Cover Image
close this bookEco-restructuring: Implications for Sustainable Development (UNU, 1998, 417 p.)
close this folder1. Eco-restructuring: The transition to an ecologically sustainable economy
close this folderControversial issues: Pollution, productivity, and biospheric stability
View the document(introduction...)
View the documentOn toxicity
View the documentThe stability of the biosphere: The impossibility of computing the odds
View the documentTechnical preconditions for sustainability

On toxicity

There is no doubt that widespread fear of exposure to toxic chemicals is one of the major driving forces behind the environmental movement. The near-hysterical media coverage of the "Love Canal" episode and the proliferation of "Superfund" sites certainly support this contention. Yet, as a basis for discussing environmental threats half a century hence, one needs a different kind of evidence. Unfortunately, methodological problems proliferate even faster than superfund sites.

First, the number of industrial chemicals produced in annual quantities greater than 1 metric ton is estimated at 60,000. The number grows by thousands each year. Only a tiny percentage of these has been tested for the whole range of toxic effects. In fact, it could be argued that none has, since new effects are being discovered all the time, often by accident or from epidemiological evidence long after the fact. For instance, mercury was not known to be harmful in the environment until the mysterious outbreak of "Minamata disease," a severe and sometimes lethal neurological disorder among cats, seabirds, and fishermen living near Minamata Bay, in Japan. It took several years before public health workers were able to trace the problem to organic mercury compounds (mainly methyl mercury) in fish from the bay. The ultimate source turned out to be inorganic mercury from spent catalysts discharged by a nearby chemical plant. The toxic effects of cadmium ("itai-itai disease") were discovered in a similar way.

Second, quantitative production and consumption data for chemicals are not published consistently even on a national basis, still less on a worldwide basis. Data can be obtained only with great difficulty, from indirect sources (such as market studies), and for only the top 200 or so chemicals. In the United States and virtually all countries with a central statistical office (or census), production and shipments data are collected, but the data are withheld for "proprietary" reasons if the number of producers is three or fewer. In Europe, the largest producer of most chemicals, all quantitative production and trade data are suppressed. Data are published in terms only of "ranges" so wide (e.g. 100-10,000 tonnes) that the official published numbers are useless for analysis.

Data on toxic chemical emissions are extremely scarce. The US Environmental Protection Agency's Office of Toxic Wastes is the only official primary source of such data in the world, and its major tool is the so-called Toxic Release Inventory (TRI), which is an annual survey that has been in effect since 1987. The survey must be filled out by US manufacturing firms (Standard Industrial Classification 20-39) with 10 or more employees and that produce, import, process, or use more than a threshold amount of any of 300 listed chemicals. The reporting threshold as regards production or processing for each chemical was initially (1987) 75,000 Ib; since 1989 it has been 25,000 lb (roughly 12 metric tons), while for use the reporting threshold is now set at 10,000 lb (roughly 4.5 metric tons) per year. Releases are reported by medium (air, water, land) and transfers for disposal purposes to other sites are also reported. There is serious doubt about both the completeness and the accuracy of the TRI reports, because published data are very difficult to reconcile with materials balance estimates, as discussed elsewhere (Ayres and Ayres 1996).

Third, a large number of manufactured chemicals - probably the vast majority in terms of numbers, if not tonnage's - are produced not because they are really needed as such, but because they are available as by-products of other chemical processes. This is particularly true of products of chlorination and ammonylation reactions, which require repeated separation (e.g. distillation) and recycling stages to obtain reasonably pure final products. For instance, it has been estimated that 400 chlorinated compounds are used for their own sake, but at least 4,000 are listed in the directories (Braungart, personal communication, 1992). This is because it is easier to treat them as "products" than as wastes. Many such chemicals are found in products such as pesticides, paint thinners and paint removers, dry-cleaning agents, and plasticizing agents.

Fourth, many of the most dangerous toxic chemicals are known to be produced by side reactions in the manufacturing process, or "downstream" reactions in the environment. Perhaps the most infamous toxic/carcinogenic chemicals are the so-called "dioxins," which are not produced for their own sake but appear to be minor contaminants of some chlorinated benzene compounds that are used for herbicide manufacturing. Thus dioxins were accidental contaminants in the well-known herbicide 2-4-D, which became known as "Agent Orange" during the Viet Nam war. They are also probably produced by incinerators and other non-industrial combustion processes, depending on what is burned. As regards downstream processes, the example of methyl mercury - produced by anaerobic bacteria in sediments - was mentioned earlier. Exactly the same problem arises in the case of dimethyl and trimethyl arsine, extremely toxic volatile compounds that are generated by bacterial action on arsenical pesticide (or other) residues left in the soil. Still other examples would be the dangerous carcinogens such as Benz(a)pyrene (BAP) and peracyl nitrate (PAN) produced by reactions between unburned hydrocarbons, especially aromatics, nitrogen oxides (NOx), and ozone. (This occurs in Los Angeles "smog", for instance.) In fact, oxides of nitrogen are themselves toxic. NOx is produced not only by most high-temperature combustion processes but also by atmospheric electrical discharges.

