drinking water quality, guideline interpretation, arsenic, uncertainties
Drinking water quality guidelines are often interpreted by the non-expert as make or break cut-off values below which drinking water is absolutely safe, and above which it is totally unacceptable. In reality there is no such knifelike cut-off limit, and there is a large grey area between safe water and undrinkable water. The uncertainty of the boundaries of the grey area for each constituent presents a serious problem, both in the creation of sound drinking water quality guidelines or standards, and in the problem of how to interpret the risk to human health when guideline values are exceeded. In this paper this problem is discussed using the case of arsenic, where the definition of the boundaries of the grey area is particularly uncertain.