The world health report

Chapter 4

Environmental risks

The environment in which we live greatly affects our health. The household, workplace, outdoor and transportation environments pose risks to health in a number of different ways, from the poor quality of the air many people breathe to the hazards we face as a result of climate change (see Table 4.6). A range of selected environmental risk factors is assessed here and some summary results are shown in Figure 4.7a and Figure 4.7b .

Unsafe water, sanitation and hygiene

Adverse health outcomes are associated with ingestion of unsafe water, lack of access to water (linked to inadequate hygiene), lack of access to sanitation, contact with unsafe water, and inadequate management of water resources and systems, including in agriculture. Infectious diarrhoea makes the largest single contribution to the burden of disease associated with unsafe water, sanitation and hygiene.

Six broad scenarios were characterized; these included populations with no access to improved water sources or no basic sanitation; those with access to fully regulated water supply and sanitation services; and an ideal scenario in which no disease transmission is associated with this risk factor. In addition, schistosomiasis, trachoma, ascariasis, trichuriasis and hookworm disease were fully attributed to unsafe water, sanitation and hygiene.

Exposure prevalence was determined from the WHO/UNICEF Global Water Supply and Sanitation Assessment 2000. This provides a synthesis of major international surveys and national census reports, which provide data for 89% of the global population. In 2000, the percentage of people served with some form of improved water supply worldwide reached 82% (4.9 billion), while 60% (3.6 billion) had access to basic sanitation facilities. The vast majority of diarrhoeal disease in the world (88%) was attributable to unsafe water, sanitation and hygiene.

Approximately 3.1% of deaths (1.7 million) and 3.7%of DALYs (54.2 million) worldwide are attributable to unsafe water, sanitation and hygiene. Of this burden, about one-third occurred in Africa and one-third in SEAR-D. In these areas, as well as in EMR-D and AMR-D, 4--8% of all disease burden is attributable to unsafe water, sanitation and hygiene. Overall, 99.8% of deaths associated with this risk factor are in developing countries, and 90% are deaths of children.

Urban air pollution

The serious consequences of exposure to high levels of urban ambient air pollution were made clear in the mid-20th century when cities in Europe and the United States experienced air pollution episodes, such as the infamous 1952 London fog, that resulted in many deaths and hospital admissions. Subsequent clean air legislation and actions reduced ambient air pollution in many regions. However, recent epidemiological studies, using sensitive designs and analyses, have identified serious health effects of combustion-derived air pollution even at the low ambient concentrations typical of Western European and North American cities (45). At the same time, the populations of the rapidly expanding megacities of Asia, Africa and Latin America are increasingly exposed to levels of ambient air pollution that rival and often exceed those experienced in industrialized countries in the first half of the 20th century (46).

Urban air pollution is largely and increasingly the result of the combustion of fossil fuels for transport, power generation and other human activities. Combustion processes produce a complex mixture of pollutants that comprises both primary emissions, such as diesel soot particles and lead, and the products of atmospheric transformation, such as ozone and sulfate particles formed from the burning of sulfur-containing fuel.

Air pollution from combustion sources is associated with a broad spectrum of acute and chronic health effects (47,48), that may vary with the pollutant constituents. Particulate air pollution (i.e. particles small enough to be inhaled into the lung,) is consistently and independently related to the most serious effects, including lung cancer and other cardiopulmonary mortality (44,49,50). Other constituents, such as lead and ozone, are also associated with serious health effects, and contribute to the burden of disease attributable to urban air pollution. The analyses based on particulate matter estimate that ambient air pollution causes about 5% of trachea, bronchus and lung cancer, 2% of cardiorespiratory mortality and about 1% of respiratory infections mortality globally. This amounts to about 0.8 million (1.4%) deaths and 7.9 million (0.8%) DALYs. This burden predominantly occurs in developing countries, with 42% of attributable DALYs occurring in WPR-B and 19% in SEAR-D. Within subregions, the highest proportions of total burden occur in WPR-A, WPR-B, EUR-B and EUR-C, where ambient air pollution causes 0.6--1.4% of disease burden. These estimates consider only the impact of air pollution on mortality, and not morbidity, due to limitations in the epidemiologic database. If air pollution multiplies both incidence and mortality to the same extent, the burden of disease would be higher.

