All constituents of the environment of our planet ultimately exert an influence on human health and well-being. However, the greatest and most direct influence is the immediate environment of home, work place and neighbourhood. Both environmental and genetic factors are involved in the incidence of disease. While genetic factors usually give rise to congenital diseases and environmental factors to acquired ones, there is often an interplay between the two.
Freedom from organic disease is usually considered synonymous with a reasonable state of health, but freedom from non-organic disease is also important. Good health demands a sound mind in a sound body. The socio-economic implications of impaired mental health in any population group cannot be ignored. Impaired mental health, like its organic counterpart, can be caused by genetic or environmental factors, or an interplay between both. Over the past two decades, evidence of the role of biochemical change in causing mental ill health has increased. Some of these biochemical abnormalities could be inherited and some environmentally induced. Certain organic causes of mental ill health are found in environmental factors--as with the group of psychoses resulting from trypanosomias and other infectious agents. Exposure to heavy metals such as mercury or lead and to certain synthetic compounds may create a predisposition to brain tumours or abnormal behaviour A study on the long-term effects of exposure to low doses of lead in childhood indicated that such exposure is associated with deficiencies in central nervous system functioning that persist into young adulthood (1).
Malnutrition is the most pervasive cause of ill health (see Chapter 11), and a major contributor to the high death rate among infants and young children in developing countries. An infant's birth weight is the single most important determinant of its early chances of survival, healthy growth and development. Because birth weight is conditioned by the health and nutritional status of the mother, the proportion of infants with a low birth weight (less than 2500 g) accurately reflects the health and social status of women and of the communities into which children are born. In communities where malnutrition is a chronic problem, or during periods of food shortages or of physical stress such as recurrent droughts, pregnant women rarely get enough to eat and foetal growth suffers. Approximately 51 per cent of pregnant women in the world suffer from nutritional anaemia (low haemoglobin levels due to poor diet); the percentage in developing countries is 59, much higher than the 14 per cent encountered in industrialized countries (Figure 18.1). Some 22 million (about 16 per cent) of the 140 million infants born each year in the world have a low birth weight. At least 20 million of these infants are born in developing countries, the majority (more than 13 million) in South Asia and the rest in Africa, Latin America and East Asia (3, 4). The Global Strategy of Health for All, launched by the World Health Assembly, aims at achieving a target birth weight of at least 2500 g for 90 per cent of newborn infants, and adequate growth of children, measured by weight-for-age goals, by the year 2000 (4).
In terms of numbers affected, malnutrition is the most serious condition affecting the health of children, particularly in developing countries. Surveys in different regions of the world indicate that at any moment an estimated 10 million children are suffering severe malnutrition, and a further 200 million are inadequately nourished (Figure 18.2). Malnutrition makes a child (or an adult) more prone to infection, and infection may exacerbate the effects of malnutrition.
The best protection for infants against both malnutrition and infection is breast feeding and the past two decades saw a heightened awareness of its importance. However, nearly all chemical compounds ingested by the mother will be found in her milk in one form or another. Cadmium, lead, mercury, DDT, its derivatives and other pesticides have all been found in human milk in several countries. Studies (5, 6, 7, 8) have revealed that in some countries, concentrations of DDT and DDE in human milk are higher than acceptable daily intake criteria and maximum residue limits established by WHO and FAO. However, no evidence has been found to suspect that the levels of DDT and DDE generally found in human milk harmed infants. In fact, breast milk tends to be much less contaminated than its substitutes. The prevalent high death and disease rates among artificially-fed infants in many developing countries can be attributed to improper preparation, as well as contamination, of formula milk and other infant foods. The acceptance of WHO's international code for marketing of breast milk substitutes has stimulated creation of energetic government-designed programmes to promote breast-feeding. Despite the increasing popularity of breast feeding in the industrialized countries, no similar increase has occurred in developing countries (9).
Different environmental conditions determine seasonal as well as regional differences in the incidence of disease. Figure 18.3 illustrates the main causes of death in the mid-1980s (10). Infectious, parasitic, perinatal and pregnancy complications predominate in developing countries. Some communicable diseases are transmitted much more easily during the rainy season. Temperature, humidity, soil, rainfall and atmospheric conditions are all important factors in the ecology of certain infectious and infectious diseases, especially because they control the distribution and abundance of their vectors.
