9.12 Assessing the significance of short-term exceedances of health-based guideline values

Most of the chemical health-based guideline values listed in Chapter 10 (Table 10.6) are based on the amount of a chemical in drinking water that a person can consume on a daily basis over a lifetime without any appreciable risk to health—these therefore generally relate to chronic exposure. Most health-based guideline values are conservative and include safety factors—a small and brief exceedance of a chemical guideline value would not necessarily result in an increased risk to health or indicate that the water is unsuitable for consumption. The amount and duration for which any guideline value can be exceeded without affecting public health depend upon the specific contaminant involved and sensitivity of exposed populations. Consideration should therefore be given to any potential effects of short-term exposure on a chemical-by-chemical basis. Identification of toxicological risks from short-term exposure (up to seven days) and derivation of short-term exposure values must only be undertaken in consultation with the relevant health authority or drinking water regulator.

The World Health Organization (WHO) has provided guidance on the derivation of short-term exposure values in the 2017 edition of the WHO Guidelines for Drinking Water Quality. The WHO recommended that short-term exposure values be based on acute reference doses (ARfD) and that the proportion of intake from water can be adjusted to 100% for exposures of 24 hours or less; this could be reduced to 50% for exposures of up to seven days (Leusch et al. 2020). The ARfD is defined as the amount of a chemical that can be ingested over a period of 24 hours or less without appreciable health risk to the consumer based on all known facts at the time of the evaluation (Solecki et al. 2005). In Australia, ARfD values are provided by the Australian Pesticides and Veterinary Medicines Authority (APVMA 2019), while the Food and Agricultural Organization/WHO Joint Meeting on Pesticide Residues (JMPR) provides an international perspective. ARfD are usually only available for pesticides, but approaches for a broader framework and examples of short-term exposure value derivation for several classes of chemicals are available in the scientific literature (Haber et al. 2016; Leusch et al. 2020). It is important to keep in mind that some chemicals pose an acute health risk and that deriving short-term exposure values for such acutely toxic chemicals is not appropriate. In these cases the relevant health-based guideline value and chemical fact sheets should be consulted in discussion with the relevant health authority or drinking water regulator.

Short-term exposure values can be useful to assess the risk posed by small and brief exceedances of health-based guideline values, providing a basis for deciding when water can continue to be supplied to consumers without adverse health risk. Short-term exposure values can also be useful to effectively communicate the risk to consumers when a guideline value exceedance has been identified.

The availability of short-term exposure values does not void the overarching ADWG principle of producing safe and high-quality drinking water while reducing unnecessary chemical exposure. Short-term exposure values assist in determining whether there is an imminent threat to public health. They are also an acknowledgment that not supplying water poses significant risks to public health (e.g. loss of water for sanitation and fire-fighting, a shift to less safe alternative water sources or poor adherence to avoidance advisories). Any exceedance of health-based guideline values should be investigated and the relevant health authority or drinking water regulator should always be involved in the response to an exceedance.

It is critical to understand that short-term exposure values are only applicable to “once off” exceedances (i.e. no more than one event in any 12-month period). Short-term exposure values are not appropriate for intermittent and recurrent events, which are indicative of recurring water quality issues that need to be investigated in consultation with the relevant health authority or drinking water regulator.