Indoor smoke from solid fuels

Although air pollutant emissions are dominated by outdoor sources, human exposures are a function of the level of pollution in places where people spend most of their time (51,53). Human exposure to air pollution is thus dominated by the indoor environment. Cooking and heating with solid fuels such as dung, wood, agricultural residues or coal is likely to be the largest source of indoor air pollution globally. When used in simple cooking stoves, these fuels emit substantial amounts of pollutants, including respirable particles, carbon monoxide, nitrogen and sulfur oxides, and benzene.

Nearly half the world continues to cook with solid fuels. This includes more than 75% of people in India, China and nearby countries, and 50--75% of people in parts of South America and Africa. Limited ventilation is common in many developing countries and increases exposure, particularly for women and young children who spend much of their time indoors. Exposures have been measured at many times higher than WHO guidelines and national standards, and thus can be substantially greater than outdoors in cities with the most severe air pollution.

Studies have shown reasonably consistent and strong relationships between the indoor use of solid fuel and a number of diseases. These analyses estimate that indoor smoke from solid fuels causes about 35.7% of lower respiratory infections, 22.0% of chronic obstructive pulmonary disease and 1.5% of trachea, bronchus and lung cancer. Indoor air pollution may also be associated with tuberculosis, cataracts and asthma.

In total, 2.7% of DALYs worldwide are attributable to indoor smoke, 2.5% in males and 2.8% in females. Of this total attributable burden, about 32% occurs in Africa (AFR-D and AFR-E), 37% in SEAR-D and 16% in WPR-B. Among women, indoor air smoke causes approximately 3--4% of DALYs in AFR-D, AFR-E, EMR-D, SEAR-D and WPR-B. The most important interventions to reduce this impact are better ventilation, more efficient vented stoves, and cleaner fuels.

Many other risks to health accumulate in the indoor environment, and housing has a key role in determining their development and impact (see Box 4.2).

Box 4.2 Housing and health

The primary purpose of buildings worldwide is to protect humans from the hazards and discomforts of outdoor environments and to offer a safe and convenient setting for living and human activity. Furthermore, people -- especially in temperate and cold climates and in industrialized societies -- spend most of their time indoors in buildings such as homes, offices, schools and day-care centres. This means that, from the perspective of exposure to environmental conditions and hazards, housing and indoor environments have important public health consequences for both physical and mental health.

The most extreme health impact of housing is found among the poorest sectors of societies in the form of a complete lack of housing, which affects millions of people worldwide. Lack of affordable housing for low-income households may mean diverting family resources from expenditure on food, education or health towards housing needs. Beyond this, both the physical structure of houses and their location can involve health risks.

Important parameters in indoor environments include the thermal climate, noise and light, and exposure to a large number of chemical, physical and biological pollutants and risk factors. While these parameters are also affected by human-related activities and outdoor sources (such as vehicle and industrial pollutants or local vegetation and insect ecology), human exposure is modified by housing characteristics such as building materials, number and size of rooms and windows, ventilation and energy technology. For example, a "leaky" house can lead to dampness and mould which may result in various forms of respiratory illness and allergic reactions; the use of building materials such asbestos or lead-based paint can increase exposure to these toxic substances; the use of inflammable or weak material such as wood, plastic or cardboard -- particularly common in urban slums -- poses increased risks of injuries; building design will influence exposure to disease vectors such as mosquitoes; inadequate ventilation or overcrowding will cause exposure to different pollutants and pathogens; poor lighting or heating will influence both physical and mental health as well as participation in activities such as education; and so on.

