Health effects associated with drinking water
The transmission of waterborne diseases is now controlled in most developed countries but a residual level of both epidemic and endemic diseases can still be observed. Recent observations have involved emerging pathogens such as Cryptosporidium which has been implicated in several very large outbreaks worldwide. To control these waterborne outbreaks many countries are proposing treatment goals that would achieve a significant reduction in the risk to the population without increasing the risks of cancer due to disinfection by-products. In the United States, the objective is zero pathogen in drinking water. This objective can be approached by appropriate treatment but monitoring cannot be done using the current biological indicators. Reliance on continuous measurement of physico-chemical parameters such as disinfectant residual and contact time, turbidity and particulate measurement in real-time are proposed solutions. Microbiological indicators remain an active mean of controlling afterwards water quality : spores of aerobic or anaerobic bacteria are extremely resistant to treatment and ofter means of assessing removal of pathogens over a range of several order of magnitude. Numerous pathogens have been involved in waterborne outbreaks and some are just emerging. Urbanisation, aging of water treatment plants, the increasing number of immunocompromised individuals are potential causes for increased risk of waterborne infectious diseases. The endemic level of gastrointestinal disease due to drinking water consumption is still significant and could contribute to pathogens in circulation in the populations affected. The dilemma of balancing microbial and cancer health risk remains a difficult one to resolve but it should not result in a reduction of treatment efficiency, because of the low risk-level for cancer used for the Maximum Admissible Concentration (MAC) values (10-6 fall life) as compared to the risk of waterborne infectious disease in absence of adequate water treatment. For setting these MAC values in water it is necessary to consider all sources of exposition of the considered compound and according to is mode of action two ways of evaluation may be followed. The first one, for the molecules with a deterministic effect (or non stochastic effect) the dose-effect relationships dose-effet on individuals and dose responses relationships on populations, are considered, and only no indesirable effect in the consumming population may be accepted. For the second one, for the molecules with a probabilistic (stochastic) effect, (absence of dose-effect relationship but increase of the incidence of cancer or genetic abnormalities in population according to levels of exposure), MAC values are setted by computation off all exposures and considering an accepable risk of apparition of the pathology ranging from 10-5 (WHO) to 10-6 (North America and Europe) for consumption of two liters of drinking water during 70 years. Because of the lack of information about these very conservative approaches, associations of consumers are often misinterpreating : in this situation water is not in accordance with drinking water standards but may be used during a short period without any risk increase for the population. Examples are developped in this paper with description of hazards due to nitrate, nitrite, pesticides and desinfection by-products which are actually frequently associated with debates within water specialists, politicians and consumers.If situation is now more clearly defined for nitrates and pesticides, the lack of scientific information for the effects of bromates combined with the conservative approach for the molecules with "probabilistic" effect, leads to a severe MAC value in comparison with the analytical capacities of laboratories and technical data of water desinfection both with ozone and bleaching agent. This situation may not lead to the diminution of adequate desinfection water treatment because of the consequences on the increase of the risk of waterborne infectious disease. More progresses are needed, both in terms of knowledges in toxicological and epidemiological data and technological ways of treatment, for being able to produce safe drinking water, with a taste and a price acceptable for the consumer. But the levels of risk considered for setting standards insures that drinking water is one of the safer products offer for consumption.