Keywords

1 Introduction

As shown in Part I (Introduction) of this compendium, human milk has served over decades as a useful matrix to assess the exposure of the general population to persistent organic pollutants (POPs). The early harmonization and strict parametrization of the former World Health Organization (WHO) field studies allowed to establish a durable framework for monitoring on POPs since the mid-1980s and early 1990s (Fürst 2023). The Stockholm Convention used this framework to monitor POPs in human tissues after defining parameters and core matrices for its Global Monitoring Plan. The range of chemicals increased from the initial focus on polychlorinated biphenyls (PCB), polychlorinated dibenzo-p-dioxins (PCDD), and polychlorinated dibenzofurans (PCDF) in the first two WHO field surveys to 30 chemicals (28 chlorinated or brominated, 2 perfluorinated substances) in the latest, seventh (2016–2019) round of milk surveys (Malisch et al. 2023a). However, there are additional challenges ahead.

2 New Candidate Chemicals, New Requirements

Originally, the POPs were predominantly lipophilic (Fürst 2023), but in recent years the candidate chemicals cover hydrophilic and amphiphilic compounds, such as poly- and perfluorinated candidates that have variable physicochemical properties and require a very different set of techniques to detect these analytes in the samples.

As no multi-method exists allowing to determine all POPs of interest to the Stockholm Convention, various analytical methods have to be applied for the presently listed 30 chemicals as shown in Part II (Analytical methods and quality control) of this compendium. This is even more true for other candidate (POPs) chemicals in the pipeline—already listed was PFHxS in 2022, and the COP will consider listing of dechlorane plus, and sunscreen/UV filter UV-328 and then methoxychlor. Next in the pipeline are chlorpyrifos, chlorinated paraffins with carbon chain lengths in the range C14–17 and chlorination levels at or exceeding 45% chlorine by weight and long-chain perfluorocarboxylic acids, their salts and related compounds (BRS 2022).

Some data on those candidate chemicals in human milk are already available and give us a hint of the magnitude of the problem as well as techniques necessary to detect the new chemicals along the existing set of standard methods. A recent review by (Martín-Carrasco et al. 2023) on several pesticides includes concentrations of methoxychlor and chlorpyrifos in human milk and infant formula from different countries. UV filters, their levels and possible sources in China were studied by (Liu et al. 2022). They report concentrations of 12 UV filters in 100 pooled milk samples from China. The mean 2-(3,5-di-tert-amyl-2-hydroxyphenyl) benzotriazole (UV-328) concentration was 2.6 ± 2.6 ng/g lipid weight. Another survey by (Kim et al. 2019) shows results on UV stabilizers from three Asian countries and finally, (Lee et al. 2015) covers UV-328 levels in human milk collected from Korea with demographic information on the lactating women.

Furthermore, as discussed in the analytical chapter on methods for various POPs including CP (Schächtele et al. 2023), there are challenges in distinguishing between the already listed short chain CP (SCCP) and the candidate chemicals of medium chain CP (MCCP). Indeed, opportunities for using new techniques more broadly are quoted by (Chi et al. 2023) pointing at chemical mixtures and unknown chemicals of potential concern. So far, biomonitoring studies have relied mostly on target analysis. Non-target analysis (NTA) is seen as a tool to improve the characterization of the chemical exposome. HRMS or TOF-MS can identify known and unknown chemicals and are useful tools for non-target screening. “Omics” biomarkers permit the observation and measurement of response modulation at different biological scales. Finally, confirmatory methods allow the unequivocal identification and quantification of POPs present in a sample and provide full information on congener basis.

Last but not least, the expansion of analytes requires continuous amendments of the GMP guidance document (UNEP 2021) to encompass technological progress. This activity regularly takes place by GMP experts once the candidate chemicals complete the assessment procedure and the Conference of the Parties to the Stockholm Convention approves their listing. At that moment experts prepare an overview of available existing scientific knowledge including analytical techniques/methods to analyse newly listed chemicals in core matrices of the Stockholm Convention.

We believe, it may be worth considering another holistic review of the guidance very soon to define further approaches for the long term. Namely, to reconsider type and number of core matrices, corresponding most efficient analytical methods and new techniques for baseline screening and detailed chemical analyses.

