HBM4EU is a joint project funded under the Horizon 2020 programme designed to advance and harmonise Human Biomonitoring (HBM) in Europe (HBM4EU, 2017–2022). A main achievement is the so-called “HBM4EU Aligned Studies”. A sampling frame has been developed and later been implemented by aligning existing national and regional HBM studies to meet a common goal, that is the assessment of human biomonitoring data on environmental chemicals in a harmonised way (Gilles et al., 2021, 2022).

One of the first prioritised chemical substance group under HBM4EU were phthalates (Ougier et al., 2021). Phthalates are used as plasticisers to soften poly vinyl chloride (PVC) and are or used to be applied in a variety of consumer products, such as cosmetics, food packages, medicinal products, textiles, toys, and footwear (European Chemicals Agency (ECHA) and Danish Environmental Protection Agency, 2016; EFSA, 2019). Several phthalates have endocrine disrupting properties and are classified in the European Union as reproductive toxicants, category 1B (“May damage fertility and/or the unborn child”) under regulation (EC) No 1272/2008 on classification, labelling and packaging (CLP) of substances and mixtures. As a result, they are identified as substances of very high concern (SVHC) (ECHA, 2022) and are subject to various regulations in the European Union. Animal studies have revealed that exposure to certain phthalates affects fertility and reproduction of both sexes (US CPSC, 2014; European Chemicals Agency (ECHA) and Danish Environmental Protection Agency, 2016; NRC, 2008; Yost et al., 2019). Most susceptible for phthalate toxicity is the male offspring if exposed prenatally: in utero exposure to e.g. diethylhexyl phthalate (DEHP), diisobutyl phthalate (DiBP), di-n-butyl phthalate (DnBP) and butyl benzylphthalate (BBzP) during the critical window of sexual development induces various irreversible malformations and alterations of the reproductive tract of the male rat offspring, which are summarized under the term “phthalate syndrome” (Conley et al., 2021; European Chemicals Agency (ECHA) and Danish Environmental Protection Agency, 2016; ECHA, 2017a; EFSA, 2019; German HBM Commission, 2011; NRC, 2008; US CPSC, 2014). Observed effects include hypospadias, cryptorchidism, testicular and epididymal malformations, but also reduced sperm count and reduced anogenital distance (Gray et al., 2000; Gray and Butterworth, 1980; Hannas et al., 2011; Howdeshell et al., 2008a, 2015). The occurrence of the same effects after exposure to different phthalates observed in animal experiments led to the assumption that this group of substances may act via the same mechanism and thus might have cumulative effects. Phthalates suppress testosterone and insulin-like 3 hormone production, androgens crucial for male sexual development (Gray et al., 2000; Howdeshell et al., 2008a). More than 10 years ago, the National Academies of Science National Research Council Committee (NRC) recommended to assess the risk from exposure to reproductive phthalates together by using a dose-addition approach (NRC, 2008). Phthalate mixture studies in rodents confirmed the cumulative effects of phthalates already at low doses for the individual chemical (Conley et al., 2021; Furr et al., 2014; Hannas et al., 2011; Howdeshell et al., 2007, 2008b, 2015, 2017). It can be assumed that the assessment of cumulative mixture effects is relevant for humans exposed to several phthalates at the same time. Besides the concurrent exposure to reprotoxic phthalates, scientist have expressed their concerns about the vast number of endocrine disrupting chemicals the general population is exposed to (Howdeshell et al., 2017; Kortenkamp, 2007, 2008; Orton et al., 2014). It has been shown in animal studies that beyond the group of phthalates also other anti-androgenic substances that disrupt male reproductive tract development act in a dose-additive manner and thus contribute to the cumulative risk. This is even true if the individual substances act via different mechanisms of action to disrupt the androgen-mediated pathway or even via completely different pathways (Conley et al., 2018, 2021; Howdeshell et al., 2017; Rider et al., 2010). To evaluate the risk of possible health effects from exposure to reprotoxic phthalates in European children and adolescents, within HBM4EU, health-related human biomonitoring guidance values (HBM-GVs) for the general population (HBM-GVGenPop) were derived for five phthalates, namely DEHP, DnBP, DiBP, BBzP and di-(2-propylheptyl)phthalate (DPHP) (Lange et al., 2021). These values refer to the urinary concentration of the specific biomarker(s) of a phthalate at and below which, according to current knowledge, no risk of health impairment is anticipated. HBM-GVs can directly be compared with the urinary biomarker concentrations gathered in HBM studies (Apel et al., 2020b). Since the developing organism is most sensitive to the toxicological effects from phthalates, it is necessary to prevent the foetus from phthalate exposure, but also to protect children and adolescents as these are among the most vulnerable populations. Therefore, in the present study, a mixture risk assessment (MRA) was conducted by using the hazard index (HI) approach as straightforward approach making use of harmonised European human biomonitoring data and the HBM-GVs already derived in HBM4EU. Generically, the HI is the sum of risk quotients of the individual substances (RQi), which represent the exposure level divided by a toxic potency measure. For HI < 1, it is assumed that there is no concern for cumulative mixture effects (EFSA Scientific Committee et al., 2019; NRC, 2008; Teuschler and Hertzberg, 1995). The five phthalates, included in the MRA (DEHP, DnBP, DiBP, BBzP and DiNP), were selected based on their common reprotoxic properties and on their co-occurrence in the European subpopulations (Cullen et al., 2017; Hond et al., 2015; Husøy et al., 2019; Santé Publique France, 2019; Schoeters et al., 2017; Schwedler et al., 2020). HBM-GVGenPop were utilised to assess the cumulative risk posed by the five reprotoxic phthalates in HBM data of European children and adolescents from the HBM4EU Aligned Studies. We also apply and discuss the use of additional “precautionary factors” on the HI to account for concurrent exposures to other anti-androgenic phthalates and other substances not included in the phthalate MRA as previously suggested by Apel et al. (2020a) and Kortenkamp and Koch (2020).