An even more indirect downstream effect is exemplified by the Waldsterben (forest die-back) in central Europe. The conifer trees of the Black Forest and much of the Alps are now being weakened and many are dying. This appears to be the result of a complex sequence of effects starting with increased acidity of the soil. As the pH drops below 6 there is a sharply increased mobilization of aluminium ions, which are toxic to plants. There is also an increased mobilization of heavy metals hitherto fixed in insoluble complexes with clay particles. Many toxic heavy metals - from pesticides, or from deposition of fly ash from coal burning - have long been immobilized by adherence to clay particles at relatively high pH levels (thanks, in part, to liming of agricultural soils). However, as the topsoil erodes as a result of intensive agriculture, it is being washed into streams and rivers and, eventually, into estuaries, bays such as the Chesapeake, or enclosed seas (such as the Baltic, the Adriatic, the Aegean, or the Black Sea), where it accumulates.

This sedimentary material is "relatively" harmless as long as the local environment is anaerobic, except for the localized risk of bacterial methylation of mercury, arsenic, and cadmium mentioned earlier. But this accumulated sedimentary stock of heavy metals (and other persistent toxic chemicals too) would become much more dangerous in the event of a sudden exposure to oxygen. For instance, sediments dredged from rivers and harbours may be rapidly acidified and could become "toxic time bombs" (Stigliani 1988).

Fifth, many toxic compounds are produced naturally by plants and animals, largely as protection against predators or as means of immobilizing prey. Nicotine, rotenone (from pyrethrum), heroin and morphine (from opium), cocaine, curare, digitalis, belladonna, and other alkaloids are well-known examples from the plant world. Recent research suggests that natural (i.e. biologically produced) compounds have about the same probability of being toxic or carcinogenic as synthetic compounds. The widespread idea that "natural" products are ipso facto safer than synthetic ones is apparently false. In fact, Bruce Ames (inventor of the "Ames test") has argued with considerable force that the use of synthetic pesticides is less dangerous, to humans, than reliance on "non-chemical" methods of agricultural production, because plants produce greater quantities of natural toxins when they are under stress. However, probably even less is known about the range of toxic effects from natural chemicals than from industrial chemicals.

In fact, there is no general theory of toxicity. It comes in many colours and varieties. The notion includes mutagenic effects visible only after generations, effects on the reproductive cycle, and carcinogenic effects (e.g. asbestos, dioxins, vinyl chloride), or chronic but minor degradation of physiological function. At the other extreme are acute effects resulting in rapid or even instantaneous death. Methyl isocyanate (MIC), the cause of the Bhopal disaster, is an example of the latter. Chlorinated pesticides and polychlorinated biphenyls (PCBs) were not even thought to be dangerous to humans until long after they had been in widespread use. It was belatedly discovered that these chemicals tend to accumulate in fatty animal tissues and to be concentrated as they move higher in the food chain. Eagles, falcons, and ospreys were nearly wiped out in some areas by DDT because their eggshells were weakened to the point of non-viability.

As noted above, soil acidification resulting from anthropogenic emissions of SO2 and NOx to the atmosphere is also releasing toxic metals (and other compounds) that were formerly immobilized in the soil. Large accumulations of toxic metals reside in the soils and sediments in some areas. For many decades lead arsenate was used as an insecticide, especially in apple orchards. Copper sulphate and mercury compounds (among others) were widely used to control fungal diseases of plants. Mercury was also used to prevent felt hats from being attacked by decay organisms. Chromium was, and still is, used for the same purpose to protect leather from decay. Copper, lead, nickel, and zinc ores were roasted in air to drive off the sulphur (and the arsenic and cadmium). Lead paint was used for more than a century, for both exterior and interior surfaces. For half a century tetraethyl lead and tetramethyl lead were used as gasoline octane additives (they still are so used in much of the world). Soft coal has been burned profusely in urban areas; usually the bottom ash was used as landfill for airports and roads. Coal ash contains trace quantities of virtually every toxic metal, from arsenic to mercury to vanadium. For decades, phosphate fertilizers have been spread on farmland without removing the cadmium contaminants. In all of these cases, increasing acidity means increased mobilization of toxic metals. These metals eventually enter the human food chain, via crops or cows' milk.

It is clear that toxicity is not simply a problem associated with the production and use of industrial chemicals or heavy metals. It is intimately linked to a number of other anthropogenic processes, not least of which is global acidification. To take another example, it is well known that CFCs emitted to the atmosphere are responsible for depleting the ozone layer in the stratosphere. The major consequence on the earth's surface is an increase in the intensity of harmful UV radiation reaching the surface. Spawning zooplankton and fish in shallow surface waters are likely to be adversely affected. This is, in effect, a form of eco-toxicity.

Is there any common factor among all these types of toxicity? It can be argued that all human toxins are, in effect, causes of physiological disturbance. All interfere with some biological process. Mutagens interfere with the replication of the DNA molecule itself. Carcinogens interfere with the immune system; neurotoxins (e.g. cyanide) interfere with the ability of the nerves to convey messages. Many toxins cause problems for the organism because they closely resemble other compounds that perform an essential function. Thus carbon monoxide causes suffocation because it binds to the haemoglobin in the blood, as oxygen does. But, when the haemoglobin carrier arrives at a cell in need of oxygen, the potential recipient "sees" only a carbon atom where an oxygen atom should be.

The point of this example is that toxicity, to an organism, is just another word for imbalance or disturbance. A toxin is an agent that causes some metabolic or biological process to go awry. Every organism has a metabolism. Metabolic processes are cyclic self-organizing systems far away from thermodynamic equilibrium. The same statement can be made of the metabolic processes - the "grand nutrient cycles" such as the carbon and nitrogen cycles - that regulate the whole biosphere. Any disturbance to the biosphere is "toxic," in principle.