Executive summary
Modern societies are reliant on chemicals for a range of uses including healthcare, manufacturing, agriculture and energy production. Yet, chemical pollution is a key contributor to biodiversity loss globally and a persistent threat to human health. Projections indicate large increases in global chemical production in the coming decades, which, without appropriate management, may further threaten the sustainability of our ecosystems.
In the UK, pollution of rivers, lakes and coastal areas is currently a topic of high public and political interest. The Government has made chemical pollution-related commitments via the Environmental Improvement Plan. While emissions of several well-known and regulated chemicals have declined, this does not reflect the full picture, as the concentrations of other emerging contaminants are increasing. The vast majority of anthropogenic chemicals currently present in the environment are not subject to monitoring and only a little is known about their potential impacts. Toxicity data are lacking for most chemicals and their biological effects are either unknown or not well understood.
Current approaches to chemical risk assessment have considerable shortcomings. This is true for individual chemicals, let alone when considering the additional complexities of mixture risk. Indeed, most chemicals are discharged into the environment as unintentional mixtures and/or co-exist once they enter the environment. The sheer number of chemicals in production has long overwhelmed our ability to assess their safety using traditional approaches. Therefore, it is important to leverage the latest science to better understand and deal with this inherent complexity and develop a more protective and efficient regulatory system.
There is clear evidence that unintentional mixtures, whether released as such or arising in the environment, can cause harm even when the constituent chemicals are present at levels deemed individually ‘safe’. Yet chemical risk is generally assessed on a substance-by-substance basis, with mixture risk unaccounted for. This is a significant shortcoming.
Although most chemical mixtures are thought to behave additively (meaning their overall toxicity is the sum of the toxicities of the constituent chemicals), mixtures remain a complex regulatory challenge due to extensive toxicity data gaps, different regulatory frameworks for different chemical types and limitations in monitoring data. Moreover, estimates of the frequency of synergism (where combined toxicity is greater than that of constituent chemicals) are based on incomplete toxicity data for many chemicals and chemical combinations, particularly in environmentally relevant scenarios.
There is currently policy and regulatory interest in addressing chemical mixtures within the UK and the EU; both are exploring new possible risk management frameworks to better manage combination effects. In terms of scientific understanding, approaches for chemicals management are converging (including machine learning and ‘omics’ techniques), meaning that in the future more informed ways of predicting hazards associated with mixtures will be possible. Yet our understanding of the effects of mixtures is currently biased towards relatively well-studied environments, species and chemicals and it is likely that significant uncertainties will remain even as the science progresses. Dealing with this uncertainty will therefore remain an inherent aspect of any risk-based regulation relating to chemical mixtures and should not be used to justify inaction.