Read first: Comparing the impact on the environment.
With so many factors coming into play, comparing impact on environment caused by various items is not easy. On this web site, we try to look carefully at the whole life cycle of the items, and figure out what resources are used and how much of pollution is created to produce, use, and dispose of the items. Look under the "Bottomline" tab on any item page to see the total amounts of every resource used and every pollution released. This is reasonably exact and reliable data, as we look into publications, books, Internet, and other sources to discover how items are produced, and how they are disposed of.
Of course, sometimes we need to estimate, average, or plainly skip some elements, as the necessary data is not easily obtainable. As we gather more information, the presented amounts become more and more precise. Thus, this site creates a framework of estimates, which will naturally change over time as the estimates improve.
However, it is difficult to compare tables like that. Ideally, we would like to have one number, and say "the bigger this number, the bigger impact on environment". To achieve that goal, we consider impact of every resource and every pollutant separately, calculate an environment impact index for each of them, and then sum them up. For the details on calculating the index, see the article on CEII.
Impact index is not an absolute measure, but relative. It allows us to compare the impact of different, unrelated items. It has no unit, and no meaning outside of a comparison of how big the environment impact of an item is.
This methodology has a number of important consequences.
- As the amounts of used resources and pollution released are calculated as exactly as possible, they are not subject to fads, fashions and the latest scares.
- These values may change as we learn more about production and disposal of items; it is more of "current estimate" rather than "absolute truth".
- The absolute value of the index is not that important, only the ratios of the indexes for different items is significant. Quoting the value of an impact index outside of the context of this web site does not makes much sense - one has to quote the comparison.
- It is possible to compare impact of items using one, overall number (CEII), its parts (depletion index DI, pollution index PI, and entropy index EI), or even the actual amounts of resources used and pollution generated. Unlike the index, the amounts have an absolute meaning and can be quoted on stand-alone basis.
In the process of calculating the environment impact index certain scaling, normalization and weighting constants are introduced. They depend on abundance of a resource, its importance, toxicity of the pollution at so on. They have been scaled arbitrarily so that the impact of burning of one pound of
coal will have the impact index of 1.