For at least two millennia natural water quality has deteriorated progressively and reached contamination levels where water uses are severely limited or the water can be harmful to humans. This deterioration is related to the socio-economic development within a river basin, but long-range atmospheric transport of contaminants has now changed this picture: even remote areas can be indirectly polluted (Meybeck and Helmer 1989).

Medieval reports and complaints about inadequate excreta disposal, foul and stinking water courses within overcrowded cities and other similar problems were an early manifestation of urban water pollution. The first time that a clear causal linkage between bad water quality and human health effects was established was in 1854, when John Snow traced back the outbreak of cholera epidemics in London to a particular drinking water source.

Since the middle of the twentieth century, and concurrent with the onset of accelerated industrial growth, various types of water pollution problems have occurred in rapid succession. Figure 1 illustrates the types of problems as they became apparent in European freshwaters.

Figure 1. Types of water pollution problems

In summarizing the European situation it can be stated that: (1) the challenges of the past (pathogens, oxygen balance, eutrophication, heavy metals) have been recognized, researched and the necessary controls identified and more or less implemented and (2) the challenges of today are of a different nature—on the one hand, “traditional” point and non-point pollution sources (nitrates) and ubiquitous environmental contamination problems (synthetic organics), and, on the other                                                                                                                       hand, “third generation” problems interfering with global cycles                                                                                                                      (acidification, climate change). 

In the past, water pollution in the developing countries resulted mainly from the discharge of untreated wastewater. Today it is more complex as a result of the production of hazardous wastes from industries and the rapidly increasing use of pesticides in agriculture. In fact, water pollution today in some developing countries, at least in the newly industrializing ones, is worse than in industrialized countries (Arceivala 1989). Unfortunately, developing countries, on the whole, are badly lagging behind in getting control over their major pollution sources. As a consequence, their environmental quality is gradually deteriorating (WHO/UNEP 1991).

Types and Sources of Pollution

There are a large number of microbial agents, elements and compounds which may cause water pollution. They can be classified as: microbiological organisms, biodegradable organic compounds, suspended matter, nitrates, salts, heavy metals, nutrients and organic micropollutants.

Microbiological organisms

Microbiological organisms are common in freshwater bodies polluted particularly by discharges of untreated domestic wastewater. These microbial agents include pathogenic bacteria, viruses, helminths, protozoa and several more complex multicellular organisms that can cause gastro-intestinal illness. Other organisms are more opportunistic in nature, infecting susceptible individuals through body contact with contaminated water or by inhalation of poor quality water droplets in aerosols of various origins.

Biodegradable organic compounds

Organic substances of either natural origin (allochthonous terrestrial detritus or autochthonous debris of aquatic plants) or from anthropogenic sources (domestic, agricultural and some industrial wastes) are decomposed by aerobic microbes as the river continues its course. The consequence is a lowering of the oxygen level downstream of the wastewater discharge, impairing the quality of the water and the survival of the aquatic biota, particularly of high-quality fish.

Particulate matter

Particulate matter is a major carrier of organic and inorganic pollutants. Most toxic heavy metals, organic pollutants, pathogens and nutrients, such as phosphorus, are found in suspended matter. An appreciable amount of the biodegradable organic material responsible for consumption of dissolved oxygen from rivers is also found in suspended particles. Particulate matter comes from urbanization and road construction, deforestation, mining operations, dredging operations in rivers, natural sources which are linked to continental erosion, or natural catastrophic events. Coarser particles are deposited on river beds, in reservoirs, in the flood plain and in wetlands and lakes.

Nitrates

The concentration of nitrates in unpolluted surface waters ranges from less than 0.1 to one milligrams per litre (expressed as nitrogen), so nitrate levels in excess of 1 mg/l indicate anthropogenic influences such as discharge of municipal wastes and urban and agricultural run-off. Atmospheric precipitation is also an important source of nitrate and ammonia to river basins, particularly in areas not affected by direct pollution sources—for example, some tropical regions. High concentrations of nitrate in drinking water may lead to acute toxicity in bottle-fed infants during their first months of life, or in the elderly, a phenomenon called methaemoglobinaemia.

Salts

Water salinization may be caused by natural conditions, such as geochemical interaction of waters with salty soils or by anthropogenic activities, including irrigated agriculture, sea water intrusion due to excessive pumping of groundwaters in islands and coastal areas, disposal of industrial wastes and of oilfield brines, highway de-icing, landfill leachates and leaking sewers.

While hampering beneficial uses, particularly for irrigation of sensitive crops or for drinking, salinity in itself may not, at even quite high levels, be directly harmful to health, but the indirect effects can be dramatic. The loss of fertile agricultural land and reduced crop yields caused by waterlogging and soil salinization of irrigated areas destroy the livelihood of whole communities and cause hardships in the form of food shortages.

Heavy metals

Heavy metals such as lead, cadmium and mercury are micro-pollutants and of special interest as they have health and environmental significance due to their persistence, high toxicity and bio-accumulation characteristics.

There are basically five sources of heavy metals contributing to water pollution: geological weathering, which provides the background level; industrial processing of ores and metals; the use of metal and metal compounds, such as chromium salts in tanneries, copper compounds in agriculture, and tetraethyl lead as an anti-knock agent in gasoline; leaching of heavy metals from domestic wastes and solid waste dumps; and heavy metals in human and animal excretions, particularly zinc. Metals released to the air from automobiles, fuel burning and industrial process emissions may settle on land and ultimately run off to surface waters.

Nutrients

Eutrophication is defined as the enrichment of waters with plant nutrients, primarily phosphorus and nitrogen, leading to enhanced plant growth (both algae and macrophytes) which results in visible algae blooms, floating algal or macrophyte mats, benthic algae and submerged macrophyte agglomerations. When decaying, this plant material leads to the depletion of the oxygen reserves of water bodies, which, in turn, causes an array of secondary problems such as fish mortality and liberation of corrosive gases and other undesirable substances, such as carbonic gas, methane, hydrogen sulphide, organoleptic substances (causing taste and odour), toxins and so on.

The source of phosphorus and nitrogen compounds is primarily untreated domestic wastewater, but other sources such as drainage of artificially fertilized agricultural land, surface run-off from intensive livestock farming and some industrial wastewaters can also substantially increase the trophic level of lakes and reservoirs, particularly in tropical developing countries.

