C. Corvalán, T. Kjellström and D. Briggs
Human beings are at the centre of concerns for sustainable development. They are entitled to a healthy and productive life in harmony with nature. (Principle 1 of the Rio Declaration; UN, 1993)
DEFINITIONS
An environmental health indicator may be defined as:
An expression of the link between the environment and human health, targeted at an issue of specific policy or management concern and presented in a form which facilitates interpretation for effective decision-making.
Several aspects of this definition are worthy of emphasis. The first is that an environmental health indicator embodies a linkage between the environment and health. As such it is more than either an environmental indicator or a health indicator. Environmental indicators represent indicators which describe the environment without any explicit or direct implications for health. The vast majority of environmental indicators so far developed are of this type, for example, indicators of atmospheric emissions, surface water quality, or threatened wildlife species. Health indicators are indicators which describe the status of, or trends in health without any direct reference to the environment. Again, the majority of health indicators so far developed are of this type. Examples include simple measures of life expectancy, or cause-specific mortality rates where no attempt has been made to estimate those health outcomes attributable to the environment.
Both environmental indicators and health indicators can be converted into environmental health indicators given knowledge of the relationship between environmental exposures and health effects.
The importance of this environment-health relationship within the concept of environmental health indicators cannot be over-emphasized. It is only through knowledge of this link that an environmental indicator or a health indicator can be translated into an environmental health indicator. An environmental health indicator is thus an environmental indicator or a health indicator plus a known environment-health relationship.
Another important characteristic of an environmental health indicator is its relationship with policy or management. Any environmental health indicator must have utility. To be useful, it must relate to aspects of environmental health which are both of relevance to the decision-maker and amenable to control. Equally, it must be expressed in a way which is pertinent to, and understandable by, the decision-makers concerned. In many circumstances, this requires that the indicator be expressed in terms of the health risk associated with a specific environmental hazard.
THE ENVIRONMENT-HEALTH HAZARD PATHWAY
The link between environment and health operates through the exposure of humans to environmental hazards. These hazards may take many forms, some are wholly natural in origin; the majority derive from human activities and interventions. In all cases, however, health effects only arise if humans are exposed, often at a specific place and time, to the hazards which exist.
The environment-health hazard pathway is most clearly seen in the case of exposure to pollution. Most environmental pollutants are the product of human activities. These may be released into the environment in a variety of ways, and may then be dispersed and accumulate in different environmental media ( e.g. the air, water, soil, food). Exposure occurs when humans encounter the contaminants within anyone of these media. A range of health effects may then occur, from minor sub-clinical effects through illness to death, depending upon the intrinsic harmfulness of the pollutant, the severity of exposure and the susceptibility of the individuals concerned. The whole process is often driven by persistent forces which both motivate the creation of the hazard and increase the likelihood of exposure. Thus, population growth, economic development, technological change and, behind these, social organization and policies, may all lie at the root of the problem. Ultimately, it is often in response to these that action needs to be addressed.
THE DPSEEA FRAMEWORK
The environment-health hazard pathway concept provides a useful organizing framework for the development and use of environmental health indicators. Over recent years, a number of attempts have been made to devise conceptual frameworks for indicator development. Of these, the one which has been most widely adopted has been the simple pressure-state-response (PSR) sequence, initially applied by OECD as a framework for state-of-the-environment reporting. In many ways, however, the PSR sequence has proved to be too limiting, and it has more recently been extended to include recognition of both the 'driving forces' responsible for pressures on the environment, and of the effects which often precede the policy response (e.g. USEPA, 1994). Figure 1 further adapts these concepts to provide a specific framework, the DPSEEA framework, for the development of environmental health indicators.
DRIVING FORCES
Within this framework, the driving forces component (D) refers to the factors which motivate and push the environmental processes involved. One of the most important of these is population growth. Almost inevitably this results in more people being exposed to environmental hazards simply by virtue of the increasing number of people living in the areas concerned. Indirectly, it tends to lead to the intensification of human activities within these areas, thereby contributing to environmental damage and resource depletion. In some cases, it also results in expansion of human populations into more marginal zones. Here, the inherent instability of the environment may mean that the population is especially vulnerable to environmental hazards, while the environment, in turn, is especially sensitive to damage. A wide range of other important driving forces exist, including technological development, economic development and policy intervention.
