Water- and wastewater-related disease and infection risks: what is an appropriate value for the maximum tolerable additional burden of disease?

The maximum additional burden of water- and wastewater-related disease of 10^_6 disability-adjusted life year (DALY) loss per person per year (pppy), used in the WHO Drinking-water Quality Guidelines and the WHO Guidelines for Wastewater Use in Agriculture, is based on US EPA's acceptance of a 70-year lifetime waterborne cancer risk of 10^_5 per person, equivalent to an annual risk of 1.4×10^_7 per person which is four orders of magnitude lower than the actual all-cancer incidence in the USA in 2009 of 1.8×10^_3 pppy. A maximum additional burden of 10^_4 DALY loss pppy would reduce this risk to a more cost-effective, but still low, risk of 1.4×10^_5 pppy. It would increase the DALY loss pppy in low- and middle-income countries due to diarrhoeal diseases from the current level of 0.0119 pppy to 0.0120 pppy, and that due to ascariasis from 0.0026 pppy to 0.0027 pppy, but neither increase is of public-health significance. It is therefore recommended that the maximum additional burden of disease from these activities be increased to a DALY loss of 10^_4 pppy as this provides an adequate margin of public-health safety in relation to waterborne-cancer deaths, diarrhoeal disease and ascariasis in all countries.