WHO Guidelines. However, it has to be decided whether a maximum tolerable additional DALY loss of 10<sup>−4</sup> pppy acceptable or not.

_Reasons in favor of a maximum tolerable additional DALY loss of 10_<sup>_−4_</sup> _pppy_

1.  The reason why the 2006 Guidelines use this value of 10<sup>−6</sup> DALY loss pppy is because, as noted in section A2.3, it is used in the third edition of the WHO _Drinking-water Quality Guidelines_ (WHO, 2004, 2008a) since it corresponds very closely to the fatal waterborne 70-year lifetime cancer risk of 10<sup>−5</sup> per person accepted by US EPA (Munro and Travis 1986).  This 70-year risk of 10<sup>−5</sup> per person is equivalent to an annual risk of 1.4 × 10<sup>−7</sup> per person. Whether this is a reasonable level of acceptable risk can only be judged by knowing how many Americans die each year from cancer. Horner _et al_ . (2009, Table 2.5) give `-` the 2006 age adjusted mortality rate from all causes of cancer for both sexes and all races as 181.07 per 100,000 population – i.e., an incidence of 1.8 × 10<sup>−3</sup> pppy. Thus the fatal waterborne‐cancer risk of 1.4 × 10<sup>−7</sup> pppy is four orders of magnitude lower than the actual fatal all `-` cancer incidence of 1.8 × 10<sup>−3</sup> pppy.

A DALY loss of 10<sup>−4</sup> pppy would be equivalent to a fatal waterborne lifetime cancer risk of 10<sup>−3</sup> per person and thus to an annual risk of ~10<sup>−5</sup> per person. This is two orders of magnitude lower than the actual fatal all `-` cancer incidence of ~10<sup>−3</sup> pppy – i.e., an additional DALY loss of 10<sup>−4</sup> pppy would increase this by only ~1%.

2. The current global incidence of diarrheal disease is extremely high: in order-of-magnitude terms it is 0.1−1 pppy (Table A3.1; see also Kosek _et al_ . 2003). A tolerable diarrheal disease risk of 10<sup>−2</sup> pppy, equivalent to a 10<sup>−4</sup> DALY loss pppy, is 1−2 orders of magnitude lower than the current diarrheal-disease incidence. For an individual it is equivalent to an additional episode of diarrheal disease once every 100 years (essentially once per lifetime), which is hardly a matter of significant public health concern − see also Haas (1996, 8) who comments on US EPA’s use of a waterborne-disease risk of 10<sup>−4</sup> pppy as follows: _It is becoming apparent that some key factors used for computing the 1:10,000 level of acceptable risk may not be correct. ... the total burden of waterborne illness associated with current water treatment practice in the United States may be as high as several million cases per year. This would translate to an annual illness rate of perhaps 1:100, suggesting that the current benchmark_ [of 1:10,000] _may be far too stringent._ 3. Low- and middle-income countries diarrheal diseases caused a total DALY loss of 59 million in 2001 (Lopez _et al_ ., 2006; Ezzati _et al_ ., 2006). Thus in 2001, for the then total developing-country population of 5,615 millions (UNFPA 2002), the DALY loss due to diarrheal diseases was: An additional DALY loss of 10<sup>−4</sup> pppy would increase this to 0.0106 pppy – i.e., an increase of just under 1%. Such an increase is not epidemiologically significant (and, in any case, would be extremely difficult to detect).

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