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 information about one fiber type to that for other fibers to facilitate an analysis of comparative exposure potentials. A quantity is a calculated numeric value that represents some physical aspect of exposure to asbestiform fibers. 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. In every case, the units of the input quantities 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 measurable quantities. General flow of computational logic for estimating exposures to fibers. The model sketched in Figure D-l is intended to apply to virtually any fiber type, but not all of the steps would apply to every type.