buildup of f ibers in tissue .
Formation of ferruginous bodies also may "remove" the fibers making them
less biologically active. The dose rate and disappearance rate together
determine the buildup of tissue burden of fibers.
SUGARY
Although the above list does not contain all the factors that define
exposure at the tissue level, and although the conceptual model captures
neither all their relationships nor the difficulty in measuring some of
them, the model does give an idea of the complexity of the exposure of an
individual to asbestiform fibers. A further difficulty for risk
assessment is to estimate the number of people falling into each category
of exposure no that the distribution of exposures over the entire U.S.
population can be described.
REFERENCES
Jaurand, M.C., J. Bignon, P. Sebastien, and J. Goni. 1977. Leaching of
chrysotile asbestos in human beings: Correlation with in vitro
studies using rabbit alveolar macrophages. Environ. Res. 14:245-254.
Klingholz, R., and B. Steinkopf . 1981. The Behavior of Synthetic Mineral
Fibers in a Physiological Model Liquid and in Water. Report No.
81-0-0B, ISOVER, Grunzweig und Hartman und Glasfaser AG, September 30
(Translated from German).
National Research Council. 1983. Drinking Water and Health. Vol. 5.
Report of the Safe Drinking Water Committee, Commission on Life
Sciences. National Academy Preen, Washington, D.C.
Representative terms from entire chapter:
conceptual model
