Health Stream Literature Summary - Issue 50 - June 2008

Health risks of micropollutants - the need for a new approach.
Fawell, J.K. (2008) Water Science & Technology, 57(2); 183-87.

There have been an increasing number of micropollutants detected in water as the analytical capability to measure substances at low concentrations has increased. However, simply identifying micropollutants does not provide information regarding the risk to either aquatic life or human health. There is a need to begin to develop new approaches for determining just how important the risks from micropollutants are and to establish the priorities for controlling micropollutants.

The process of risk assessment requires knowledge of the key toxic effects of a substances and the dose response to determine a safe or acceptable level of exposure and to compare this with the actual levels of exposure. However, it is difficult to obtain a suitable assessment of overall exposure which is compounded by the increasing number of substances being identified and the problems of taking an adequate range of samples. It can also be difficult to find adequate data on the toxicity and the dose response. There is also an essential requirement for high quality data and for the scientific contribution to be completely objective. Scientists need to embrace a multidisciplinary approach for the identification of hazards and subsequent risk assessment; this will result in improved design of studies.

In the toxicological community there has been considerable interest on how to prioritise substances for further examination and testing. A methodology that has emerged is the “threshold of toxicological concern” or TTC. This approach involves determining a dose below which there is reasonable evidence that there will be no appreciable risk of adverse health effects. This approach provides a good basis for prioritisation with regard to potential toxins, and has been used for establishing priorities for testing and further study of contaminants in food. It has been suggested that this is a way of determining whether there is a need for more detailed analytical and toxicological data for chemicals that may potentially leach from materials used in contact with drinking water.

The TTC concept considers chemicals under three structural classes. Class I: substances with simple chemical structures and for which efficient modes of metabolism exist, suggesting a low order of toxicity; Class II: substances which possess structures that are less innocuous than class I but do not contain structural features suggesting toxicity; Class III: substances with chemical structures that permit no strong initial presumption of safety or may even suggest significant toxicity or have reactive functional groups. Using toxicity data for well characterised chemicals, the generic TTCs derived for human exposure are 1.8 mg/kg per person per day for class I, 0.54 mg/kg per person per day for class II and 0.09 mg/kg per person per day for class III. This approach is suitable for most chemicals but for some substances (eg high potency carcinogens) a more conservative approach has been recommended, with a proposed threshold for regulation of 1.5 microg/day. For pharmaceuticals, comparison of detected levels with a 100-fold or 1000-fold fraction of the usual daily dose level provides a pragmatic threshold for triggering investigation, although again particularly toxic drugs may need a special approach. When considering implications of micropollutants for drinking water supplies, the removal of substances by water treatment processes is also relevant.