subject in several forms, EPS allows for the individual consideration of each of these consequences, called unit effects. Two criteria are applied when establishing which impacts will become unit effects: How important the impact is on the sustainability of the environment; and is it possible to establish a quantitative value for that impact within traditional economic grounds. Examples of unit effects for human health include mortality due to increased frequency of cancer, mortality due to increased maximum temperatures, and decreases in food production (and hence increased incidence of starvation) due to global warming.

Once the individual unit effects are established, their values must be determined. This is accomplished by expressing each unit effect in terms of its economic worth and associated risk factors. Formally, the value of each unit effect is equal to the product of five factors, F1 through F5. F1 is a monetary measure of the total cost of avoiding the unit effect. The extent of the affected area (F2), how frequently the unit effect occurs in the affected area (F3), and the duration of the unit effect (F4) represent risk factors similar to those used in toxicological risk evaluations. F5 is a normalizing factor, constructed so that the product F1 x F5 equals the cost of avoiding the unit effect that would arise through the use or production of one kilogram of material. The product of all five factors yields the contribution of a particular unit effect to environmental load. Summing the values of each unit effect yields the environmental load index (ELI) in units of environmental load per unit of material consumed or processed (ELU per kilogram), as summarized in Figure 4. Because these unit effects were specified according to their relevance to the five safeguard subjects, the ELI represents the total environmental load (or impact) for all five safeguard subjects.

For example, consider Table 1, which illustrates how to estimate the ELI for the release of carbon monoxide (CO) to the air. The second and third columns of data demonstrate how the impact of two specific unit effects, nuisance and morbidity, are incorporated into the overall ELI for a CO release to air. Based on a variety of studies, the value of excess nuisance and morbidity are estimated at 102 and 105 ELU/person-year, respectively. (Note that according to the definition of Fl, these values are the estimated costs, in ELUs, of avoiding these unit effects.) Furthermore, the incidence of these impacts is estimated for the world urban population, assuming that hazardous levels of CO occur only 10 percent of the time, and that 10 and 0.1 percent, respectively, of the exposed population is affected at the nuisance or morbidity level. Finally, given that 1,600 million metric tons of CO are already being released, the incremental effect of one additional kilogram released is 1/1,600,000,000,000, the F5 term. These terms, F1-F5, are multiplied together and then summed over all unit effects to develop the ELI for CO release to the atmosphere, in ELUs per kilogram released. With this number, any life-cycle data that reveal the release of some amount of CO can be valued by multiplying that release by the ELI. The EPS system is designed to develop ELIs for all releases, as well as for all human activities that consume resources, so that the relative ELUs for any two life-cycle inventories can be computed and compared.

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