PRESSURES
The driving forces within the DPSEEA model result in the generation of pressures (P) on the environment. These pressures are normally expressed through human occupation or exploitation of the environment. Pressures are thus generated by all sectors of economic activity, including mining and quarrying, energy production, manufacturing, service industries, transport, tourism, agriculture and forestry as well as households. Moreover, pressures arise at all stages in the supply chain, from initial resource extraction, through processing and distribution, to final consumption and waste release.
STATE
In response to these pressures, the state of the environment (S) (or environmental quality) is often modified. The changes involved may be complex and far-reaching, affecting almost all aspects of the environment and all environmental media. Thus changes occur: in the frequency or magnitude of natural hazards ( e.g. in flood recurrence intervals or in rates of soil erosion); in the availability and quality of natural resources (e.g. soil fertility, biodiversity); and in levels of environmental pollution (e.g. air quality, water quality). These changes in the state of the environment also operate at markedly different geographic scales. Many changes are intense and localized, and are 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 characterize the environment, almost all these changes have far-reaching secondary effects. The modifications of one area or one component of the environment feeds through to affect others.
EXPOSURE
Environmental hazards, however, only pose risks to human well-being when humans are involved. Exposure (El) thus refers to the intersection between people and the hazards inherent in the environment. Exposure is rarely an automatic consequence of the existence of a hazard. It requires that people are present both at the place and at the time that the hazard occurs.
The concept of exposure is best developed in relation to pollution. Exposure to pollution 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.
EFFECTS
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 recognized. The earliest, and least intense, effects are sub-clinical, merely involving some reduction in function or some loss of well-being. More intense effects may take the form of illness or morbidity. Under the most extreme conditions, the result is death.
In this context, the concept of sentinel diseases is of particular interest. Some diseases are very specific to environmental (or occupational) exposures, for example: asbestosis and mesotheliomas as indicators of asbestos exposure, silicosis as an indicator of exposure to silica dust, or leukaemia as an indicator of exposure to ionizing radiation. In practice, however, there are few diseases which can be used as sentinels of environmental exposures.
The concept of risk analysis is in many cases of greater utility. Given knowledge of the relationship between environmental exposures and health effects. Given known exposures and knowledge of dose-response functions, it is possible to make reasonable estimates of the health burden of specific pollutants. The further elaboration of risk analysis methods will be an important contribution to the development of indicators, by providing inexpensive and rapid estimates of the health impact of specific environmental exposures at the aggregate level.
ACTIONS
In the face of these effects, society typically attempts to invoke a range of actions (A). These may take many forms and be targeted at different points within the environment-health chain. In the short term, actions are often primarily remedial (e.g. the treatment of affected individuals). In the longer term, actions may be protective (e.g. by trying to change individual behaviour and lifestyle to prevent exposure). Alternatively, actions may be taken to reduce or control the hazards concerned ( e.g. by limiting emissions of pollutants or introducing measures of flood control). Perhaps the most effective long-term actions, however, are those that are preventive in approach, aimed at eliminating or reducing the forces which drive the system.
POSITION IN THE DPSEEA FRAMEWORK
One of the most fundamental questions in developing any environmental health indicator is clearly at what position it should be targeted within the DPSEEA framework. In terms of environmental epidemiology, the links within this framework, which have often generated most attention, are those between exposure and effect. In terms of health policy and management, however, it is often the earlier steps in the framework which are the most interesting. Many environmental health problems ultimately derive from relatively remote causal forces and events. Immediate sources of exposure thus represent little more than symptoms of the problem. Desertification, for example, is often a consequence of deep-seated social and economic causes. Pollution, equally, is often a symptom of inadequacies in industrial technology and policy controls. If the aim is to identify the underlying cause of the problem, and to take effective action at source, it is therefore essential to have indicators which allow the effects on health to be traced back to their underlying sources and causes.
Indicators from higher up the DPSEEA framework also tend to provide a better early warning, both of impending environmental problems and of the effects of intervention. Detectable changes in the state of the environment and in human health, for example, typically lag some way behind changes in source activity or emissions. In the case of some effects, such as cancers, the lag is often by many years. Most preventive action, similarly, occurs at or close to the source of the hazard (e.g. by controlling emissions at the source or through hazard management).
A further reason for relying on indicators from higher up the DPSEEA framework is the practical one of data availability. Typically, data become more difficult to acquire as one passes down the framework. Thus, while there are normally abundant data on social and economic conditions and trends, much less is known about the actual pressures on the environment, less still about environmental conditions and almost nothing about actual exposures. As a consequence, proxy indicators of exposure commonly have to be used, derived from higher up the DPSEEA framework.
