WHO Report Calls for Monitoring and Modelling of Chemical Mixtures in Water

There is a lack of systematic data on human exposure to chemical mixtures in water systems, according to a WHO report.

The expert report on Chemical mixtures in source water and drinking water calls for investigative monitoring and modelling to improve understanding of human and environmental exposures.

The report points out there is currently no "regulatory-applicable" accepted risk assessment methodology for conducting risk assessments of chemical mixtures in water.

A significant gap in exposure data is made worse by a lack of standardised sampling and analytical methods for many substances, particularly contaminants of emerging concern, write the experts.

Interactions within chemical mixtures are difficult to predict, particularly for long-term effects at very low exposure levels. Research is therefore needed to define criteria for predicting whether chemicals may act together in synergistic or antagonistic ways, the report says.

Challenges

The authors acknowledge that assessing combined risks to human health from exposure to chemical mixtures is much more complex than for single substances.

Regulators, they say, face the challenge of determining the degree to which humans are co-exposed to chemicals, which interactions may occur, and which specific human health impacts are associated with chemical mixtures, says the report.

Grouping chemicals for risk assessment and management can help, the authors say. This is already done, for example, for disinfection byproducts trihalomethane and haloacetic acids.

The report uses perfluorinated substances in water systems as a possible grouping example. These substances often occur together, and now that more data on potential toxicity are emerging, it may be possible to consider them as mixtures. Such consideration would be based on PFOS being the most toxic substance in the group and using a toxicity equivalent approach, it says.

The report provides a set of questions that can help to decide whether to treat a group of substances as a mixture and which management approaches to consider. These include:

> how frequently do the chemicals occur together?

> are the substances of similar water solubility? and

> how stable is the mixture?

"In assessing the possible risks of exposure to combinations of chemicals a number of frameworks are currently in place, but significant knowledge gaps, complexity, limited practical experience and resource intensity, precludes this type of approach being systematically introduced into drinking-water standards at the present time," the report concludes.