3 New Approaches

Building on the two rounds of field studies in human milk coordinated by WHO in the mid-1980s and 1990s on exposure to PCB and PCDD/PCDF, five expanded studies on POPs were performed between 2000 and 2019. To date, the seven rounds of WHO- and/or UNEP-coordinated human milk exposure studies on POPs are the largest global surveys on human tissues with a harmonized protocol spanning over the longest time period and carried out in a uniform format. As shown in Parts III and IV of this compendium, the surveys yielded a comprehensive set of global data covering all targeted POPs listed under the Stockholm Convention in all five UN regions over up to three decades. Therefore, assessments are possible from various perspectives. For the human milk samples of the 2016–2019 period, results for the full set of 32 POPs of interest for the Convention until 2019 (30 listed, 2 proposed for listing as of 2019) are available providing the basis for discussion of the relative importance (“ranking”) of the quantitative occurrence of POPs—a unique characteristic among the core matrices under the GMP (Malisch et al. 2023b).

Expanding the list of chemicals of concern (HBM4EU 2016; HBM4EU 2022; WHO 2015) also brings additional regulatory responsibilities/needs. Fast interventions and promotion of different products, and exposure to mixtures, raise awareness and concerns in public and policy-makers (UNEP 2019). Regulatory decisions on chemicals require more scientific information, including on exposure, as a priority (WHO 2021; EU 2020).

In addition, there have been also other parallel developments related to a broader use of human biomonitoring as a tool contributing to better protection of human health and the environment from negative effects of toxic compounds. The EU has initiated several projects that examined the human biomonitoring (HBM), such as COPHES/DEMOCOPHES in 2009–2012. There are many human biomonitoring surveys and projects in many countries around the world and their number is increasing (Choi et al. 2015). Just in Europe, a total of 192 HBM surveys were reported from 29 European countries in 2017, but not all surveys/projects are using human milk as a matrix (HBM4EU 2017). More recently, the European Initiative for Human Biomonitoring (HBM4EU) in 2016–2021 has aimed at fully harmonizing the accumulated experience (HBM4EU 2022). Last but not least, the EU also declares the importance of human biomonitoring in the EU Chemical Strategy for Sustainability (EU 2020). The same document also emphasizes risk assessment and needs to have a rapid transfer of progress in science directly to decision-making. For implementation of the EU Green Deal and EU Chemicals Strategy, the EU has established a science-to-policy partnership on assessment of risks of chemicals with the aim to develop and endorse new methods for new generation risk assessment (EU 2023).

Further, use of exposure assessments in decision-making gained more importance in recent years, as demonstrated by strengthening regional policy actions, and harmonizing the human biomonitoring surveys has been a chemical safety priority in the Parma and Ostrava Declarations endorsed by Ministers of Health and Environment at the European continent (WHO 2010; WHO 2017). Other examples could be the European Human Exposome Network (EHEN) of nine research projects that addresses health impacts in relation to exposures to air quality, noise, chemicals, and urbanization (EHEN 2022).

What can be the future for such projects and how to define their sustainability? One example of the way forward could be a roadmap for the European Strategy Forum on Research Infrastructures (ESFRI) to develop a pan European network addressing a gap in knowledge on linking environment factors affecting human health—an exposome research (ESFRI 2021). The Environmental Exposure Assessment Research Infrastructure (EIRENE RI) is a network aiming at sustainable research infrastructure enabling the advancement of exposome research in Europe by bringing together complementary capacities available in the member states, harmonizing them and upgrading to address current scientific and societal challenges in the areas of chemical exposures and population health, be it on data mining or further developing monitoring tools and approaches (EIRENE 2022).

4 Needs for the Future

Our ability to assess exposure in the future and derive the effectiveness of adopted measures depends on the availability of comprehensive datasets. We need to adapt to new methods and opportunities, but we also need to maintain the ability to understand and encompass well-established methods—e.g. maintain and regularly repeat the WHO/UNEP human milk survey on POPs, adapting to new POPs of interest that are of potential public health concern, as it has been done over the past decades.

In our view, we see an irreplaceable role for UNEP and WHO coordinated studies performed every four or five years using standard protocols and reliable analytical methods to generate reliable, harmonized, and validated datasets which allow to detect changes in concentrations of target chemicals over time in this core matrix of the Stockholm Convention for evaluation of the effectiveness of policy and management measures.

This includes the need to promote the repeated participation of countries that had already participated in the milk studies as it broadens the dataset to evaluate temporal trends. Furthermore, to increase the number of representatives, other countries should be encouraged to close gaps in the regional/global coverage.

Finally, there is also a need to continue the development of modern technology with (computer based) sustainable tools to support data mining efforts, such as science-policy research partnership PARC activities on data or the development of networks for exposome research such as the EIRENE infrastructure, which would provide open access to interested researchers, but also to other stakeholder communities.