The main problems associated with eutrophication of lakes, reservoirs and impoundments are: oxygen depletion of the bottom layer of lakes and reservoirs; water quality impairment, leading to treatment difficulties, particularly for the removal of taste- and odour-causing substances; recreational impairment, increased health hazards to bathers and unsightliness; fisheries impairment due to fish mortality and the development of undesirable and low-quality fish stocks; ageing and reducing the holding capacity of lakes and reservoirs by silting; and increase of corrosion problems in pipes and other structures.

Organic micropollutants

Organic micropollutants can be classified in groups of chemical products on the basis of how they are used and consequently how they are dispersed in the environment:

• Pesticides are substances, generally synthetic, that are deliberately introduced into the environment to protect crops or control disease vectors. They are found in various distinct families, such as organochloride insecticides, organophosphate insecticides, herbicides of the plant hormone type, triazines, substituted ureas and others.

• Materials for widespread household and industrial use comprise volatile organic substances used as extraction solvents, solvents for degreasing metals and dry-cleaning clothes, and propellants for use in aerosol containers. This group also includes halogenated derivatives of methane, ethane and ethylene. As they are widely used their rates of dispersion in the environment, compared with the amounts produced, are generally high. The group also contains the polycyclic aromatic hydrocarbons, whose presence in the environment results from the extraction, transport and refining of petroleum products and the dispersion of combustion products resulting from their use (petrol and heating oil).

• Materials used essentially in industry include substances which are direct or intermediate agents of chemical synthesis, such as carbon tetrachloride for synthesizing freons; vinyl chloride for polymerizing PVC; and chlorinated derivates of benzene, naphthalene, phenol and aniline for manufacturing dyestuffs. The group also contains finished products used in closed systems, such as heat-exchange fluids and dielectrics.

Organic micropollutants are generated from point and diffuse sources, either urban or rural. The largest part originates in major industrial activities such as petrol refining, coal mining, organic synthesis and the manufacture of synthetic products, the iron and steel industries, the textile industry and the wood and pulp industry. Effluents from pesticides factories may contain considerable quantities of these manufactured products. A significant proportion of organic pollutants are discharged into the aquatic environment as run-off from urban surfaces; and in agricultural areas, pesticides applied to crops may reach surface waters through rainwater run-off and artificial or natural drainage. Also, accidental discharges have led to severe ecological damage and temporary closure of water supplies.

Urban Pollution

Owing to this continuously expanding, aggressive and multi-faceted pollution scenario, the problem of maintaining the quality of water resources has become acute, particularly in the more urbanized areas of the developing world. Maintaining water quality is hampered by two factors: failure to enforce pollution control at the main sources, especially industries, and inadequacy of sanitation systems and of garbage collection and disposal (WHO 1992b). See some examples of water pollution in different cities in developing countries.

Examples of water pollution in selected cities

Karachi (Pakistan)

The Lyari river, which runs through Karachi, Pakistan’s largest industrial city, is an open drain from both the chemical and the microbiological point of view, a mixture of raw sewage and untreated industrial effluents. Most industrial effluents come from an industrial estate with some 300 major industries and almost three times as many small units. Three-fifths of the units are textile mills. Most other industries in Karachi also discharge untreated effluents into the nearest water body.

Alexandria (Egypt)

Industries in Alexandria account for around 40% of all Egypt’s industrial output, and most discharge untreated liquid wastes into the sea or into Lake Maryut. In the past decade, fish production in Lake Maryut declined by some 80% because of the direct discharge of industrial and domestic effluents. The lake has also ceased to be a prime recreational site because of its poor condition. Similar environmental degradation is taking place along the seafront as a result of the discharge of untreated wastewater from poorly located outfalls.

Shanghai (China)

Some 3.4 million cubic metres of industrial and domestic waste pour mostly into the Suzhou Creek and the Huangpu River, which flows through the heart of the city. These have become the main (open) sewers for the city. Most of the waste is industrial, since few houses possess flush toilets. The Huangpu has essentially been dead since 1980. In all, less than 5% of the city’s wastewater is treated. The normally high water table also means that a variety of toxins from industrial plants and local rivers find their way into groundwater and contaminate wells, which also contribute to the city water supply.

São Paulo (Brazil)

The Tiete River, as it passes through Greater São Paulo, one of the world’s largest urban agglomerations, receives 300 tonnes of effluents each day from 1,200 industries located in the region. Lead, cadmium and other heavy metals are among the main pollutants. It also receives 900 tonnes of sewage each day, of which only 12.5% is treated by the five sewage treatment stations located in the area.

Source: Based on Hardoy and Satterthwaite 1989.

Health Impacts of Microbial Pollution

Diseases arising from the ingestion of pathogens in contaminated water have the greatest impact worldwide. “An estimated 80% of all diseases, and over one-third of deaths in developing countries are caused by the consumption of contaminated water, and on average as much as one-tenth of each person’s productive time is sacrificed to water-related diseases” (UNCED 1992). Water-borne diseases are the largest single category of communicable diseases contributing to infant mortality in developing countries and second only to tuberculosis in contributing to adult mortality, with one million deaths per year.

The total annual number of cholera cases reported to the WHO by its member states has reached levels unprecedented during the seventh pandemic, with a peak of 595,000 cases in 1991 (WHO 1993). Table 1 shows the global morbidity and mortality rates of the major water-related diseases. These figures are, in many cases, grossly underestimated, since reporting of disease cases is done quite erratically by many countries.

Table 1. Global morbidity and mortality rates of main diseases related to water

Source: Galal-Gorchev 1994.

Health Impacts of Chemical Pollution

The health problems associated with chemical substances dissolved in water arise primarily from their ability to cause adverse effects after prolonged periods of exposure; of particular concern are contaminants that have cumulative toxic properties such as heavy metals and some organic micropollutants, substances that are carcinogenic and substances that may cause reproductive and developmental effects. Other dissolved substances in water are essential ingredients of dietary intake and yet others are neutral with regards to human needs. Chemicals in water, particularly in drinking water, may be classified into three typical categories for the purpose of health impact (Galal-Gorchev 1986):

• Substances exerting an acute or chronic toxicity upon consumption. The severity of the health impairment increases with the increase of their concentration in drinking water. On the other hand, below a certain threshold concentration no health effects can be observed—that is, the human metabolism can handle this exposure without measurable long-term effects. Various metals, nitrates, cyanides and so on fall within this category.

• Genotoxic substances, which cause health effects such as carcinogenicity, mutagenicity and birth-defects. According to present scientific thinking there is no threshold level which could be considered safe, since any amount of the substance ingested contributes to an increase in cancer and similar risks. Complex mathematical extrapolation models are used to determine such risks, since very little epidemiological evidence exists. Synthetic organics, many chlorinated organic micropollutants, some pesticides and arsenic fall within this category.

