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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.