While, reductions in GHG emissions are directed principally at achieving long-term global benefit by mitigating climate change, secondary health effects of mitigation technologies are likely to occur at a local level and more immediately. This has important implications for "joint implementation" and "clean development mechanisms". Governments can thus act to optimise health as well as GHG emissions reduction in their populations.

Through the Clean Development Mechanism, investment in GHG mitigation strategies that promote more efficient or low-carbon energy generation can improve health in less developed countries. Fossil fuel combustion produces air pollutants that have both short- and long-term impacts on mortality and morbidity rates (Katsouyanni et al., 1997). The secondary health benefit of reducing air pollutant concentrations can be substantial, particularly for the impacts of particulates, nitrogen oxides and sulphur dioxide. For example, the Working Group on Public Health and Fossil Fuel Combustion (1997) estimated the global health benefit of reduced outdoor exposure to particulates as 700,000 fewer premature deaths per year by 2020 under a Kyoto-like mitigation scenario compared to a business-as-usual scenario. The authors emphasised that simplifying assumptions in the model precluded precise predictions of the number of avoidable deaths and that the estimate of avoided deaths is merely indicative of the approximate magnitude of the likely health benefits of the climate policy scenario. Moreover, comparisons of premature mortality are difficult to interpret across differing populations.

Some country-specific estimates of air pollution-related secondary benefits have also been undertaken. China is an important source of GHG emissions and already suffers a high burden of ill health due to air pollution (WRI, 1998). Reductions in GHGs emissions would have large benefits for the Chinese population through reductions in indoor air pollution (Wang and Smith, 1999a,b). Several studies have evaluated other secondary health benefits associated with air pollution reduction - such as the direct costs of health services used (e.g., Aaheimet al., 1997); or costing lives lost or years-of-life lost (e.g., Ontario Medical Association, 1998; see also IPCC TAR Working Group III (Chapter 9), forthcoming).

The degree of health benefit depends markedly on the particular mitigation scenario that is used in the assessment (Wang and Smith, 1999a). Furthermore, in countries with high levels of air pollution, mitigation strategies will have a greater health benefit per unit GHG emission reduction than in those countries with low levels of air pollution. The degree of health benefit also depends on the current source of energy and the proposed alternative. Switching from natural gas power plants to wind or solar power sources has little near-term health benefit because gas burns relatively cleanly. Reductions in sectors where emissions occur near human activities, e.g., in transport and household/domestic sectors, will have more near-term health benefit per unit GHG reduction than in other sectors (see Box 14.1).

The adverse impacts on health of mitigation technologies must also be considered. An increased demand for hydropower may increase the building of large dams; yet, there has been growing concern about the social and health impacts of large hydropower projects (Goodland, 1997). In 1998, a World Commission on Dams was set up by the World Bank and the World Conservation Union to set new international guidelines. Large water projects in tropical and sub-tropical countries have resulted in increases in the prevalence of schistosomiasis and other diseases, loss of food security and social problems that negatively influence health (Oomen et al., 1994; Brantly and Ramsey, 1998; Lerer and Scudder, 1999). Social impacts include the dislocation of the rural population living in the area to be inundated; over 40 million people are estimated to have been displaced by dam projects over the past 10 years (Cernea, 1996). Resettled families lose homes, land, food sources and employment. Communities that host the resettlers face increased population densities, which places severe pressure on natural resources and water and sanitation infrastructure. Reduction of fish populations downstream has affected indigenous populations that rely on fish as their main source of animal protein. For example, downstream of the Tucurui dam, Brazil, affected communities along the Tocantins River complained that seven fish species have almost disappeared (Confalonieri, personal communication, 1998). The health impact of dams may also include increases in the transmission of vector-borne diseases. For example, an increase in the population of mosquito vectors of malaria due to the availability of more breeding sites has been observed at the Tucurui dam (Tadei, 1993).

Small-scale hydroelectric power generation schemes rarely use dams, but instead collect water from smaller structure such as weirs. Such schemes may also have undesirable local effects, such as providing vector breeding sites (Ghebreyesus et al., 1999), although small-scale dams are generally less environmentally damaging than are large-scale projects. The most promising application of small-scale hydropower appears to be in isolated communities, to provide electricity for limited uses (e.g., lighting, communications, refrigeration) when there is no other feasible means of providing a continuous supply. In these conditions hydro schemes may have positive effects (including benefits for public health such as better vaccine storage, telemedicine, education) that outweigh relatively minor, local environmental and health impacts (EECA, 1996).

The evaluation and full-cost accounting of GHG mitigation technologies should include an assessment of the health impacts of these technologies. Regrettably, although environmental health impact assessments are an important part of environmental impact assessment, they are often omitted and are all too rarely a prime determinant of the ultimate policy decision. Lack of awareness of long-term objectives and of a political will to value human well-being and health over material gain contribute to this problem (Last, 1997).