Improving health requires understanding how environmental conditions foster disease. While the causal connections are clear for some diseases and conditions, for others scientific evidence can only identify associations and likely contributors. This section focuses on both biological and chemical hazards in the environment. Biological factors lead to infectious diseases. Although many of these diseases have proven difficult to eradicate, enough is known about them to identify actions that will drastically reduce their incidence. Chemical hazards in the environment can cause immediate, dangerous health effects and can also contribute to chronic, or long-term, problems. In contrast to infectious diseases, our understanding of how chemical exposures influence health, especially very low-level exposures typically found in the environment, remains incomplete.
Of all the environmental hazards humans encounter, the most formidable adversaries remain the microorganisms – viruses, bacteria, protozoa, and helminths (parasitic worms.) Up to 17 million deaths per year are attributable to these infectious and parasitic agents, almost all in the developing world, along with hundreds of millions of cases of sickness (85). Indeed, the history of humankind has been a struggle between humans and microbes (86)(87). Years of concerted efforts have revealed that while it is very difficult to eradicate microbial threats, it is possible to live in balance with them. However, human activities that change the environment and disrupt natural ecosystems can tip the scales in favor of the microbes (88).
Why consider infectious diseases “environmental” in origin? Because most, though certainly not all, are intimately connected with conditions in the physical environment. (See Environmental Hazards and Disease.) Cholera and other diarrheal diseases, for instance, are associated with inadequate access to clean water and sanitation and poor hygiene. Intestinal worms, which debilitate hundreds of millions at any given time, are associated with contaminated or undercooked food – which in turn arise from inadequate water supplies and improper food preparation or storage. Malaria, schistosomiasis, and other vector-borne diseases require certain ecological conditions for the vector – mosquito, fly, or snail – to persist. Environmental conditions increase the biological organisms’ ability to thrive or spread. Although some supporting conditions exist in the natural environment, many are created or enhanced by human activities, as will be described more fully in Changing Environments, Changing Health. Other diseases, such as acute respiratory infections or tuberculosis or measles, are linked with poor conditions within the household environment, including overcrowding, soot and smoke, and air pollution. The role of environmental factors here seems to be to weaken the body’s natural defenses to organisms that are often present.
The agents that cause infectious disease require not only favorable environmental conditions but also a susceptible host. The most vulnerable people tend to be those with low or reduced immunity, such as those weakened by malnutrition or other infections. Children are particularly vulnerable, especially infants who are not breast-fed and thus do not have the advantage of the mother’s immunity. (See Children’s Special Vulnerability). For some infectious diseases, new migrants to an area are also at heightened risk because they have not been previously exposed to diseases endemic in the area and thus have not built up any defenses against them. The next section reviews the major infectious diseases and the environmental conditions that influence their incidence.
Water-, Food-, and Soilborne Diseases
Of all the infectious diseases, diarrhea has perhaps the clearest links to the environment and some of the most deadly repercussions. Diarrhea is spread by both bacteria and viruses through contaminated food or water, and these disease-causing agents represent one of the most widespread health problems in the contemporary world. Diarrhea killed roughly 2.5 million people in 1996, according to WHO, most of whom were children under age 5 (89). In 1990, diarrhea accounted for 8 percent of DALYs globally (90). Diarrhea kills through dehydration. Fortunately, it is easily and fairly inexpensively managed with oral rehydration therapy (ORT), for those who have access to adequate medical care. Despite concerted efforts to make ORT widely available, the huge death toll from diarrhea clearly shows that much more remains to be done. Short of death, the roughly 4 billion episodes of diarrhea each year cause widespread debilitation (91). Diarrhea is intimately connected to malnutrition, which increases the frequency and severity of diarrheal episodes; repeated bouts of diarrhea, in turn, exacerbate malnutrition (92). Among causative agents, one of the most common is the intestinal bacteria E. coli, although a variety of other pathogens can cause diarrhea
Diarrheal diseases arise through contact with feces and are spread by what is known as the fecal-oral route. Until recently, contaminated water supplies were thought to be the chief culprit, but now the pathway is known to be more complex. An even greater factor appears to be insufficient water for washing, especially hand-washing, which makes proper hygiene impossible (93). When inadequate water supplies are coupled with shared latrines – or none at all – it creates conditions rife for transmitting diarrhea. Today, an estimated 2.9 billion people lack access to adequate sanitation and roughly 1.4 billion people do not have access to safe drinking water (94). This situation has persisted despite investments of more than US$100 billion during the International Water and Sanitation Decade.
References and notes
85. World Health Organization (WHO), The World Health Report 1997: Conquering Suffering, Enriching Humanity (WHO, Geneva, 1997), p. 20.
86. Anne E. Platt, “Infecting Ourselves: How Environmental and Social Disruptions Trigger Disease,” Worldwatch Paper 129 (Worldwatch Institute, Washington, D.C., 1996), p. 21.
87. William H. McNeill, Plagues and Peoples (Anchor Books, Garden City, New York, 1976).
88. Anne E. Platt, “Infecting Ourselves: How Environmental and Social Disruptions Trigger Disease,” Worldwatch Paper 129 (Worldwatch Institute, Washington, D.C., 1996), p. 21.
89. World Health Organization (WHO), The World Health Report 1997: Conquering Suffering, Enriching Humanity (WHO, Geneva, 1997), p. 15.
90. Christopher J. L. Murray and Alan D. Lopez, eds., The Global Burden of Disease: Volume 1 (World Health Organization, Harvard School of Public Health, and The World Bank, Geneva, 1996), p. 573.
91. World Health Organization (WHO), The World Health Report 1996: Fighting Disease, Fostering Development (WHO, Geneva, 1996), p. 24.