• For some elements, such as fluoride, iodine and selenium, the contribution made by drinking water is crucial and, if deficient, causes more or less severe health effects. At high concentrations, however, these same substances cause equally severe health effects, but of a different nature.

Environmental Impacts

The impacts of environmental pollution on freshwater quality are numerous and have existed for a long time. Industrial development, the advent of intensive agriculture, the exponential development of human populations and the production and use of tens of thousands of synthetic chemicals are among the main causes of water quality deterioration at local, national and global scales. The major issue of water pollution is the interference with actual or planned water uses.

One of the most severe and ubiquitous causes of environmental degradation is the discharge of organic wastes into watercourses (see “Biodegradable organic compounds” above). This pollution is mainly of concern in the aquatic environment where many organisms, for example fish, require high oxygen levels. A serious side effect of water anoxia is the release of toxic substances from particulates and bottom sediments in rivers and lakes. Other pollution effects from domestic sewage discharges into watercourses and aquifers include the build-up of nitrate levels in rivers and groundwaters, and the eutrophication of lakes and reservoirs (see above, “Nitrates” and “Salts”). In both cases, the pollution is a synergistic effect of sewage effluents and agricultural run-off or infiltration.

Economic Impacts

The economic consequences of water pollution can be rather severe due to detrimental effects on human health or on the environment. Impaired health often lowers human productivity, and environmental degradation reduces the productivity of water resources used directly by people.

The economic disease burden can be expressed not only in costs of treatment, but also in quantifying the loss of productivity. This is particularly true for primarily disabling diseases, such as diarrhoea or Guinea Worm. In India, for example, there are about 73 million workdays per year estimated to be lost due to water-related diseases (Arceivala 1989).

Deficiencies in sanitation and the resulting epidemics can also lead to severe economic penalties. This became most apparent during the recent cholera epidemic in Latin America. During the cholera epidemic in Peru, losses from reduced agricultural exports and tourism were estimated at one billion US dollars. This is more than three times the amount that the country had invested in water supply and sanitation services during the 1980s (World Bank 1992).

Water resources affected by pollution become less suitable as sources of water for municipal supply. As a consequence, expensive treatment has to be installed or clean water from far away has to be piped to the city at much higher costs.

In the developing countries of Asia and the Pacific, environmental damage was estimated by Economic and Social Commission for Asia and the Pacific (ESCAP) in 1985 to cost about 3% of the GNP, amounting to US$250 billion, while the cost of repairing such damage would range around 1%.

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The WHO Commission on Health and Environment (1992a) Panel on Energy considered four energy-related issues to be of the highest immediate and/or future concern for environmental health:

1. exposure to noxious agents in the course of domestic utilization of biomass and coal
2. exposure resulting from urban air pollution in numerous large cities of the world
3. the possible health-related impacts of climate change
4. serious accidents with environmental impacts on the health of the general public.

Quantitative assessment of the health risks from different energy systems requires system-wide evaluation of all steps in a fuel cycle, starting with extraction of raw resources, and concluding with the final consumption of energy. For valid intertechnology comparisons to be made, methods, data and end-use demands must be similar and specified. In quantifying the effects of end-use demands, differences in conversion efficiencies of energy- and fuel-specific devices to useful energy must be evaluated.

Comparative assessment is built around the idea of the Reference Energy System (RES), which depicts fuel cycles step-by-step, from extraction through processing to combustion and ultimate disposal of wastes. The RES provides a common, simple framework for defining flows of energy and related data used for risk assessment. An RES (figure 1) is a network representation of the main components of an energy system for a given year, specifying resource consumption, fuel transportation, conversion processes and end uses, thereby compactly incorporating the salient features of the energy system while providing a framework for assessment of major resource, environmental, health and economic effects that can result from new technologies or policies.

Figure 1. Reference energy system, year 1979

Based on their health risks, energy technologies can be classified into three groups:

1. The fuels group is characterized by use of large amounts of fossil fuels or biomass—coal, oil, natural gas, wood and so on—the collection, processing and transport of which have high accident rates that dominate occupational risks and the burning of which produces large amounts of air pollution and solid waste that dominate public risks.
2. The renewable group is characterized by use of diffuse renewable resources with low energy density—sun, wind, water—which are available in enormous quantities at no cost, but the capturing of which requires large areas and construction of expensive facilities capable of “concentrating” them into useful forms. Occupational risks are high and dominated by construction of the facilities. Public risks are low, mostly confined to low-probability accidents, such as dam failures, equipment failures and fires.
3. The nuclear group includes nuclear fission technologies, distinguished by extremely high energy densities in the processed fuel, with corresponding low quantities of fuel and wastes to process, but with low concentrations in the earth’s crust, necessitating a large mining or collection effort. Occupational risks are, therefore, relatively high and dominated by mining and processing accidents. Public risks are small and dominated by routine operations of reactors. Special attention must be given to public fears of risks from exposure to radiation from nuclear technologies—fears which are relatively large per unit risk to health.

The significant health effects of technologies for generating electricity are shown in table 1, table 2 and table 3.

Table 1. Significant health effects of technologies for generating electricity - fuels group

* As an energy source, usually considered as renewable.

Table 2. Significant health effects of technologies for generating electricity - renewable group

Table 3. Significant health effects of technologies for generating electricity - nuclear group

Studies of health effects of wood burning in the United States, like analyses of other energy sources, were based on the health effects of supplying a unit amount of energy, that is, that needed to heat one million dwelling years. This is 6 ×10⁷ GJ heat, or 8.8 ×10⁷ GJ wood input at 69% efficiency. Health effects were estimated in gathering, transport and combustion stages. Oil and coal alternatives were scaled from earlier work (see figure 2). The uncertainties in gathering are ± a factor of ~2, those in home fires ± a factor of ~3, and those in air pollution ± a factor greater than 10. If the hazards of nuclear electric were plotted on the same scale, the total risk would be approximately one-half that of mining for coal mining.

Figure 2. Health effects per unit amount of energy

A convenient way to help understand the risk is to scale it to a single person supplying one dwelling with wood over 40 years (figure 3). This results in a total risk of fatality of ~1.6 x 10^–3 (i.e., ~0.2%). This can be compared with the risk of death in an automobile accident in the United States during the same time, ~9.3 x 10^–3 (i.e., ~1%), which is five times greater. Wood burning presents risks which are of the same order as more conventional heating technologies. Both are well below the overall risk of other common activities, and many aspects of the risk are clearly amenable to preventive measures.

