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    The indicators are arranged in terms of the now widely-used DPSEEA framework (Figure 1) (Corvalán et al. 1996). Within this framework, the driving forces component (D) refers to the factors which motivate and push the environmental processes involved. Of these, possibly the most important is population growth; others include technological development, economic development and policy intervention. 

    The driving forces within the DPSEEA model result in the generation of pressures (P) on the environment. These are normally expressed through human occupation or exploitation of the environment, and may be generated by all sectors of economic activity, including mining and quarrying, energy production, manufacturing, service industries, transport, tourism, agriculture and forestry. In each case, pressures arise at all stages in the supply chain - from initial resource extraction, through processing and distribution, to final consumption and waste release. 





    In response to these pressures, the state of the environment (S) is often modified. The changes involved may be complex and far-reaching, affecting almost all aspects of the environment and all environmental media. They are expressed, therefore, in terms of the frequency or magnitude of natural hazards, the availability and quality of natural resources, and the levels of environmental pollution. These changes in the state of the environment also operate at markedly different geographic scales. Many changes are intense and localised, and often concentrated close to the source of pressure (e.g. habitat loss, urban air pollution, contamination of local water supplies). Many others are more widespread, contributing to regional and global environmental change (e.g. desertification, marine pollution, climate change). Because of the complex interactions which characterise the environment, almost all these changes have far-reaching secondary effects. 

    When people are exposed to these environmental hazards, then risks to health may occur. Exposure (E1) thus refers to the intersection between people and the hazards inherent in the environment. The National Academy of Sciences (1991) defines exposure as ‘an event that occurs when there is contact at a boundary between a human and the environment with a contaminant of a specific concentration for an interval of time’. In the case of environmental pollution, therefore, exposure can occur in a number of different ways — by inhalation, ingestion or dermal absorption — and may involve a wide range of different organs. External exposure refers to the quantity of the pollutant at the interface between the recipient and the environment. It is often measured either

    using some form of personal monitor (e.g. passive sampling tubes for air pollution) or by modelling techniques (e.g. based upon knowledge of concentrations in the ambient environment). The amount of any given pollutant that is absorbed is often termed the absorbed dose, and may be dependent on the duration and intensity of the exposure. Target organ dose refers specifically to the amount that reaches the human organ where the relevant effects can occur (Sexton et al. 1995). 

    Exposure to environmental hazards, in turn, leads to a wide range of health effects (E2). These may vary in type, intensity and magnitude depending upon the type of hazard to which people have been exposed, the level of exposure and the number of people involved. For convenience, a simple spectrum of effects can often be recognised. The earliest, and least intense, effects are sub-clinical, merely involving some reduction in function or some loss of wellbeing. More intense effects may take the form of illness or morbidity. Under the most extreme conditions, the result is death. 

    It must be said that the DPSEEA framework works well for risks associated with environmental pollution, where the chain from driving force to source activity and thence to health effect via emissions and exposure is evident. It can also be applied to the many psychological and perceptual health effects which may be generated by the fear, rather than the eventuality, of a hazard (e.g. stress or anxiety caused by fear of exposure to radiation from a nuclear power station, or of physical injury from war). It is less appropriate, however, in the case of physical risks, as presented by natural hazards (e.g. flooding) or technology (e.g. traffic accidents), where the concept of ‘pressure’ is less meaningful. Nor can it easily be applied in full to those environmental hazards, such as famine, which affect health more by omission than comission. Like other aspects of environmental health indicators, therefore, the DPSEEA framework should be seen as an aid, not a straight-jacket; it needs to be adapted and modified according to circumstance. 

    Partly as a reflection of this, it may be noted that the indicators presented here do not in most cases occupy every point in the DPSEEA chain for every issue. Because of the way they are conceived, different issues tend to focus on different parts of the DPSEEA framework. Some are more source-based (i.e. focusing on the driving forces and pressures which lead to exposure); many are exposure-based; others are effect- (i.e. health-) based. The indicators are thus presented either as ‘chains’ (i.e. a set of linked indicators from different parts in the DPSEEA framework) or as ‘clusters’ (i.e. a group of related indicators from one point in the framework). In almost all cases, however, the indicators are likely to be most meaningful and effective if interpreted together. 

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