A growing area of immunotoxicology is the study of how contaminants in drinking water can impact human health. Sanah Majid, Daniel Duarte, Tessa Pronk, Corine Houtman, Insam Al Saify, Merijn Schriks, Janine Ezendam, Raymond Pieters, and Milou Dingemans make the case for the inclusion of immunotoxicity in water quality assessments.
The drinking water sector faces constant challenges of protecting public health from a growing number of existing and new water contaminants. Recent amendments in EU Drinking Water Directives, including the inclusion of per- and polyfluoroalkyl substances (PFAS) and bisphenol A (BPA), reflect the growing concern regarding the health impacts of contaminants known for their immunotoxic potential. These substances can disrupt the body’s ability to protect itself from infections and diseases and, depending on their concentrations, can pose significant risks to public health. Despite this, immunotoxicity is not yet a standard endpoint in chemical risk assessments for water quality because of limited regulatory requirements. This article emphasises the need to integrate immunotoxicity assessment into water quality assessments to enhance safety and better protect public health.
Why immunotoxicity matters
The immune system is essential for protecting the human body from infections and disease. It consists of a complex network of cells, tissues and organs that work together to defend the body against harmful substances and to remove damaged or abnormal cells.
When the immune system is impaired, the body becomes more susceptible to infections and other serious health conditions. This impairment, known as immunotoxicity, can be a result of exposure to certain chemicals that disrupt the normal functioning of the immune system. This can happen directly or indirectly.
Direct immunotoxicity occurs when a toxic substance directly damages components of the immune system (e.g., lymphatic nodes), often weakening its ability to protect the body from infections or abnormal cells, which can increase the risk of incidence of certain diseases such as cancers. Indirect immunotoxicity happens when alterations in other physiological systems (namely the nervous or hormone systems) indirectly affect the immune system (e.g., autoimmune disease induced by endocrine disruptors), disrupting its normal function. These systems work together in a complex manner to maintain a healthy immune response, and if one system is affected, it can impact the others.
Cumulative concerns
A key concern with immunotoxicity is its subtle and cumulative nature. Unlike acute health risks, the effects of immunotoxic chemicals often develop gradually, making them harder to detect early. Over time, these subtle compounding effects can lead to significant immune dysfunction. For instance, exposure to PFAS – compounds that are common in industrial discharges and detected in drinking water – have been linked to reduced vaccine effectiveness, lowered resistance to infections, and a higher risk of certain cancer types (EFSA, 2020). Similarly, bisphenol A (BPA) – widely used in plastics – is associated with endocrine disruption and an increased risk of autoimmune diseases (Chen et al., 2018).
Immunotoxicants pose an especially severe risk to vulnerable populations, including children, pregnant women, and individuals with weakened immune systems, particularly during critical developmental windows when the immune system is more vulnerable. These critical windows are moments when the immune system is developing specific cells or organs and establishing immune repertoires (T-cells and antibodies). Given the immune system’s vital role in maintaining overall health, immunotoxicity represents a significant public health concern. Therefore, identifying immunotoxic substances, understanding their long-term effects, and preventing their presence at harmful concentrations in drinking water and in other sources of exposure is critical to the protection of public health.
Immunotoxicity testing of contaminants
Chemical contaminants from industrial, agricultural and domestic sources are commonly present in drinking water sources. Monitoring these contaminants is essential to ensure the quality of water intended for human consumption.
Although many contaminants are regulated and anticipated by drinking water companies, others remain undetected, unquantified and toxicologically uncharacterised. This is particularly concerning for (potentially) immunotoxic contaminants, as immunotoxicity is not yet systematically considered when deriving health-based limits for chemical compounds, because of limited regulatory requirements.
While chemicals such as PFAS and BPA are increasingly recognised for their harmful effects on the immune system, many other substances are either inadequately studied during the authorisation phase or entirely overlooked in the context of water safety.