Figure 3. Risk, to a single person, of fatality due to supplying one dwelling with wood fuel for 40 years

The following comparisons for health risks can be made:

• Acute occupational risk. For the coal cycle, occupational risk is distinctly higher than that associated with oil and gas; it is about the same as that associated with renewable energy systems, when their construction is included in the assessment, and it is about 8-10 times higher than the corresponding risks for nuclear. Future technological advances in renewable solar and wind energy sources may result in a significant reduction in the acute occupational risk associated with these systems. Hydro-electricity generation entails comparatively high acute occupational risk.
• Late occupational risk. Late fatalities arise mainly in coal and uranium mining, and are roughly of the same size. Underground coal mining, however, appears to be more dangerous than underground uranium mining (calculation from the basis of a normalized unit of electricity generated). Use of surface-mined coal, on the other hand, leads in total to fewer late fatalities than does use of nuclear energy.
• Acute public risk. These risks, mostly due to transportation accidents, are highly dependent on distance travelled and mode of transport. The risk of nuclear is 10-100 times lower than those of all the other options, mainly because of the relatively low quantity of materials to be transported. The coal cycle has the highest acute public risk because of the large material transport using the same reasoning.
• Late public risk. There are great uncertainties associated with late public risks associated with all the energy sources. Late public risks for nuclear and natural gas are about equal and are, at least ten times lower than those associated with coal and oil. Future developments are expected to result in significant decreases in late public risks for renewables.

Clearly, health effects of different energy sources depend on the quantity and type of energy use. These vary greatly geographically. Fuelwood is the fourth largest contribution to world energy supply, after petroleum, coal and natural gas. Close to half the world population, especially those living in the rural and urban areas of developing countries, depend on it for cooking and heating (either wood or its derivative, charcoal, or, in the absence of either of these, on agricultural residues or dung). Fuelwood constitutes more than half the world’s consumption of wood, rising to 86% in developing countries and 91% in Africa.

In considering new and renewable sources of energy such as solar energy, wind power, and alcohol fuels, the idea of a “fuel cycle” must encompass industries such as solar photovoltaics, where virtually no risk attaches to the operation of the device but a substantial amount—often ignored—may be involved in its manufacture.

Attempts were made to deal with this difficulty by expanding the fuel cycle concept to include all stages in developing an energy system—including, for example, the concrete that goes into the plant that manufactures the glass for the solar collector. The issue of completeness has been addressed by noting that the backward analysis of manufacturing steps is equivalent to a set of simultaneous equations whose solution—if linear—is expressible as a matrix of values. Such an approach is familiar to economists as input-output analysis; and the appropriate numbers, showing how much each economic activity draws on the others, have already been derived—although for aggregate categories that may not exactly match the manufacturing steps one wishes to scrutinize for measuring health damage.

No single method of comparative risk analysis in the energy industry is fully satisfactory by itself. Each has advantages and limitations; each provides a different kind of information. Given the level of uncertainty of health risk analyses, results from all methods should be examined to provide as detailed a picture as possible, and fuller understanding of the magnitudes of associated uncertainties.

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Urbanization is a major feature of the contemporary world. At the beginning of the nineteenth century there were some 50 million people living in urban areas. By 1975 there were 1.6 billion, and by the year 2000 there will be 3.1 billion (Harpham, Lusty and Vaugham 1988). Such figures outstrip by far the growth of rural population.

However, the process of urbanization has often had hazardous impacts on the health of those who work and live in cities and towns. To a greater or lesser extent, the production of adequate housing, the provision of urban infrastructure and the control of traffic has not kept pace with the growth of urban population. This has generated a myriad of health problems.

Housing

Housing conditions throughout the world are far from adequate. For example, by the mid-1980s, 40 to 50% of the population in many cities in developing countries were living in substandard accommodations (WHO Commission on Health and Environment 1992b). Such figures have increased ever since. Although the situation in industrialized countries is less critical, housing problems such as decay, overcrowding and even homelessness are frequent.

The major aspects of the residential environment which influence health, and their associated hazards, are presented in table 1. The health of a worker is likely to be affected if his or her residence is deficient in one or more of these aspects. In developing countries, for instance, some 600 million urban dwellers live in health- and life-threatening homes and neighbourhoods (Hardoy, Cairncross and Satterthwaite 1990; WHO 1992b).

Table 1. Housing and health

Source: Hardoy et al. 1990; Harpham et al. 1988; WHO Commission on Health and Environment 1992b.

Housing problems may also have a direct effect on occupational health, in the case of those who work in residential environments. Those include domestic servants and also a growing number of small-scale producers in a variety of cottage industries. These producers may be further affected when their production processes generate some form of pollution. Selected studies in these types of industries have detected hazardous wastes with consequences such as cardiovascular diseases, skin cancer, neurological disorders, bronchial cancer, photophobia and infant methaemoglobinaemia (Hamza 1991).

Prevention of home-related problems includes action in different stages of housing provision:

1. location (e.g., safe and vector-free sites)
2. house design (e.g., spaces with adequate size and climatic protection, use of non-perishable building materials, adequate protection for equipment)
3. construction (prevention of construction defects)
4. maintenance (e.g., proper control of equipment, proper screening).

The insertion of industrial activities in the residential environment may require special measures of protection, according to the particular process of production.

The specific housing solutions may vary widely from place to place, depending on the social, economic, technical and cultural circumstances. A great number of cities and towns do have local planning and building legislation that includes measures to prevent health hazards. However, such legislation is often not enforced due to ignorance, lack of legal control or, in most cases, lack of financial resources to build proper housing. Therefore, it is important not only to design (and update) adequate codes, but also to create the conditions for their implementation.

Urban Infrastructure: The Provision of Environmental Health Services

Housing may also affect health when it is not properly supplied with environmental health services such as garbage collection, water, sanitation and drainage. The inadequate provision of these services, however, extend beyond the housing realm, and may cause hazards for the city or town as a whole. Standards of provision of these services are still critical in a large number of places. For example, 30 to 50% of solid waste generated within urban centres is left uncollected. In 1985 there were 100 million more people without water service than in 1975. More than two billion people still have no sanitary means to dispose of human waste (Hardoy, Cairncross and Satterthwaite 1990; WHO Commission on Health and Environment 1992b). And the media have frequently shown cases of floods and other accidents connected to inadequate urban drainage.

Hazards derived from deficient provision of environmental health services are presented in table 2. Cross-service hazards are also common—e.g., contamination of water supply due to lack of sanitation, dissemination of refuse through non-drained water. To give one illustration of the extent of infrastructural problems among many, a child is killed worldwide every 20 seconds due to diarrhoea—which is a major outcome of deficient environmental health services.