Communicable diseases account for a large proportion of illness and death in developing countries, where billions of people still lack such basic necessities as adequate shelter, access to safe water supplies and sanitation, and refuse disposal facilities. The deteriorating environmental conditions in which people live propagate the spread of infective agents and the breeding of their vectors. Overcrowding accelerates the spread of tuberculosis and other respiratory infections. The absence of sanitation and lack of a safe water supply provides fertile ground for outbreaks of water- and food-borne enteric diseases. and the larger the population exposed, the greater the risk and subsequent extent of infection. In the mid-1980s, it was estimated that 17 million people (of which 10.5 million were infants under the age of five) in the developing countries died every year of infectious and parasitic diseases (Figure 18.4) as compared to about half a million in developed countries (10).
Although cholera has subsided in Asia. it has made its way into the Americas, resulting in a dramatic increase in the number of cases reported to the WHO (about 250 000 cases in 1991). In Africa the total number of cholera cases has been almost stable for the past two decades. Occasional local outbreaks of cholera have occurred in some countries mainly as a result of the contamination of drinking water and food.
Malaria continues to be one of the most serious public health and environmental problems in large parts of the developing world. The disease is endemic in 102 countries, placing over half the world's population at risk. Since 1980 there has been a general decrease in malaria in Africa, South-East Asia and the Western Pacific, but a gradual increase in the Americas (Figure 18.5). In 1988, 8 million cases of malaria were reported to WHO, but it is believed that the overall number of cases in the world is in the order of 100 million (12). Of those reported in 1988, 39 per cent were in Africa and 32 per cent in South-East Asia (Figure 18.6). It is believed that 43 per cent of the world population lives in areas where malaria occurs, and some 445 million of them live in areas where no specific transmission control measures are undertaken and where the prevalence of the disease remains virtually unchanged (12).
Schistosomiasis remains a major health threat in some 76 developing countries. The largest number of cases occur in Brazil, Central Africa, China, Kampuchea, Egypt and the Philippines. It is estimated that some 200 million people are infected and another 600 million are at risk of infection. The creation of man-made lakes, fish-ponds and irrigation schemes has contributed to an increase in the incidence of the disease. Following the construction of the Diama dam on the Senegal River in 1986, It was found that intestinal schistosomiasis had increased since early 1988; by 1989, 71.5 per cent of samples were positive (15, 16).
The first reported cases of HIV infection and AIDS (acquired immunodeficiency syndrome) occurred in the past decade. AIDS kills people of all ages, but is a growing threat to newborn children and infants. At least 1.5 million women worldwide--of whom about one million live in Africa--are infected with HIV. Babies born to such women have a 25-40 per cent chance of being infected before or during birth. These children are almost certain to die by the age of five (17). It is estimated that worldwide some 5-10 million people are infected with the AIDS virus and about 400 000 have full-blown AIDS (14). Estimates are that by the end of 1991, more than a million cases of AIDS will have occurred worldwide, and that by the year 2000 the cumulative total could exceed five million (14).
People are exposed to a number of hazardous chemicals at home and at work. A vast amount of scientific information is available on the short-term effects of exposure to high levels of hazardous chemicals--but little is known about what happens to individuals exposed to very low concentrations of chemicals over longer periods of 20-30 years. However, the consequences of such exposure can be measured in the population at large in terms of physiological change, disease and death. Genetic mutations--the production of new, mostly detrimental, hereditary traits--can also have chemical causes, and they are permanent. Cancer and birth defects are among the other health hazards that can result from long-term exposure to toxic substances. Birth defects occur in 2-3 per cent of all births. Of these, 25 per cent have underlying genetic causes, while 5-10 per cent result from the influence of four main known external agents: chemicals, drugs, radiation and viruses. The remaining 65-70 per cent arise from unknown causes. but may result from the interaction of a number of environmental agents with genetic factors (18, 19, 20).
The effects of exposure to a chemical pollutant depend on the length and severity of exposure and the chemical involved. There are two main types of exposure. The first includes exposure to abnormally high levels of pollutants--as in accidental chemical releases (see Chapter 9), exposure at work and severe air pollution episodes. The second type of exposure is to general ambient pollutants.
In the first case, the effects are evident and include death and an increase in morbidity. The accidental release of methyl isocyanate in the Bhopal accident (see Chapter 9) led to both deaths and a high rate of morbidity. Exposure of workers to high concentrations of chemicals has also led to various occupational diseases. Lead poisoning, pneumoconiosis (a lung disease caused by dust inhalation), pesticide poisoning and various cancers can result from such exposure. WHO has estimated that the number of unintentional cases of acute poisoning due to pesticide exposure was half a million in 1972 and increased to one million in 1985, because of greater use of pesticides. About 60-70 per cent of these cases are due to occupational exposure. Some 20 000 deaths a year occur as a result of pesticide poisoning (21).
