As noted already, the major results of our analysis are summarized in tabular form in the Appendix (see tables A-H).
The lower part of figure 1 displays, for chromium and copper, the ratio of consumption-related dissipative losses to production-related emissions (not including losses at the mine) in each decade. For these two metals, whose major uses are in metallic form or, in the case of chromite, as bricks for blastfurnace liners, production-related emissions are still dominant, but the consumption share is increasing steadily.
In the upper part of figure 1 the same data are shown for five other toxic heavy metals: arsenic, cadmium, lead, mercury, and zinc.
In two cases, arsenic and mercury, the consumption share has always been high. Arsenic has been used (until very recently) almost exclusively because of its biotoxic properties. Such uses are inherently dissipative. This is also partly true for mercury. For instance, mercury is the basis of a number of commercial fungicides, germicides, and preservatives. The major dissipative uses of cadmium, in the past, were in pigments and as a contaminant of zinc oxide used in tyres. The use of cadmium for red and orange pigments has declined sharply, while metallic usage (mainly in batteries) has increased even more sharply. This accounts for the inverted "U" shape of the cadmium curve. (As electronic uses of arsenic, in gallium arsenide, may grow in the future, a similar downturn may be expected in the future.)
The increasingly dissipative usage of lead is only partly due to its role as a gasoline additive (largely phased out since 1980, of course). In earlier decades lead was the basis of one of the most widely used agricultural insecticides (lead arsenate). In the nineteenth and early twentieth century, lead was also extensively used as a white pigment for oil-based paints. So-called white lead was later replaced by a zincbased white pigment (lithopone), which was subsequently replaced by the white pigment now used most widely, titanium dioxide. Red lead was the major metal-protective paint until the last decade or so. The yellow paints currently used on roadways and to protect heavy machinery - such as bulldozers - are largely chromium-based, which accounts in part for the rapid rise in dissipative uses of chromium. Zinc is also used in large quantities in tyres and paper.
As we indicated at the outset, for three of these five metals investigated the dissipative consumption-related emissions far outweigh the production-related emissions; in fact the consumption shares for arsenic, lead, and mercury are close to 100 per cent. In the case of zinc, that share is rising rapidly; for cadmium the consumptive share is still about 50 per cent of the total.
One of the eight metals included in the study was silver. Production-related emissions data are non-existent. However, since silver is a rather valuable metal, and since almost all of it is now obtained as a by-product of lead, zinc, or copper smelting and refining, one could probably argue that productionrelated emissions are essentially non-existent. On the other hand, one major consumptive use of silver is still in photography. While commercial photographic studios do recycle some silver, a significant fraction is lost. Thus, for silver, too, the consumption-related share of total emissions is probably close to 100 per cent.
The foregoing analysis was entirely historical. But one or two points worth considering for the future emerge clearly. One of them is the fact that several of these toxic heavy metals play a major and increasing role in electronics. These include lead (solder), arsenic (semi-conductors), cadmium (batteries), mercury (switches and batteries), and silver (batteries and connectors). Electronic wastes are accumulating in obsolete equipment at an enormous rate in the United States, and all around the world. Much of this electronic "junk" might be dumped in landfills in future years, and some will be inadvertently incinerated. Many states already classify such wastes as hazardous. Leaching - especially that due to increasingly acid rainfall and combustion will mobilize some of these toxic materials. There is, therefore, a strong need for more research on ways and means of closing the materials cycle.
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