Table 2. Urban infrastructure and health

Source: Hardoy et al. 1990; WHO Commission on Health and Environment 1992b.

Those labourers whose immediate or wider working environment is not adequately supplied with such services are exposed to a profusion of occupational health risks. Those who work in the provision or maintenance of services, such as garbage pickers, sweepers and scavengers, are further exposed.

There exist indeed technical solutions capable of ameliorating the provision of environmental health services. They encompass, among many others, garbage recycling schemes (including support to scavengers), use of different kinds of garbage collection vehicles to reach different types of roads (including those of informal settlements), water-saving fittings, tighter control of water leakages and low-cost sanitation schemes such as ventilated pit latrines, septic tanks or small-bore sewers.

However, the success of each solution will depend on its appropriateness to the local circumstances and on the local resources and capacity to implement it. Political willingness is fundamental, but not enough. Governments have frequently found it difficult to provide urban services adequately by themselves. Success stories of good supply have often included cooperation between the public, private and/or voluntary sectors. A thorough involvement and support of the local communities is important. This often requires official recognition of the large number of illegal and semi-legal settlements (especially but not only in developing countries), which bear a heavy part of the environmental health problems. Workers directly involved in services such as garbage collection or recycling and sewerage maintenance need special equipment for protection, such as gloves, overalls and masks.

Traffic

Cities and towns have depended heavily on ground transport for the movement of people and goods. Thus, the increase in urbanization throughout the world has been accompanied by a sharp growth in urban traffic. However, such a situation has generated a large number of accidents. Some 500,000 people are killed in traffic accidents each year, two-thirds of which occur in urban or peri-urban areas. In addition, according to many studies in different countries, for every death there are ten to twenty persons injured. Many cases suffer permanent or prolonged loss of productivity (Urban Edge 1990a; WHO Commission on Health and Environment 1992a). A large share of such data relates to people on their way to or from work—and such a type of traffic accident has lately been considered an occupational hazard.

According to World Bank studies, the main causes of urban traffic accidents include: poor condition of vehicles; deteriorated streets; different types of traffic—from pedestrians and animals to trucks—sharing the same streets or lanes; non-existent foot-paths; and reckless road behaviour (both from drivers and pedestrians) (Urban Edge 1990a, 1990b).

A further hazard generated by the expansion of urban traffic is air and noise pollution. Health problems include acute and chronic respiratory diseases, malignancies and hearing deficiencies (pollution is also dealt with in other articles in this Encyclopaedia).

Technical solutions to ameliorate road and car safety (as well as pollution) abound. The major challenge seems to be changing the attitudes of drivers, pedestrians and public officials. Road safety education—from elementary school teaching to campaigns throughout the media—has often been recommended as a policy to target drivers and/or pedestrians (and such programmes have often had some degree of success when implemented). Public officials have the responsibility to design and enforce traffic legislation, inspect vehicles and design and implement engineering safety measures. However, according to the aforementioned studies, these officials seldom perceive traffic accidents (or pollution) as a top priority, or have the means to act dutifully (Urban Edge 1990a, 1990b). Therefore, they have to be targeted by educational campaigns, and supported in their work.

The Urban Fabric

In addition to the specific issues already noted (housing, services, traffic), the overall growth of the urban fabric also has had an impact on health. Firstly, urban areas are usually dense, a fact which facilitates the spread of communicable diseases. Secondly, such areas concentrate a large number of industries, and their associated pollution. Thirdly, through the process of urban growth, natural foci of disease vectors may get entrapped within new urban areas, and new niches for disease vectors may be established. Vectors may adapt to new (urban) habitats—for example, those responsible for urban malaria, dengue and yellow fever. Fourthly, urbanization has often had psychosocial consequences such as stress, alienation, instability and insecurity; which, in their turn, have led to problems such as depression and alcohol and drug abuse (Harpham, Lusty and Vaugham 1988; WHO Commission on Health and Environment 1992a).

Past experiences have demonstrated the possibility (and the need) to tackle health problems via improvements in urbanization. For instance, “¼ the remarkable decline in mortality rates and improvements in health in Europe and North America at the turn of the last century owe more to improved nutrition and improvements in water supply, sanitation and other aspects of housing and living conditions than to medical establishments” (Hardoy, Cairncross and Satterthwaite 1990).

Solutions for the mounting problems of urbanization need sound integration between (often separated) urban planning and management, and the participation of the different public, private and voluntary actors which operate in the urban arena. Urbanization affects a wide range of workers. Contrary to other sources or types of health problems (which might affect specific categories of workers), occupational hazards derived from urbanization cannot be dealt with through single trade union action or pressure. They require inter-profession action, or, even more broadly, action from the urban community in general.

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Climate Change

The major greenhouse gases (GHGs) consist of carbon dioxide, methane, nitrous oxide, water vapour and chlorofluorocarbons (CFCs). These gases allow sunlight to penetrate to the earth’s surface, yet prevent infrared radiant heat from escaping. The Intergovernmental Panel on Climate Change (IPCC) of the United Nations has concluded that emissions, primarily from industry, and destruction of greenhouse gas sinks, via poor land use management, especially deforestation, have substantially increased the concentrations of GHGs beyond natural processes. Without major policy shifts, pre-industrial carbon dioxide levels are expected to increase, yielding a 1.0-3.5°C rise in average global temperature by the year 2100 (IPCC in press).

The two primary components of climate change include (1) temperature elevation with concomitant weather instability and extremes and (2) rising sea-level due to thermoexpansion. These changes may result in an increased frequency of heat waves and hazardous air pollution episodes, reduced soil moisture, higher incidence of disruptive weather events, and coastal inundation (IPCC 1992). Subsequent health effects may include an increase in (1) heat-related mortality and morbidity; (2) infectious diseases, particularly those that are insect borne; (3) malnutrition from food supply shortages; and (4) public health infrastructural crises from weather disasters and sea-level rise, coupled with climate-related human migration (see figure 1).

Figure 1. Public health effects from the major components of global climate change

 Humans have an enormous capacity to adapt to climatic and environmental conditions. However, the rate of predicted climatic and potential ecological change is of great concern to medical and earth scientists alike. Many of the health effects will be mediated through ecological responses to altered climate conditions. For example, spread of vector-borne diseases will depend on shifts in vegetation and availability of reservoir or intermediate hosts, in conjunction with the direct effects of temperature and humidity on parasites and their vectors (Patz et al. 1996). Understanding the hazards of climate change will, therefore, require an integrated ecological risk assessment which demands new and complex approaches compared to conventional single-agent cause-and-effect risk analysis from empirical data (McMichael 1993).

