28 October 2025

Unregulated and understudied cocktails of chemical pollutants in UK rivers, lakes and coastal environments can have harmful effects on aquatic ecosystems even when their individual components are present at ‘safe’ levels, the Royal Society has warned.

The Chemical mixtures in aquatic environments: understanding and preventing harm report warns existing regulatory approaches, focusing on individual chemicals, often in laboratory settings, are not fit for purpose.

It calls for the UK to adopt a pragmatic, yet precautionary approach, limiting the release of chemical contaminants into the environment while closing gaps in our monitoring and understanding of the risks from chemical mixtures.

Longer term, the report calls for the UK to take a leading role in embedding emerging science and technology to support monitoring and regulation, including the use of environmental DNA and machine learning, to make monitoring more accurate and cost effective.

It also highlights the need to increase the training of existing and future environmental toxicologists, to fill expertise gaps and meet the increased regulatory burden since leaving the European Union.

Chemical pollution is a key contributor to biodiversity loss globally and a persistent threat to human health. The UK government has made commitments to tackling pollutants via the Environmental Improvement Plan, and the report comes as both the UK and EU are looking at new approaches to managing these combination effects.

However, despite reductions in some key contaminants, concentrations of other emerging contaminants are increasing. With more than 350,000 chemicals in production globally, only a fraction are routinely monitored and there is limited data on their toxicity and biological effects.

Where data exists it is often limited to well-studied chemicals, such as pesticides, and freshwater environments.

Preliminary analysis by the UK Centre for Ecology and Hydrology, published alongside the report, shows that in UK waters, the highest mixture risk comes from metal pollutants and polycyclic aromatic hydrocarbons (PAHs), a byproduct of combustion in car engines or industrial processes.

However the analysis did not include emerging groups such as PFAS, pharmaceuticals and plasticisers which can have a range of effects from outright toxicity, to disrupting hormonal cycles, behaviour and fertility in a range of aquatic species.

Additionally, the analysis found 85% of available UK data came from river monitoring with comparatively little reporting on estuary and marine settings where pollutants may build up as sediments.

Professor Michael Depledge CBE FRS Emeritus Professor of Environment and Human Health, European Centre for Environment and Human Health, University of Exeter and co-chair of the report, said:

“Individually, many registered chemicals can harm humans and wildlife, and are already entering the water environment through run off, direct industrial, and waste water discharges.

“But to date we have been blind to the combined harmful effects of mixtures of these chemicals in aquatic ecosystems. Our report presents ample evidence to suggest the harms are cumulative – and sometimes greater than the sum of individual chemical toxicities.

“Recent advances in sampling and data techniques mean, for the first time, it is possible to address these gaps – but with chemical production set to triple by 2050 – managers and regulators need new skills, data and resources now to protect our rivers, lakes, estuaries and coastal seas, and the people and wildlife who use them.”

Professor Louise Heathwaite CBE FRS, Distinguished Professor at Lancaster Environment Centre, Lancaster University, and co-chair of the report, said:

“Chemical mixtures have long been considered an intractable problem. However, given their scale and widespread presence in the environment, the risks they pose can no longer be ignored by policymakers and regulators.

“Our current approaches to assessing chemical mixtures and their associated risks are very limited. The UK now has a real opportunity to lead the way in implementing an innovative, pragmatic and precautionary approach to managing chemical mixture risk.”