In the European Union (EU), the REACH (Registration, Evaluation, Authorisation, and Restriction of Chemicals) mandates comprehensive safety assessments of chemicals. However, immunotoxicity testing is not routinely required. Immunotoxicity studies under REACH are only conducted when concern-driven scientific triggers arise, meaning potential immunotoxic effects may go unassessed.
Currently, water quality health limits are primarily based on toxicological risk assessments, considering endpoints such as carcinogenicity, reproductive toxicity, and organ-specific damage. However, there is still an unmet need for guidelines that also address immunotoxicity and the toxicity of other sensitive organ systems, such as the brain (neurotoxicity) and the endocrine system.
While standardised testing methods exist to assess the immunotoxic properties of individual chemicals for regulatory approval, standardised methods that can be incorporated into water quality assessments are lacking. A key difficulty lies in detecting low-level chemical mixtures in water, where multiple contaminants may interact in unpredictable ways.
There is a significant gap in understanding how these mixtures might affect the immune system compared with individual substances. A major complication is determining whether changes in immune system components, such as specific cells or proteins, actually indicate harm to immune function. This challenge applies to both individual substances and mixtures of contaminants, as well as variations in factors such as age and gender, with different methodologies potentially further complicating the process.
In addition, the immune system has built-in backup mechanisms that can compensate for damage, potentially masking the effects of immunotoxicity. This makes it difficult to establish clear, standardised guidelines for identifying and interpreting immunotoxic effects, as the immune system may adapt or compensate in ways that obscure the true extent of the damage.
Assessing risk
There is a tendency to assume that health effects are unlikely to occur at the low concentrations typically found in drinking water. But this perspective overlooks the potential long-term risks associated with low-level, chronic exposure to contaminants. Even at low concentrations, chemicals in drinking water, such as disinfection by-products or environmental contaminants, can accumulate in the body over time, potentially weakening the immune system and increasing vulnerability to infections or diseases. To address these gaps, there is a pressing need for water quality monitoring and risk assessment approaches that include immunotoxicity as an endpoint.
Emerging approaches
One promising approach to immunotoxicity testing is the evaluation of adverse outcome pathways (AOPs). AOPs are a framework for understanding how chemicals interact with biological systems, potentially leading to adverse health outcomes such as diseases (Nymark et al., 2021). AOPs map the sequence from a chemical’s initial interaction with the body, referred to as a molecular initiating event (MIE), to its final adverse impact on health, the adverse outcome (AO), through several intermediate key events (KEs). A single MIE can trigger a cascade of downstream KEs, which can diverge and lead to various toxicological outcomes (Spinu et al., 2019). Alternatively, multiple MIEs can converge into a single adverse outcome.
In the context of drinking water, prolonged low exposures to contaminants can lead to MIE, which may contribute to KEs, leading ultimately to AOs. For example, drinking water containing organohalogen disinfection by-products (DBPs), such as chloroform, trichloroacetic acid, and trichlorophenol, has been linked to mitochondrial toxicity (McMinn et al., 2019). The key event in this case is the excessive production of free radicals (reactive oxygen species), which can overwhelm the body’s antioxidant defences, leading to oxidative stress and associated cellular damage.
Despite this growing understanding of how contaminants trigger these molecular mechanisms, the application of AOP frameworks to immunotoxicity is still limited. New approach methodologies (NAMs), which include non-animal testing methods, such as in vitro bioassays and computational models, can play a critical role in bridging these gaps by providing the tools to assess key events within AOPs.
Operational approaches
AOPs may seem very technical and difficult to integrate into the daily operations of water quality managers. However, gaining a basic understanding of key concepts such as MIEs and KEs, which trigger adverse effects such as immunotoxicity, can be highly useful. This knowledge can help inform risk management decisions and assumptions, guiding more effective strategies for managing water quality across various environments, including drinking water, surface water, groundwater and wastewater.
Effect-based monitoring (EBM), for example, has gained recognition as a valuable approach for evaluating drinking water quality, complementary to chemical analytical approaches.