The use of indicators from higher up the framework, whether in their own right or as proxies, is not without its dangers. As noted earlier, to be effective any environmental health indicator must be based on a clear and firm relationship between the environmental hazard and the health effect. Unfortunately, the further removed the indicator is from the health effect, the weaker this link is liable to be. The DPSEEA cycle is an imperfect one. Each link in the framework is itself dynamic and uncertain. Each step is subject to a wide range of influences and controls. The extent to which the driving forces are translated into active pressures on the environment, for example, depends upon the policy context, social attitudes and the pre-existing economic infrastructure of the area concerned. Whether these pressures cause detectable changes in the environment depends upon the ability of the environment to absorb and dampen down the changes involved. Whether the environmental hazards, in turn, lead to health effects is determined by all the factors that control exposure and human susceptibility to its effects. It depends, therefore, on the form, duration, intensity and timing of exposure; on the social, economic and prior health status of the individuals concerned; and on the quality and accessibility of the health system. Equally, of course, there is no certainty that action will be taken in response to the existence of environmental health problems. This depends not only on adequate recognition of the problems concerned, but also on political will, economic and technological capability, and public acceptance of the actions involved. As a consequence, indicators from higher up the framework must invariably be used and interpreted with care.
INDICATOR REPORTING
The way in which indicators are reported may have significant consequences for decision-making. An indicator can be measured as a point in time and reported as such or it can be measured at several points in time and reported as 'change' over the time period. For example, the information provided by an indicator of health status differs if the health indicator is presented as: the number of persons with the health outcome of interest, the proportion of the population with this health outcome; or the annual change in that proportion. Similarly, for an environmental indicator, such as desertification, very different information is provided if desertification is described as the proportion of the area of a country that is classified as a desert, or the annual change in that proportion.
In the health field, available epidemiological tools provide a number of options for reporting on indicators. The occurrence of ill health (or good health) in a population is reported as either the number of existing cases of a disease (prevalence) or as the number of new cases of the disease occurring in a set time period (incidence). Because the number of cases depends on the size of the population studied, prevalence and incidence are mostly reported as rates (e.g. number of cases per 1000 population). Moreover, these measurements can be reported as 'change' when a comparison rate (expected 'background' rate or the rate at some earlier time) is available.
CONCLUSIONS
Population growth, technological and economic development, changing lifestyles and social attitudes, natural processes of change in the physical environment and the long-term impacts of past human interventions are all contributing to increasing problems of environmental health. To address these problems effectively, decision- makers require better information. This information needs to be reliable, consistent and targeted at the issues of real concern. It needs to be made available quickly and available in an understandable and usable form.
Environmental health indicators offer a means of providing this information. To be effective, however, they need to be carefully designed and subject to rigorous quality control. In recent years, much progress has been made in developing indicators in a wide range of fields and for many different applications, much of it driven by the Earth Summit of 1992 and the adoption of Agenda 21. Progress on developing environmental health indicators, however, has so far been slower, partly due to the lack of consensus about the key issues which need to be addressed. Usable environmental health indicators also depend upon the existence of known and definable links between environment and health. Difficulties in establishing these relationships, due, for example, to the complexity of confounding effects and the problems of acquiring reliable exposure data, inhibit the practical utility of many potential indicators and make it difficult to establish core indicator sets. It is also important to appreciate that environmental health indicators are not panaceas. Indicators are only as good as the understanding which was used in developing them, and the models and data on which they are based.
Environmental health indicators consequently have limits, but if used within these limits, and with awareness of the limits, they can still make a major contribution to improved management and protection of public health. Well-designed and well-constructed indicators provide the capability to define more clearly the environmental health issues which need to be addressed, to prioritize these issues, to identify where action can best be taken, to compare the potential cost-effectiveness of different actions and strategies, to assess the effects of past or current action, to define the remaining research needs, and to inform the various stakeholders involved. The development and use of purpose-designed indicators to meet specific needs, therefore, remains a priority.
REFERENCES
United Nations (1993) Agenda 21: Programme of Action for Sustainable Development. United Nations, New York.
USEPA (1994) A Conceptual Framework to Support the Development and Use of Environmental Information (draft). USEPA.
World Commission on Environment and Development (1987) Our Common Future. Oxford University Press, Oxford