Many studies have looked into the effects of chemicals on humans and the environment. However, such effects are traditionally evaluated based on single substances, chemical-by-chemical.
It's a point that has been exercising the Council of Environment Ministers of the European Union (EU) for some time. In December 2009, the Council invited the European Commission to assess how — and whether — existing legislation addresses the problem, and to suggest appropriate modifications and guidelines.
Researchers from the University of Gothenburg, Sweden, and the University of London, U.K., were contracted to review the state-of-the-art of mixture toxicology and ecotoxicology. "The Study: State of the Art Report on Mixture Toxicity," published in April, shows that all relevant research is unambiguous: the combined "cocktail effect" of environmental chemicals is greater and more toxic than the individual impact of each chemical.
"The number of chemical combinations that the Earth's living organisms are exposed to is enormous," says Thomas Backhaus, researcher at the University of Gothenburg's Department of Plant and Environmental Sciences and co-author of the report. "Assessing every conceivable combination is not therefore realistic, and predictive approaches must be implemented in risk assessment. We need guidelines on how to manage the chemical cocktail effect so that we can assess the risks to both humans and the environment."
The study makes several recommendations. First, the authors point out that current mixture guidelines, as for example those issued by the U.S. Environmental Protection Agency (EPA) or the recently-suggested ones from the World Health Organization (WHO), are limited to assessment of potential human health risks from chemical mixtures. In contrast, the European regulatory system considers protecting the environment equally important. Therefore, they say that a future European guideline to assess chemical mixtures should go beyond currently existing regulatory approaches and extend to protection of ecosystem structure and function from detrimental effects of chemical mixtures.
Second, the study calls for strengthening the legal mandate for mixtures risk assessment in the EU. How this scientific knowledge might be best transferred into appropriate regulatory approaches is, however, not at all trivial. The authors point out that the U.S. EPA spent many years developing guidelines for the health risk assessment of chemical mixtures — something that wouldn't have happened without an explicit legal mandate.
The third recommendation focuses on existing environmental legislation. The authors explain that regulations control single- and multi-constituent substances, preparations of chemicals and products containing chemicals that are intentionally produced and placed on the market. Typically, they assess hazards and risks of these substances and products as if they were present in isolation. Assessment of complex exposure situations of humans and the environment from multiple substances and products is out of their scope and difficult to integrate. The authors suggest the best starting point for assessing those mixtures should be given by corresponding media-, site-, or population-oriented elements of legislation, such as the Water Framework Directive, the Marine Strategy Directive, or the proposed Soil Directive. Options for advancing this legislation with the aim of taking account of, and improving, risk assessments of realistic complex exposure scenarios should be explored, they say.
The authors also say that dual use of single substance data should already be considered when designing and implementing studies for risk assessment of individual chemicals. Specifically, this calls for the use of benchmark doses instead of using no observable adverse effect levels (NOAEL) or no observed effect concentrations (NOEC) as the preferred method for defining thresholds of regulatory concern and points of departure.
More information on typical exposure to chemical mixtures needs to be compiled and systematized, say the authors: "Beyond the lists of priority chemicals that are currently defined in certain areas, we need to know priority chemical mixtures that are present in the environment and might have an impact on human health and ecosystems. Furthermore, our understanding of the determinants of synergistic effects needs to be improved scientifically, with a view of being able to anticipate synergisms in the future."
The authors acknowledge that the scientific state-of-the-art of mixture toxicology has been significantly advanced, not least as a result of EU-funded research: "Because the protection of human health and the environment are goals of equal importance in EU regulations, Europe is uniquely placed to set the agenda worldwide for a truly integrated mixture risk assessment, provided there is the political will."
The full report can be downloaded at http://ec.europa.eu/environment/chemicals/effects.htm.
Seán Ottewell is Chemical Processing's Editor at Large. You can e-mail him at firstname.lastname@example.org.