Stratospheric Ozone Depletion

Stratospheric ozone depletion is occurring primarily from reactions with halogen free radicals from chlorofluorocarbons (CFCs), along with other halocarbons and methyl bromide (Molina and Rowland 1974). Ozone specifically blocks the penetration of ultravioletB radiation (UVB), which contains the most biologically destructive wavelengths (290-320 nanometres). UVB levels are expected to rise disproportionately in temperate and arctic zones, since a clear relationship has been established between higher latitudes and the extent of ozone thinning (Stolarski et al. 1992).

For the period 1979-91, average ozone loss has been estimated at 2.7% per decade, correcting for solar cycle and other factors (Gleason et al. 1993). In 1993, researchers using a sensitive new spectroradiometer in Toronto, Canada, discovered that current ozone depletion has caused local increases in ambient UVB radiation of 35% in winter and 7% in summer, relative to 1989 levels (Kerr and McElroy 1993). Earlier estimates by the UN Environment Programme (UNEP) predicted a 1.4% rise in UVB per 1% drop in stratospheric ozone (UNEP 1991a).

The direct health impacts from stratospheric ozone depletion, which leads to increased ambient UVB radiation, include (1) skin cancer (2) ocular diseases and (3) immunosuppression. Indirect effects to health may occur from crop damage by ultraviolet radiation.

Health Effects of Temperature and Precipitation Change

Heat-related morbidity and mortality

Physiologically, humans have a great capacity for thermoregulation up to a threshold temperature. Weather conditions exceeding threshold temperatures and persisting for several consecutive days cause increased mortality in the population. In large cities, poor housing combined with the urban “heat island” effect further exacerbate conditions. In Shanghai, for instance, this effect can be as high as 6.5 °C on a windless evening during winter (IPCC 1990). Most heat-related fatalities occur in the elderly population and are attributed to cardiovascular and respiratory disorders (Kilbourne 1989). Key meteorological variables contribute to heat-related mortality, the most significant being high night-time readings; the greenhouse effect is predicted to especially elevate these minimum temperatures (Kalkstein and Smoyer 1993).

Temperate and polar regions are expected to warm disproportionately more than tropical and subtropical zones (IPCC 1990). Based on predictions by the US National Aeronautics and Space Administration (NASA), average summer temperatures in New York and St. Louis, for example, would rise by 3.1 and 3.9 °C, respectively, if ambient CO₂ doubles. Even with adjustment for physiological acclimatization, annual summer mortality in temperate cities such as these could rise over fourfold (Kalkstein and Smoyer 1993).

Atmospheric chemistry is an important contributing factor in the formation of urban photochemical smog, whereby photodecomposition of NO₂ in the presence of volatile organic compounds results in the production of tropospheric (ground-level) ozone. Both increased ambient UV radiation and warmer temperatures would further drive these reactions. Adverse health effects from air pollution are well known, and continued fossil fuel use will extend acute and chronic health impacts. (see “Air pollution” in this chapter).

Infectious diseases and climate/ecosystem change

Coupled atmosphere-ocean general circulation models predict that high latitudes in the northern hemisphere will experience the largest surface temperature elevation based on current IPCC scenarios (IPCC 1992). Minimum winter temperatures are expected to be disproportionately more affected, allowing for certain viruses and parasites to extend into regions where they previously could not live. In addition to direct climate effects on vectors, transformation of ecosystems could have marked implications for diseases whereby the geographic range of vector and/or reservoir host species is defined by these ecosystems.

Vector-borne diseases may spread to temperate regions in both hemispheres and intensify in endemic areas. Temperature determines vector infectivity by affecting pathogen replication, maturation and the period of infectivity (Longstreth and Wiseman 1989). Elevated temperature and humidity also intensify the biting behaviour of several mosquito species. Extreme heat, on the other hand, can shorten insect survival time.

Infectious diseases which incorporate a cold-blooded species (invertebrate) within their life cycles, are most susceptible to subtle climate variations (Sharp 1994). Diseases whose infectious agents, vectors or hosts are affected by climate change include malaria, schistosomiasis, filariasis, leishmaniasis, onchocerciasis (river blindness), trypanosomiasis (Chagas’ and African sleeping sickness), dengue, yellow fever and arboviral encephalitis. Current figures of the number of people at risk of these diseases are listed in table 1 (WHO 1990d).

Table 1. Global status of major vector-borne diseases

^a The numbers refer to explanations in the text. ^b Based on a world population estimated at 4.8 billion (1989).
0 = unlikely; + = likely; ++ = very likely; +++ = no estimate available; ? = not known.

Worldwide, malaria is the most prevalent vector-borne disease and causes one to two million deaths annually. An estimated one million additional annual fatalities may arise from climate change by the middle of the next century, according to Martens et al. (1995). The Anopheline mosquito which carries malaria can extend to the 16 °C winter isotherm, since parasite development does not occur below this temperature (Gilles and Warrell 1993). Epidemics occurring at higher altitudes generally coincide with above average temperatures (Loevinsohn 1994). Deforestation also affects malaria, since cleared areas provide an abundance of freshwater pools in which Anopheline larvae can develop (see “Species extinction, biodiversity loss and human health” in this chapter).

Over the past two decades, efforts to control malaria have made only marginal gains. Treatment has not improved as drug-resistance has become a major problem for the most virulent strain, Plasmodium falciparum, and antimalarial vaccines have shown only limited efficacy (Institute of Medicine 1991). Great capacity for antigenic variation of protozoans has thus far prevented acquisition of effective vaccines for malaria and sleeping sickness, leaving little optimism for readily available new pharmaceutical agents against these diseases. Diseases which involve intermediate reservoir hosts (e.g., deer and rodents in the case of Lyme disease) make human herd immunity from vaccination programmes essentially unattainable, representing another hurdle to preventive medical intervention.

As climate change alters habitat, causing a potential reduction of biodiversity, insect vectors will be forced to find new hosts (see “Species extinction, biodiversity loss and human health”). In Honduras, for example, blood-seeking insects such as the assassin beetle, which carries incurable Chagas’ disease (or American Trypanosomiasis), has been forced to seek human hosts as biodiversity decreases from deforestation. Of 10,601 Hondurans studied in endemic regions, 23.5% are now seropositive for Chagas’ disease (Sharp 1994). Zoonotic diseases are frequently the source of human infections, and generally affect man after an environmental change or alteration of human activity (Institute of Medicine l992). Many “newly emerging” diseases in humans are actually long-standing zoonoses of animal host species. For example, Hantavirus, recently found to be the cause of human fatalities in the southwest United States, has long been established in rodents and the recent outbreak was felt to be related to climatic/ecological conditions (Wenzel 1994).

