The impact on human health

health from increased malnutrition and from a prevalence of conditions more likely to lead to the spread of diseases from a variety of causes.

How about direct effects ofthe climate change itselfon human health? Humans can adapt themselves and their buildings so as to live satisfactorily in very varying conditions and have great ability to adapt to a wide range of climates. The main difficulty in assessing the impact of climate change on health is that of unravelling the influences of climate from the large number of other factors (including other environmental factors) that affect health.

The main direct effect on humans will be that of heat stress in the extreme high temperatures that will become more frequent and more widespread especially in urban populations (see box and Figure 6.6). In large cities where heatwaves commonly occur death rates can be doubled or tripled during days of unusually high temperatures.58 Although such episodes may be followed by periods with fewer deaths showing that some of the deaths would in any case have occurred about that time, most of the increased mortality seems to be directly associated with the excessive temperatures with which old people in particular find it hard to cope. On the positive side, mortality due to periods of severe cold in winter will be reduced. The results of studies are equivocal regarding whether the reduction in winter mortality will be greater or less than the increase in summer mortality. These studies have largely been confined to populations in developed countries, precluding a more general comparison between changes in summer and winter mortality.

A further likely impact of climate change on health is the increased spreading of diseases in a warmer world. Many insect carriers of disease thrive better in warmer and wetter conditions. For instance, epidemics of diseases such as viral encephalitides carried by mosquitoes are known to be associated with the unusually wet conditions that occur in the Australian, American and African continents associated with different phases of the El Niño cycle.60 Some diseases, currently largely confined to tropical regions, with warmer conditions could spread into mid latitudes. Malaria is an example of such a disease that is spread by mosquitoes under conditions that are optimum in the temperature range of 15-32 °C with humidities of 50-60%. It currently represents a huge global public health problem, causing annually around 300 million new infections and over 1 million deaths. Under climate change scenarios, most predictive model studies indicate a net increase in the geographic range (and in the populations at risk) of potential transmission of malaria and dengue infections, each of which currently impinge on 40-50% of the world's population. Other diseases that are likely to spread for the same reason are yellow fever and some viral encephalitides. In all cases, however, actual disease occurrence will be strongly influenced by local environmental conditions,

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