Content of the Guidelines

Similar to the 1984 Guidelines, the 1993 Guidelines place the greatest importance on the microbiological quality of drinking water and repeatedly, as a Delenda est Carthago, emphasize the destruction of pathogenic microorganisms. The health risk due to toxic chemicals in drinking water differs from that caused by microbiological contaminants. There are few chemical constituents of water that can lead to acute health problems, except through massive accidental contamination of a supply. Moreover, experience shows that, in such instances, the water usually becomes undrinkable due to unacceptable taste, odor, and appearance.

The fact that chemical contaminants are not normally associated with acute effects places them in a lower priority category than microbial contaminants, the effects of which are usually acute and widespread. Indeed, it can be argued that chemical standards for drinking water are of secondary consideration in a water supply subject to severe microbial contamination.

Contrary to the 1984 Guidelines, the 1993 Guidelines do not propose GVs for substances and parameters that may affect the acceptability of drinking water to consumers. The review groups decided that GVs should be recommended only for constituents that represent a potential hazard to health.

The problems associated with chemical constituents of drinking water arise primarily from their ability to cause adverse health effects after prolonged periods of exposure. Of particular concern are contaminants that have cumulative toxic properties, such as heavy metals; contaminants that are carcinogenic, and contaminants that may cause reproductive and developmental effects. Excessive levels of such contaminants need to be controlled; however, time is usually available to take remedial action.

Many of the inorganic and aesthetic constituents evaluated in the Guidelines are known to be essential for life. Chromium, copper, fluoride, iodine, manganese, molybdenum, and selenium are essential elements in human nutrition. Of the aesthetic constituents, iron, chloride, calcium and magnesium (hardness), sodium, and zinc are essential elements. No attempt has been made in the Guidelines to define a minimum desirable concentration of essential elements in drinking water.

The WHO-recommended GVs for potentially toxic chemicals in drinking water should not be regarded as precise: there are considerable uncertainties regarding these GVs, including, among other things, extrapolation of toxicological data from animals to humans and from high dose to low dose, assumptions made regarding exposure to drinking water relative to other sources, inherent problems of epidemiological studies due to possible random and systematic biases, and limited information on individual exposure. Uncertainties regarding the GVs are clearly stated in the Guidelines, particularly in volume 2.

The recommended GVs for chemical contaminants in drinking water, including disinfectants and DBPS, are of an advisory nature and are not to be regarded as regulations or standards, which are the responsibility of individual governments.