The location of housing and the organization of neighbourhoods also have public health implications, in particular in rapidly urbanizing developing countries, where a growing proportion of the population live in informal settlements or slums, often on the periphery of major cities. If housing is located on floodplains or steep hillsides, near sources of traffic, industrial activity, solid waste dumps or vector breeding sites, and away from services such as sanitation, transportation, schools or health facilities, public health will be affected directly (for example, through sanitation) or indirectly through access to food and education. In addition, organization of neighbourhoods has been shown to have an effect on mental and physical health, school attendance and performance, or prevalence of violence and crime.

Referring to housing as a "risk factor" would mask the important role that it plays in providing a setting for daily household and community activities. At the same time, it is important to acknowledge the important and complex roles that housing and neighbourhood design play in public health and to promote systematic inclusion of health in the design of housing, housing technology and the urban and regional planning processes.

Sources: (54,56).

Lead exposure

Lead, because of its multiplicity of uses, is present in air, dust, soil and water. Lead enters the body mainly by ingestion or inhalation. Contamination of the environment has increased with industrial development and particularly the use of leaded petrol. Currently about 60 countries have phased out leaded petrol and approximately 85% of petrol sold worldwide is lead-free. Other important lead sources are more difficult to control, such as leaded kitchenware ceramics, water pipes and house paints.

Following control measures, lead levels have been steadily declining in industrialized countries but at least 5% of children still have elevated blood lead levels, with even higher rates in children of poorer households (57). In many developing countries, where leaded gasoline is still used, lead can present a threat to more than half of children (58). Rapidly increasing traffic loads have the potential to further increase blood lead levels. Worldwide, 120 million people are estimated to have lead levels of 5--10 g/dl, with similar numbers above 10 g/dl, and 40% of children have blood lead levels above 5 g/dl. Overall, 97% of affected children live in developing regions. Industrial or cottage exposure to lead, such as from smelters or battery recycling, could only partly be assessed here, but can represent a large additional burden in certain regions.

Lead affects practically all body systems. Most toxic exposures occur at chronic low levels and can result in reductions in intelligence quotient (IQ) (59), increased blood pressure, and a range of behavioural and developmental effects. The range and extent of adverse health effects has been appreciated only relatively recently. Furthermore, lead is now understood to be toxic, especially to children, at levels previously thought to be safe (60). In more severe cases of poisoning, adverse health effects include gastrointestinal symptoms, anaemia, neurological damage and renal impairment (61). Other adverse effects, such as reduction in IQ levels, behavioural disorders or renal function, can be discerned only through special examinations. These analyses estimate that lead results in about 234 000 (0.4%) deaths and 12.9 million (0.9%) DALYs. About one-fifth of this entire burden occurs in SEAR-D, and a further one-fifth in WPR-B.

Climate change

Humans are accustomed to climatic conditions varying daily, seasonally and yearly. The recent concern over global climate change arises from accumulating evidence that, in addition to this natural climate variability, average climatic conditions measured over extended periods (conventionally 30 years or longer) are now also changing (62). The most recent report (2001) from the United Nations Intergovernmental Panel on Climate Change (IPCC) estimates that the global average land and sea surface temperature has increased by 0.6 0.2 C since the mid-19th century, with most change occurring since 1976 (63). The 1990s was the warmest decade on record. Warming has been observed in all continents, with the greatest temperature changes occurring at middle and high latitudes in the northern hemisphere. Patterns of precipitation have also changed: arid and semiarid regions are apparently becoming drier, while other areas, especially mid-to-high latitudes, are becoming wetter. There is also evidence that where precipitation has increased, there has been a disproportionate increase in the frequency of the heaviest precipitation events. The causes of this climate change are increasingly well understood. The IPCC concluded that "most of the warming observed over the last 50 years is likely to be attributable to human activities", most importantly the release of greenhouse gases from fossil fuels.

Climate model simulations have been used to estimate the effects of past, present and future greenhouse gas emissions on future climate. Based on a range of alternative development scenarios and model parameters, the IPCC concluded that if no specific actions are taken to reduce greenhouse gas emissions, global temperatures are likely to rise between 1.4 C and 5.8 C from 1990 to 2100. Such a rise would be faster than any rise encountered since the inception of agriculture around 10 000 years ago. Predictions for precipitation and wind speed are less consistent, but also suggest significant changes.