EBM refers to a set of bioanalytical tools (bioassays) that assess water quality by capturing the combined effects of the complex low-level mixture of known and unknown chemicals present in water, if they are active in the applied bioassays. This approach is particularly important given the complex mixtures of chemical contaminants found in water bodies, which traditional targeted chemical analyses may not be able to capture adequately.
Knowledge of AOPs can aid in identifying the most relevant effect-based method to detect immunotoxic or other effects of low-level chemical mixtures in water. In addition, it can support the establishment of effect-based trigger values (EBTs), which are used as benchmarks to assess potential health risks and guide regulatory decisions to ensure drinking water is safe. This enables water companies to implement more focused and efficient monitoring strategies, especially when time, budget or resource constraints are present. Prioritising bioassays based on AOPs may ensure that the most adequate bioassays provide relevant information based on the most critical indicators.
Integrating immunotoxicity into water quality monitoring
To address the limitations of conventional effect-based monitoring techniques in detecting the specific immunotoxic effects of complex mixtures of legacy and emerging contaminants, there is a pressing need to make use of immunotoxicological information of individual substances – and relevant mixtures – and consider integrating immunotoxicity testing methods into the routine evaluation of drinking water sources. The following recommendations outline a clear path forward:
Implementation of a tiered approach to testing, starting with broad screening bioassays and moving to more detailed studies on high-risk contaminants. This will help prioritise which chemicals to focus on, based on their potential to affect immune health.
Establish EBTs for chemical mixtures with immune effects. EBTs are the thresholds that indicate whether a chemical concentration requires further investigation. This will enable quicker decision-making when assessing water safety using effect-based methods.
Prioritise substances not routinely tested for immunotoxicity, from sources such as chemical industries, pharmaceuticals and microplastics, based on factors such as environmental persistence, potential for human exposure, and possible health risks.
Develop scientifically validated testing protocols for immunotoxicity aligned with both next generation risk assessment (NGRA) and water quality monitoring, to ensure that practices reflect the latest advancements in immunotoxicological science.
Further research to develop standardised immunotoxicity bioassays for drinking water.
Conclusion
Immunotoxicity is an essential, but overlooked aspect of drinking water safety and chemical safety in general. Chemicals that disrupt the immune system may not show immediate effects, but their long-term impacts can be adverse, especially for vulnerable populations.
The lack of standardised methods for detecting immunotoxicity in water emphasises a significant gap in current water quality practice, which leaves the public’s health at potential risk from contaminants via this route. While it is not yet definitively established whether immunotoxic effects from drinking water are likely or widespread, certain populations may be more susceptible to potential risks. With emerging contaminants posing new challenges, it is crucial that water utilities continue to take proactive measures to assess and mitigate risks, including those resulting from exposure to immunotoxic contaminants. Collaboration between scientific researchers and water utilities is crucial for conducting research that addresses knowledge gaps about the immunotoxic potential of emerging water contaminants.
Acknowledgement
The research presented in this article was funded by the Waterwijs collective research programme of Dutch water companies, Flemish water company De Watergroep, and the Association of Drinking Water Companies, the Netherlands (Vewin).
The authors: Sanah Majid is a scientific researcher and toxicologist, Daniel Duarte is a scientific researcher and project leader, and Tessa Pronk is a scientific researcher, all at the KWR Water Research Institute;
Corine Houtman is a toxicologist at Het Waterlaboratorium and VU University;
Insam Al Saify is a toxicologist at Waternet;
Merijn Schriks is a specialist drinking water quality toxicologist at Vitens;
Janine Ezendam is Head of the Department of Innovative Testing Strategies at the National Institute for Public Health and the Environment;
Raymond Pieters is Associate Professor at Utrecht University and full Professor at Utrecht University of Applied Sciences;
Milou Dingemans is Chief Science Officer and Principal Toxicologist at KWR Water Research Institute and guest researcher at the Institute for Risk Assessment Sciences, Utrecht University;
All are based in the Netherlands
