6.4 Differences between Australian and WHO guideline values

The guideline values in the Guidelines take as their point of reference the WHO Guidelines for Drinking-water Quality (2004) and subsequent addenda in 2006 and 2008. When the guideline values derived for chemicals in the Guidelines differ from those recommended by the WHO, the difference usually arises in one of two ways:

• The Guidelines use an average adult weight of 70 kg, consistent with developed countries such as Canada, whereas the WHO figure is 60 kg to cater for lighter body weights in developing countries. The use of a higher average weight can sometimes yield slightly higher guideline values, but the difference is not significant given the large safety factors used.

• For genotoxic carcinogenic compounds, WHO uses a risk assessment calculation, with the guideline value set at the concentration that would give rise to a risk of one additional cancer per 100,000 people. The Australian guideline values for these types of compounds are based on a consideration of:

  • the limit of determination based on the most common analytical method;

  • the concentration, calculated by the WHO using a risk assessment model, that could give rise to a risk of one additional cancer per million people, if water containing the compound at that concentration were consumed over a lifetime;

  • a value based on a threshold effect calculation, with an additional safety factor for potential carcinogenicity.

Frequently the values determined from these two types of calculations are very similar. The balance between these considerations is assessed as follows:

• If the limit of determination gives an adequate degree of protection (i.e. is within a factor of 10 of values determined from health considerations), it has been used as the guideline value. If the limit of determination is much lower than values determined from health considerations, then the lower of the two calculated values has been used. If, conversely, the calculated value is much lower than the limit of determination, then the calculated value is used, but with a note that it is lower than the practical limit of determination. Improved limits of determination are required for such compounds.

• The approach used for carcinogenic compounds in the Guidelines is believed to lead to a more balanced assessment of the health risks, and is similar to that adopted in other countries (e.g. Canada). Whether the assumed risk should be one in 100,000 or one in a million is a value judgment. However, the greater degree of protection afforded by a risk of one in a million is generally consistent with calculations based on a threshold approach, and is in line with the high expectations of Australian consumers.