Effect of investments in water supply and sanitation on health status: a threshold-saturation theory.

A general theory on the relationship between water supply and sanitation investments and health, the threshold-saturation theory, is proposed. The theory takes into consideration three variables: health status, socioeconomic status, and sanitation level, and attempts to encompass, for the first time in one general theoretical framework, numerous conflicting empirical findings. The two-tiered S-shaped logistic form of the relationship that is proposed assumes that at the lower end of the socioeconomic spectrum there is a threshold below which investments in community water supplies and/or excreta disposal facilities alone result in little detectable improvement in health status. Similarly, at the higher end of the socioeconomic scale, it is suggested that a point of saturation is reached beyond which further significant health benefits cannot be obtained by investments in conventional community sanitation facilities.A preliminary attempt to validate this model using published data on sanitation level (defined as access to water supply), life expectancy, and adult literacy rates, for 65 developing countries, appears to provide preliminary support for the threshold saturation theory but further empirical validation is required before a quantitative predictive model can be developed.