Potential risks to human health from climate change would arise from increased exposures to thermal extremes (cardiovascular and respiratory mortality) and from increases in weather disasters (including deaths and injuries associated with floods). Other risks may arise because of the changing dynamics of disease vectors (such as malaria and dengue fever), the seasonality and incidence of various food-related and waterborne infections, the yields of agricultural crops, the range of plant and livestock pests and pathogens, the salination of coastal lands and freshwater supplies resulting from rising sea-levels, the climatically related production of photochemical air pollutants, spores and pollens, and the risk of conflict over depleted natural resources. Effects of climate change on human health can be expected to be mediated through complex interactions of physical, ecological, and social factors. These effects will undoubtedly have a greater impact on societies or individuals with scarce resources, where technologies are lacking, and where infrastructure and institutions (such as the health sector) are least able to adapt. For this reason, a better understanding of the role of socioeconomic and technological factors in shaping and mitigating these impacts is essential. Because of this complexity, current estimates of the potential health impacts of climate change are based on models with considerable uncertainty.

Climate change was estimated to be responsible in 2000 for approximately 2.4% of worldwide diarrhoea, 6% of malaria in some middle income countries and 7% of dengue fever in some industrialized countries. In total, the attributable mortality was 154 000 (0.3%) deaths and the attributable burden was 5.5 million (0.4%) DALYs. About 46% this burden occurred in SEAR-D, 23% in AFR-E and a further 14% in EMR-D.

Other environmental risks to health

Traffic and transport form another component of environmental hazard in society. Traffic-related burden includes not only injury, but also the consequences of pollution with lead and the effects on urban air quality. Furthermore, as with many exposures assessed here, there are complex interactions with other exposures -- for example, the lost opportunity for physical activity and the economic effects of transport and traffic. Considerations related to road traffic injuries are outlined in Box 4.3.

Box 4.3 Road traffic injuries

Road traffic injuries were estimated to account for over 1.2 million deaths worldwide in 2000, amounting to 2.3% of all deaths. Many such deaths occur in young adults, with significant loss of life, so the proportion of disease burden measured in disability-adjusted life years (DALYs) is greater -- about 2.8% of the total. Over 90% of these deaths occur in the middle and low income countries, where death rates (21 and 24 deaths per 100 000 population, respectively) are approximately double the rates in high income countries (12 per 100 000 population).

Differences in road use between industrialized and developing countries have implications for intervention policies. Driver or occupant deaths accounted for approximately 50--60% of national road traffic fatalities in industrialized countries in 1999, with the vast majority occurring on rural roads. Pedestrian involvement was higher in urban areas, with evidence for increased risk among children and over-60-year-olds. In developing countries, a far higher proportion of road deaths occurs among vulnerable users (pedestrians, bicyclists, other non-motorized traffic, and motor cyclists and moped riders) and among passengers of buses and trucks.

Road traffic crashes are largely preventable. Approaches to improving road safety fall into three broad groups: engineering measures (e.g. road design and traffic management), vehicle design and equipment (e.g. helmets, seat belts and day-time running lights) and road user measures (e.g. speed limits, and restrictions on drinking and driving).

The prospects for prevention can be estimated from some interventions. For example, in Thailand the introduction of a new motor cycle helmet law was followed by a reduction in fatalities of 56%; in Denmark, improved traffic management and provision of cycle tracks was followed by a 35% drop in cyclist fatalities; and in Western Europe it was estimated that lowering average vehicle speeds by 5km/hour could yield a 25% reduction in fatalities. Based on a model developed in the United Kingdom, which takes into account the numbers of cars per capita, it is estimated that, if the countries with the higher road traffic injury rates were to lower these rates to those of other countries in each region, death rates would fall by between 8% and 80%. The scope for improvement is highest in the poorest countries. Worldwide, it is estimated that 44% of road traffic fatalities -- or 20 million DALYs -- per year could be avoided by this method.

Sources: (64,70)