The Failure Of Dilute And Disperse

This example illustrates just how dangerous assumptions about safe levels of release into the environment can be. Our best knowledge of the behaviour of materials released into the environment is extremely sketchy. Perhaps the most extensive knowledge we have about global material behaviour concerns the major nutrient cycles such as the carbon cycle. This is the cycle which is so critical to the vexing question of global warming. But even in this area, the answers to crucial questions are haunted by uncertainty. How much carbon can we release into the atmosphere without affecting the global temperature? Will some of the extra carbon be absorbed by vegetation or the oceans? What feedback effects will increase or decrease the warming effect of increased levels of atmospheric carbon? The accepted wisdom in these issues still holds that there are major uncertainties associated with all of these questions.

Much more obvious dangers are associated with the release into the environment of materials which do not occur freely in nature. In this case there are no well-established material cycles at all, so that any link with the natural materials cycling of ecosystems is extremely tenuous. There are now so many synthetic chemicals that we do not even have a complete grasp of their toxicity, let alone their behaviour in the environment.5 We do know, however, that some extremely serious environmental problems have occurred as a result of releasing synthetic substances into the environment. For example, the use of the chemical DDT as a pesticide has raised so many concerns about human and environmental health that Western nations have now banned its use in agriculture.

In spite of these failures, a dilute-and-disperse approach played a rather central role in responses to a wave of environmental management

Plate 1 'Dilute and disperse' or 'foul and flee'? Atmospheric emissions from industry in Wales Source: © The Environmental Picture Library/David Hoffman problems which emerged during the second half of the twentieth century. For instance, clean air legislation introduced in the 1950s in response to heavy city smogs in many industrialised countries was largely based on this philosophy. This legislation did take the unprecedented step of setting aside certain areas as 'smokeless zones'. But the main plank of the clean air legislation was to regulate the height of industrial chimneys, under the belief that releasing pollutants higher into the atmosphere would reduce the environmental impacts by diluting and dispersing them (Plate 1).

Even this simple expedient of dispersing atmospheric pollution through tall chimney stacks has proved ineffectual. Under adverse meteorological circumstances—such as prolonged high pressure and temperature inversion episodes—the pollution becomes trapped for days at a time within a local region, leading to severe respiratory problems for the local population. Even if the strategy is successful in carrying pollutants away from the immediate vicinity of the chimney, they may still eventually cause damage. That damage may be far away, even overseas. But this is not a solution. And sometimes the final environmental burden can be greater, if the pollution happens to be transported to very sensitive ecosystems. Acid pollution from sulphur dioxide emissions provides a clear example of this. Some very sensitive woodland areas such as those in northern Scandinavia are now threatened by acid pollution which has travelled hundreds of miles from Central and Eastern Europe.6

We can see from these examples that the concept of dilute and disperse is an extremely problematic one. Even though it may take some of its rationale from the behaviour of some materials in certain ecosystems, there is often very little justification for the assumption that material releases from the economic system will be absorbed within such cycles. Belief in the assimilative capacity of the environment often amounts to little more than blind faith that nature will deal with our waste disposal problems for us.

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