Marine effects

Climate change may further impact public health through effects on harmful marine phytoplankton (or algae) blooms. Increases in phytoplankton globally has been a consequence of poor erosion control management, liberal agricultural application of fertilizers, and coastal sewage release, all resulting in effluents rich in nutrients which promote algae growth. Conditions that favour this growth could be augmented by warmer sea surface temperatures expected with global warming. Overharvesting of fish and shellfish (algae consumers) coupled with widespread pesticide use toxic to fish, further contribute to plankton overgrowth (Epstein 1995).

Red tides causing diarrhoeal and paralytic diseases and amnesic shellfish poisoning are prime examples of diseases stemming from algal overgrowth. Vibrio cholera has been found to be harboured by marine phytoplankton; thus blooms could represent an expanded reservoir from which cholera epidemics may initiate (Huq et al. 1990).

Food supply and human nutrition

Malnutrition is a major cause of infant mortality and childhood morbidity due to immunosuppression (see “Food and agriculture”). Climate change could adversely affect agriculture both by long-term changes, such as reducing soil moisture through evapotranspiration, and, more immediately, by extreme weather events such as droughts, flooding (and erosion) and tropical storms. Plants may initially benefit from “CO₂ fertilization”, which can enhance photosynthesis (IPCC 1990). Even accounting for this, agriculture in developing countries will suffer most, and it is estimated that in these nations, 40-300 million additional people will be at risk from hunger due to climate change (Sharp 1994).

Indirect ecological changes affecting crops will need to be considered as well, since agricultural pests may change in distribution (IPCC 1992) (see “Food and agriculture”). Considering complex ecosystem dynamics, complete assessment will need to extend beyond the direct impacts of changing atmospheric and/or soil conditions.

Health Effects of Weather Disasters and Sea Level Rise

Thermal expansion of oceans may cause sea level to rise at a relatively rapid rate of two to four centimetres per decade, and projected extremes of the hydrologic cycle are expected to produce more severe weather patterns and storms. Such events would directly disrupt dwellings and public health infrastructures, such as sanitation systems and stormwater drainage (IPCC 1992). Vulnerable populations in low-lying coastal areas and small islands would be forced to migrate to safer locations. Resulting overcrowding and poor sanitation among these environmental refugees could amplify the spread of infectious diseases such as cholera, and vector-borne disease transmission rates would escalate due to crowding and potential influxes of infected individuals (WHO 1990d). Flooded drainage systems may further exacerbate the situation, and psychological impacts must also be considered from post-traumatic stress syndrome following major storms.

Fresh water supply would diminish due to saline intrusion of coastal aquifers and coastal farmland lost to salination or outright inundation. For example, a sea-level rise of one metre would destroy 15% and 20% of agriculture in Egypt and Bangladesh respectively (IPCC 1990). As for droughts, adaptive irrigation methods could affect arthropod and invertebrate breeding sites of vectors (e.g., similar to schistosomiasis in Egypt), but cost/benefit evaluation of such impacts will be difficult.

Health Effects of Stratospheric Ozone Depletion

Direct health effects of ultravioletB radiation

Ozone specifically blocks the penetration of ultravioletB radiation, which contains the most biologically destructive wavelengths of 290-320 nanometres. UVB induces the formation of pyrimidine dimers within DNA molecules, which if unrepaired can evolve to cancer (IARC 1992). Non-melanoma skin cancer (squamous and basal cell carcinoma) and superficial spreading melanoma are correlated with sunlight exposure. In Western populations, melanoma incidence has increased by 20 to 50% every five years over the past two decades (Coleman et al. 1993). While there is no direct relationship between cumulative ultraviolet exposure and melanoma, excessive UV exposure during childhood is associated with incidence. For a sustained 10% decline in the stratospheric ozone layer, non-melanoma skin cancer cases could rise by 26%, or 300,000 globally per year; melanoma could increase by 20%, or 4,500 more cases annually (UNEP 1991a).

Eye cataract formation causes half of the world’s blindness (17 million cases annually) and is associated with UVB radiation in a dose-response relationship (Taylor 1990). Amino acids and membrane transport systems in the lens of the eye are especially prone to photo oxidation by oxygen radicals generated by UVB irradiation (IARC 1992). A doubling of UVB exposure could cause a 60% increase in cortical cataracts over current levels (Taylor et al. 1988). UNEP estimates that a 10% sustained loss of stratospheric ozone would result in nearly 1.75 million extra cataracts annually (UNEP 1991a). Other ocular effects of UVB exposure include photokeratitis, photokerato-conjunctivitis, pinguecula and pterygium (or overgrowth of the conjunctival epithelium) and climatic droplet keratopathy (IARC 1992).

The ability of the immune system to function effectively depends on “local” antigen processing and presentation to T-cells, as well as augmentation of the “systemic” response via lymphokine (biochemical messenger) production and resultant T-helper/T-suppressor cell ratios. UVB causes immunosuppression at both levels. UVB in animal studies can affect the course of infectious skin diseases, such as onchocerciasis, leishmaniasis and dermatophytosis, and impair immunosurveillance of transformed, precancerous epidermal cells. Preliminary studies further show an influence on vaccine efficacy (Kripke and Morison 1986; IARC 1992).

Indirect public health effects of UVB

Historically, terrestrial plants became established only after the formation of the shielding ozone layer, since UVB inhibits photosynthesis (UNEP 1991a). Weakening of food crops susceptible to UVB damage could further extend the impacts on agriculture due to climate changes and sea-level rise.

Phytoplankton are at the foundation of the marine food chain and also serve as an important carbon dioxide “sink”. UV damage to these algae in polar regions, therefore, would detrimentally affect the marine food chain and exacerbate the greenhouse effect. UNEP estimates that a 10% loss of marine phytoplankton would limit the oceans’ annual CO₂ uptake by five gigatonnes, which equals the yearly anthropogenic emissions from fossil fuel combustion (UNEP 1991a).

Occupational Hazards and Control Strategies

Occupational hazards

With regard to reduction in GHG emissions from fossil fuels, alternate renewable energy sources will need to be expanded. The public and occupational hazards of nuclear energy are well known, and safeguarding plants, workers and spent fuel will be necessary. Methanol may serve to replace much gasoline usage; however, formaldehyde emission from these sources will present a new environmental hazard. Superconducting materials for energy efficient electricity transfer are mostly ceramics comprised of calcium, strontium, barium, bismuth, thallium and yttrium (WHO in press).

