The concentrations of polychlorinated naphthalenes (PCN) were determined in 40 pooled human milk samples from 39 countries covering all five of the United Nations regional groups. The samples were collected in the 2016–2019 exposure studies on persistent organic pollutants coordinated by the United Nations Environment Programme (UNEP).The median concentration of the sum of 26 PCN was 55 pg/g lipid (range 27 pg/g to 170 pg/g). Human milk from European countries showed considerably higher levels than those found in milk from countries in the African, Asia-Pacific, and Latin America/Caribbean regions. The most abundant congeners were the congener pairs PCN 52/60 and PCN 66/67 (inseparable by conventional chromatography) and to a lesser extent PCN 28/36, PCN 42, PCN 46, PCN 48, PCN 59, and PCN 69.Among other adverse biological effects, a critical response of many PCN congeners is dioxin-like toxicity. So, in addition to reporting concentrations of individual congeners, the toxic equivalents (TEQ) were also calculated in these samples, using two sets of relative effect potency (REP) values: a set that has been used in a number of human exposure studies and another set reported by Falandysz et al. (J Environ Sci Health, Part C: Environ Carcinogenesis Ecotoxicol Rev 32(3):239–272, 2014). The median PCN-TEQ concentration in human milk was 0.07 pg PCN-TEQ/g lipid (range 0.03 pg/g to 0.23 pg/g), when calculated using the human biomonitoring study REPs, and 0.03 pg PCN-TEQ/g lipid (range 0.01 pg/g to 0.10 pg/g), when calculated with other suggested REPs. The vast majority, about 90%, of this TEQ can be attributed to the PCN 66/67 congener pair. Individual REPs for PCN 66 and 67 from in vivo studies are quite different, but a chromatographic separation of these two congeners is not possible under routine GC conditions. Different approaches to estimate the uncertainties showed that the value of the REPs used is more important than the analytical problem to separate PCN 66 and PCN 67. PCN-TEQ based on the two sets of REPs differ approximately by a factor of 2.2, whereas the congener-specific determination was estimated to result in approximately 30% lower concentrations in comparison with the standard method.The assessment of PCN 66 and PCN 67 in order to obtain confirmed TEF would be most important for calculations of the dioxin-like toxicity of PCN, followed by PCN 69. Minor contributions to PCN-TEQ concentrations in human milk come from PCN 52/60, PCN 64/68, PCN 70, and PCN 73.On average, the contribution of PCN-TEQ to the cumulative TEQ (including the overall sum of toxic equivalents of PCDD, PCDF, and dioxin-like PCB [WHO2005-TEQ]) is between 1% and 2%, with a wider range of up to 5% for the 39 countries of this study. This is about an order of magnitude lower than the contribution of dioxin-like PCB to the cumulative TEQ (median 26%). In line with the observed higher total PCN concentrations, European countries also showed considerably higher levels of PCN-TEQ than found in the other regions. PCN-TEQ calculated with REPs used in human biomonitoring studies add on average about 2% to the cumulative TEQ of dioxin-like contaminants in Africa, the Asia-Pacific region, and Latin American and Caribbean countries and about 4% in European countries. The corresponding contribution of PCN-TEQ calculated using the other set would be 1% in non-European countries and 2% in European countries.