Many traditional occupational diseases are declining in developed countries, as a result of the strict enforcement of protection measures. But occupational diseases are on the increase in several developing countries, because of a lack or non-enforcement of regulatory measures to protect workers, and also because of a lack of awareness and cooperation among workers. There is also mounting concern about the increase of occupational diseases in small-scale industries, including repair workshops, especially among children who constitute a large proportion of the work force (see Chapter 17).
The effects of air pollution episodes, such as the London smog of 1952, are well documented; children and the elderly (in particular those with respiratory and circulatory problems) were most affected.
Assessing the health impacts of exposure to chemical pollutants in the general environment is difficult because an individual is generally exposed to several pollutants at the same time. Total exposure includes inhalation, ingestion and skin absorption of the pollutants from air, water, food or soil. In many cases the effect of an individual pollutant can be increased or decreased through interactions with other pollutants: the adverse health effects of sulphur dioxide are known to increase in the presence of particulate matter; tobacco smoking increases the incidence of cancer due to exposure to indoor radon (see Chapter 1).
Various attempts have been made in the past two decades to estimate the health impacts of total human exposure. Models have been used to calculate the environmental distribution, transformation and fate of chemical pollutants; human exposure via different routes; and the toxicological and dynamic properties of chemical substances in humans (22, 23). In 1984, WHO/UNEP set up the Human Exposure Assessment Locations programme (HEALs), as part of GEMS, in order to monitor total human exposure to pollutants. The results should enable countries to assess the combined risks from air, food and water pollutants, and to take appropriate action to safeguard human health.
Cause and effect have been established for several pollutants. The health effects of carbon monoxide, tropospheric ozone, sulphur oxides combined with particulates, and lead in ambient air are well documented (see Chapter 1). Epidemiological research over the past two decades has established that indoor air pollution could increase the incidence of cancer due to radon exposure and tobacco smoke. In rural areas of developing countries it could increase respiratory diseases and cancer due to exposure to emissions from biomass fuel. The increase in nitrates in groundwater has become a cause for concern in several countries as nitrates constitute a health risk, especially for infants. WHO, UNEP and ILO have been working together since the early 1970s to establish health criteria for various pollutants (see Chapter 10).
There is now widespread agreement that roughly 85 per cent of all cancers are caused by environmental factors, such as ionizing radiation, carcinogenic chemicals in air, food or water, smoking, alcohol and drugs, including chemotherapeutic agents. The rest, presumably, have a hereditary basis or else arise from spontaneous metabolic events. Although the percentage of deaths from cancer is higher in developed than in developing countries (Figure 18.3), the pattern of cancer incidence in both groups of countries is similar. However variations in the incidence of different types of cancer are encountered (Figure 18.7). Tobacco smoking (including passive smoking) is the most important cause of lung cancer. In spite of this well-established fact, the global use of tobacco has grown by nearly 75 per cent over the past two decades and smoking has increased markedly among young people.
The different responses outlined in previous chapters contribute. directly and indirectly, to the improvement of human health and to reducing the health risks associated with exposure to different pollutants. Results have demonstrated that prevention is better than cure. Though the original goals of the IDWSSD were not met by 1990, the 1980s did provide hundreds of millions of people with safe drinking water and sanitation facilities (see Chapter 5) and this has largely contributed to health improvements in the areas provided with such facilities. Figure 18.8 illustrates how improved water supplies and sanitation facilities can reduce the rate of illness related to diarrhoea. Further preventive measures include actions taken to reduce air emissions (see Chapter 1) and to protect the ozone layer (see Chapter 2). However, there is still a long way to go before the health risks of environmental pollution and deterioration are significantly reduced. Much research is required to clarify the cause and effects of total human exposure to establish practical guidelines protect human health.
There is still much to be done to reduce the incidence of communicable diseases in developing countries, though several communicable diseases were brought under control in the past two decades. Smallpox was eradicated. The incidence of onchoceriasis (river blindness) has been reduced sharply in West Africa. An increase in the use of oral rehydration therapy (ORT) reduced the mortality of children under the age of five due to diarrhoeal diseases. In 1985, about 18 per cent of children with diarrhoea were treated with ORT; by 1989 the percentage of these children reached 25 per cent (26), saving the lives of some one million children each year (27). The six vaccine-preventable diseases of childhood--poliomyelitis, tetanus, measles, diphtheria, pertussis and tuberculosis--have declined through increased immunization. In the 1970s, these diseases killed about five million children a year, but in the 1980s the figure dropped to about three million a year and continues to reduce through the expanded programme of immunization.
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