Less is known about the occupational safety in the manufacturing units for solar energy capture. Silicon, gallium, indium, thallium, arsenic and antimony are the primary elements used to build photovoltaic cells (WHO in press). Silicon and arsenic adversely affect the lungs; gallium is concentrated in the kidney, liver, and bone; and ionic forms of indium are nephrotoxic.

The destructive effects of CFCs on the stratospheric ozone layer were recognized in the 1970s, and the US EPA banned these inert propellants in aerosols in 1978. By 1985, widespread concern erupted when an Antarctic-based British team discovered the “hole” in the ozone layer (Farman, Gardiner and Shanklin 1985). Subsequent passage of the Montreal Protocol in 1987, with amendments in 1990 and 1992, has already mandated sharp cuts in CFC production.

The replacement chemicals for CFCs are the hydrochlorofluorocarbons (HCFCs) and the hydrofluorocarbons (HFCs). The presence of the hydrogen atom may more readily subject these compounds to degradation by hydroxyl radicals (OH^–) in the troposphere, thus reducing potential stratospheric ozone depletion. These CFC replacement chemicals are, however, more biologically reactive than CFCs. The nature of a C-H bond makes these chemicals prone to oxidation via the cytochrome P-450 system (WHO in press).

Mitigation and adaptation

Meeting the public health challenges presented by global climate change will require (1) an integrated ecological approach; (2) reduction of greenhouse gases through industrial emission control, land use policies to maximize the extent of CO₂ “sinks” and population policies to achieve both; (3) monitoring of biological indicators on both regional and global scales; (4) adaptive public health strategies to minimize the impacts from unavoidable climate change; and (5) cooperation between developed and developing nations. In short, increased integration of environmental and public health policies must be promoted.

Climate change and ozone depletion present a vast number of health risks at multiple levels and underscore the important relationship between ecosystem dynamics and sustained human health. Preventive measures must therefore be systems based, and must anticipate significant ecological responses to climate change as well as the direct physical hazards predicted. Some key elements to consider in an ecological risk assessment will include spatial and temporal variations, feedback mechanisms and use of lower level organisms as early biological indicators.

Reduction of greenhouse gases by diverting from fossil fuels to renewable energy resources represents primary prevention of climate change. Similarly, strategic land use planning and stabilization of population stress on the environment will preserve important natural greenhouse gas sinks.

Because some climate change may be unavoidable, secondary prevention through early detection by monitoring of health parameters will require unprecedented coordination. For the first time in history, attempts are being made to monitor the earth system in its entirety. The Global Climate Observing System incorporates the World Weather Watch and Global Atmosphere Watch of the World Meteorological Organization (WMO) with parts of UNEP’s Global Environmental Monitoring System. The Global Ocean Observing System is a new joint endeavour by the Intergovernmental Oceanographic Commission of UN Educational, Scientific and Cultural Organization (UNESCO), WMO and the International Council of Scientific Unions (ICSU). Both satellite and underwater measurements will be utilized to monitor changes in marine systems. The Global Terrestrial Observing System is a new system sponsored by UNEP, UNESCO, WMO, ICSU and the Food and Agricultural Organization (FAO), and will provide the terrestrial component of the Global Climate Observing System (WMO 1992).

Adaptive options to reduce unavoidable health consequences include disaster preparedness programmes; urban planning to reduce the “heat island” effect and improve housing; land use planning to minimize erosion, flash flooding and unnecessary deforestation (e.g., halting the creation of rangeland for meat exportation); personal adaptive behaviours, such as avoiding sun exposure; and vector-control and expanded vaccination efforts. Unintended costs of adaptive control measures of, for example, increased pesticide use will require consideration. Over-dependence on pesticides not only leads to insect resistance but also eliminates natural, beneficial, predatory organisms. The adverse effect on public health and the environment due to current pesticide use is estimated to be between US$100 billion and US$200 billion annually (Institute of Medicine 1991).

Developing countries will suffer disproportionately more from the consequences of climate change, though industrialized nations are presently more responsible for GHGs in the atmosphere. In the future poorer countries will influence the course of global warming significantly more, both through the technologies they choose to adopt as their development accelerates, and by land use practices. Developed nations will need to embrace more environmentally sound energy policies and promptly transfer new (and affordable) technology to developing countries.

Case Study: Mosquito-borne viruses

Mosquito-borne encephalitis and dengue fever are prime examples of vector-borne diseases whose distributions are limited by climate. Epidemics of St. Louis encephalitis (SLE), the most common arboviral encephalitis in the United States, generally occur south of the 22°C June isotherm, but northerly outbreaks have occurred during unseasonably warm years. Human outbreaks are highly correlated with several-day periods when temperature exceeds 27°C (Shope 1990).

Field studies on SLE indicate that a 1°C increase in temperature significantly shortens the elapsed time between a mosquito blood-meal and viral replication to the point of infectivity within the vector, or the extrinsic incubation period. Adjusting for reduced adult mosquito survival at elevated temperatures, a 3 to 5 °C temperature increase is predicted to cause a significant northern shift of SLE outbreaks (Reeves et al. 1994).

The range of the primary mosquito vector of dengue (and yellow fever), Aedes aegypti, extends to 35° latitude because freezing temperatures kill both larvae and adults. Dengue is widespread in the Caribbean, tropical America, Oceania, Asia, Africa and Australia. Over the past 15 years, dengue epidemics have increased in both numbers and severity, especially in tropical urban centres. Dengue haemorrhagic fever now ranks as one of the leading causes for hospitalization and mortality of children in Southeast Asia (Institute of Medicine 1992). The same increasing pattern observed in Asia 20 years ago is now occurring in the Americas.

Climate change can potentially alter dengue transmission. In Mexico in 1986, the most important predictor of dengue transmission was found to be the median temperature during the rainy season, with an adjusted fourfold risk observed between 17 °C and 30 °C (Koopman et al. 1991). Lab studies support these field data. In vitro, the extrinsic incubation period for dengue type-2 virus was 12 days at 30 °C and only seven days at 32 to 35 °C (Watts et al. 1987). This temperature effect of shortening the incubation period by five days translates to a potentially threefold higher transmission rate of disease (Koopman et al. 1991). Finally, warmer temperatures result in the hatching of smaller adults, which must bite more frequently to develop an egg batch. In summary, increased temperatures can lead to more infectious mosquitoes that bite more frequently (Focks et al. 1995).

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