Many studies have shown that providing microbiologically safe drinking water can result in a substancial reduction in mortality and morbidity (WHO 1992). Disinfection, especially with chlorine, is necessary to provide microbiologically safe drinking water. The potencial human risks associated with drinking water disinfection are largely unknown, even though some information is available from toxicological and epidemiological studies. More research is needed to determine the risks associated with DBPS, but current scientific evidence suggests that risks are very small compared to microbiological risks when water is not disinfected.

In their pioneering work on the comparison of estimated risks from known pathogens in untreated surface water and DBPs in drinking water, Regli et al. (1993) concluded the following:

• The risk of death from pathogens is at least 100 to 1000 times greater 
  than the risk of cancer from DBPS.
   
• The risk of illness from pathogens is at least 10,000 to 1 million times greater than
  the risk of cancer from DBPS.
  
• Morbidity and mortality rates from pathogens, compared with those from DBPS,
  may be considerably higher in developing countries where the sanitary
  and healthstatus is not as good.
   
• In societies where infant mortality is high and life expectancy is low, many people
  would not be expected to live long enough to incur cancer, which
  also causes much higher differences in risk resulting from exposure to pathogens
  versus DBPs cited above.

While this last conclusion may seem cynical, it does reflect the true situation in many developing countries.

IARC has concluded that there is inadequate evidence for the carcinogenicity of chlorinated drinking water in experimental animals and humans (IARC 1991). A review of the subsequent studies by an internacional panel of scientists during a conference on the safety of water disinfection concluded that the evidence was not sufficient to alter the IARC conclusions (Craun 1993).

Although stated in qualitative way, the WHO Guidelines provide a similar message. Review of the toxicological evidence has resulted in recommended GVs for some disinfectants and their by-products. Regarding microbial risks, however, the message is clear: The estimated risks to health from disinfectants and their by-products are extremely small in comparison to the real risks associated with inadequate disinfection, and it is important that disinfection should not he compromised in attempting to control such by-products. The destruction of microbiological pathogens through use of disinfectants is essential for protecting public health.