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ecosystems, and rivers utilized by others. In practice it is not possible to finetune policies to reflect all variations in each environmental medium. Moreover, few emissions have no environmental impact whatsoever and many can have long-range or long-term impacts that are difficult to identify. Moreover, governments must maintain consistent policies for all affected persons to avoid arbitrary decisions and ensure predictability.

Different policies may sometimes even be needed to achieve identical environmental outcomes. An example is the preservation of lobster stocks in the United States and Canada through size limitations.5 In this case, a smaller size limitation in Canada achieved better conservation than larger US limits because lobsters mature faster (i.e., at smaller size) in Canadian waters, which are warmer on account of the Gulf Stream. Other examples concern the control of photochemical smog in Southern California, which experiences long periods of sunshine and limited atmospheric exchange. Western Europe has less sunshine, lies at more northerly latitudes and has greater wind movement, and consequently experiences different smog events.

In some instances the impact of environmental conditions can be observed in the phasing of environmental measures or in differences in priority-setting. The United Kingdom (like Japan but unlike the rest of Europe) is an island with short, swift-flowing rivers. Modest levels of water treatment can achieve dramatic improvements in water quality, as demonstrated by the Thames. In continental Europe, even vigorous water treatment can still result in limited water quality, as demonstrated by the Rhine.6 Even though the lack of water treatment in the United Kingdom is essentially transferring pollution to the oceans, it has taken twenty years to demonstrate the impact of such policies and to induce a shift in priorities to more closely resemble those of the countries most affected by deterioration of the North Sea.

Certain chemical substances react differently under different ambient conditions. For example, pesticides will typically volatilize more rapidly under hot climatic conditions, requiring larger applications to achieve comparable levels of receptor exposure.

These differences in environmental conditions are frequently overlaid by long distance effects of emissions, which are hard to detect and have tended to be identified only when they reach crisis proportions, as in the case of acid rain in Europe or the transport of toxic substances into the Great Lakes region of North America, or when their presence is very unusual, as with particulates from the United Kingdom found in remote Swedish lakes in the late 1960s or industrial chemicals in the tissue of Arctic and Antarctic animals.

Past Emissions

Environmental policy does not originate in a pristine environment. In many countries, accumulated environmental impacts, some of which are reversible only

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