7. Total Social Costs ...
THE INTERRELATION OF HEALTH AND IMPROVEMENTS
The epidemiological basis for the prevention of infections related to water by improving supplies may be summarized in the following way. The general model is illustrated in figure 7.1. Under conditions of appalling water supply the amount of illness is A. Increasing the volume supplied will decrease the morbidity until a point is reached where no matter how much more water is supplied the situation will remain static with V units of disease. On the other hand, improving water quality alone will reduce the disease amount to level P. Improving all aspects of water supply will, at optimal purity and volume, reduce the disease level to M. This residual amount will be unaffected by changes in water supply. The values of A, M, P, and V will depend on the physical and social environment and on the disease being considered. An aggregate model for all disease in a particular area will have a particular form depending on the environment. A similar model for improvement costs may be constructed (fig. 7.2), and the two may be compared. Quantitative data for such disease models are unavailable, but qualitatively the position is as follows.
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The waterborne diseases in the strict sense are due to polluted supplies. The classical waterborne diseases have a low infecting close of organisms and may produce dramatic epidemics following relatively light pollution of a large common source. They reach their greatest importance in urban areas where the number of households per source is highest and for their prevention require completely pure supplies. The nonclassical waterborne infections are those requiring a larger infecting dose of microbes and are usually commoner diseases than the preceding ones, though often not waterborne in more developed countries. They are transmitted under conditions of heavy pollution even when sources do not supply large groups of households, and they may produce sporadic disease. We know little about their extent but suspect that they are most important in urban areas with very heavy pollution of unimproved sources. They are prevented by moderate protection of sources short of absolute sterility.
The water-washed diseases are of two types. The bulk of such diseases as seen in the outpatient clinic are superficial infections of the skin and eyes. These reach their peak in dry areas with both a scarcity of water for washing and a dusty environment, though a generally unsanitary habitat makes matters worse. These are clearly of the water-washed category and the prime need in most rural areas is for a more accessible supply of greater volume. Purity is a secondary consideration and should not delay attempts to increase the quantities available.
The diarrheal diseases also seem to diminish when water supplies are made more accessible. Their precise etiology is still far from clear and other factors are important, so that there are variations in prevalence between areas with comparable water supplies. A hot, dry climate and an unsanitary environment are both associated with much diarrhea, which is therefore common both in crowded urban and arid rural areas.
Where the population density rises, whether in towns or in some densely inhabited rural areas, purity becomes of greater relative importance, not only because more people use each common source but also because dense rural settlement tends to be in high-rainfall areas. On the basis of evidence presented elsewhere, gastroenteric disorders appear to have both water-washed and waterborne components and reach their peak in overcrowded periurban zones where, fortunately, most can be done about them, though so far municipal authorities have paid more attention to purity than to availability of improved supplies. The diarrheal diseases and typhoid cause hospital admissions and death far more often than the superficial water-washed infections; they affect treatment costs more and also provide the bulk of lost work time and the economic losses from premature death. …