Chemical mixtures in aquatic environments

In this context, chemical or contaminant mixtures are the combinations of chemicals that end up together in the environment, for example, residues from agriculture, industry, sewage and household products that mix in rivers and seas. Organisms are almost never exposed to just one chemical at a time, but rather to shifting “cocktails” of substances. These mixtures of chemicals can interact with one another and with organisms in a number of ways (see below).

Chemicals within mixtures interact with one another in three different ways. Their effects can be additive, meaning their overall toxicity is roughly the sum of each chemical’s individual effect. In some cases, they may interact in a synergistic way, where the chemicals enhance the toxicity of one another and the combined toxic effect is greater than expected, or antagonistic, where one chemical reduces the toxicity of another. Because we only have toxicity data for a small fraction of the estimated 350,000 of chemicals in use, and even less on their combinations, predicting toxic mixture effects is extremely challenging.

Additivity is of concern to regulators because if many chemicals target the same biological pathway, or mode of action, their combined effects can result in harmful effects even when the concentration of each individual chemical is deemed ‘safe’. Synergistic effects, while less common, are also of concern due to their potential to amplify toxic effects. Early indications suggest that synergistic effects are less common than additive and synergistic effects, but far more research is required to determine this for sure.

Chemicals in the environment are almost always part of mixtures, and their combined effects can cause harm to species and ecosystems even when present at individually safe levels. Chemical mixtures can impair growth, reproduction, immune function, behaviour, development, and stress tolerance in aquatic organisms. Effects may be seen either at an individual, species or population-level (eg reduced abundance, altered species diversity, lessened resilience to other stressors). Combinations of chemicals may be particularly harmful when they act on the same biological pathway (eg endocrine disruption). Theoretically chemical mixtures could similarly be harmful to human health, but our report did not explore this in any detail.

Alongside these potentially harmful effects, the sheer complexity of possible chemical combinations, fluctuating concentrations and environmental conditions, and multiple exposure routes makes evaluation and control difficult.

The spectrum is wide: pharmaceuticals, industrial intermediates, solvents, detergents, pesticides, consumer products, cleaning agents, personal care products, transformation products, and by-products from manufacturing. Sources include industrial discharge, agricultural runoff, sewage treatment plants, leaching from urban surfaces, domestic effluent, stormwater, and atmospheric deposition.

It’s a mixed picture, for legacy pollutants such as heavy metals and certain types of now banned pesticides, concentrations have declined. However, for many emerging chemicals - especially newer industrial compounds, pharmaceuticals, micro-pollutants and personal care product residues – these are becoming increasingly prevalent. Monitoring and regulation tend to focus on a very limited set of chemicals, meaning that much of the chemical pollution in aquatic environments remains invisible. Therefore, water quality may appear to be stable or improving in reported metrics, but this masks the fuller picture.

Chemicals are predominantly regulated in the UK via UK REACH (Registration, Evaluation, Authorisation, and Restriction of Chemicals). However, separate legislation exists for some sector-specific pollutants such as pesticides and pharmaceuticals. For all of these, chemicals are regulated on an individual basis based on a laboratory risk assessment on a few test species.

In short, no. Chemicals in the UK are regulated on an individual basis, which largely ignores the fact that chemicals accumulate and interact in the natural environment, and that chemical mixtures can have harmful effects above and beyond that of their individual chemical constituents.

The convergence of several new and developing areas of science offer potential opportunities for monitoring, managing and regulating chemical mixtures, beyond what has previously been possible.

These include:

• Non-targeted and high-throughput chemical screening, enabling detection of many substances at once, including unknowns
• ‘Omics approaches (transcriptomics, metabolomics, proteomics) to detect early biochemical response patterns
• Machine learning/AI/modelling frameworks to predict mixture toxicity from limited data and identify emerging patterns from complex, interacting data from a large number of chemicals and species
• Bayesian or probabilistic exposure modelling aggregating multiple sources of data
• New opportunities for databases and shared chemical/biological data platforms to pool and reuse information
• Mechanistic approaches which can group chemicals by common modes of action, enabling cumulative risk models

These innovations may reduce uncertainty, help prioritise testing, and allow more responsive regulatory decisions. They could also make monitoring and risk assessment faster, cheaper and more comprehensive. We recommend that UK government and agencies become early adopters of these innovative approaches, alongside strategic research investment to address critical evidence gaps.