Human Milk as a Biomonitor of Toxic Metal Exposure: Sources, Transfer Mechanisms, and Implications for Infant...

Human milk (HM) is recognized as the optimal source of nutrition for infants, providing essential nutrients, bioactive compounds, and immunological protection crucial for proper growth and development. However, due to increasing environmental pollution, HM may also serve as a vector for exposure to toxic substances, including heavy metals. These contaminants originate from both current environmental exposure and long-term accumulation in maternal tissues, which may be mobilized during pregnancy and lactation. Objectives: The aim of this review was to comprehensively analyze the occurrence, sources, and determinants of heavy and toxic metals in human milk, with particular emphasis on maternal-infant transfer pathways and geographical variability of exposure. Methods: A structured narrative review with systematic literature search elements was conducted using PubMed, Scopus, and Web of Science databases. The search covered studies published between 2010 and 2025 and was limited to articles written in English. The search strategy included terms related to human milk and heavy metal exposure (Pb, Cd, Hg, As, Cr, Al). Predefined inclusion and exclusion criteria were applied, and a qualitative synthesis of environmental, dietary, physiological, and lifestyle-related determinants, as well as geographical variability, was performed. Results: The available evidence indicates that heavy metals are commonly detected in human milk worldwide, with concentrations strongly influenced by environmental pollution, maternal diet, and lifestyle factors. Under typical exposure conditions, reported concentration ranges are approximately 2-5 µg/L for lead (Pb), 1.4-1.7 µg/L for mercury (Hg), and below 1 µg/L for cadmium (Cd). However, substantially higher levels have been reported in highly contaminated regions, with extreme values exceeding 1000 µg/L for Pb and 100 µg/L for Hg in isolated cases. Key exposure pathways include contaminated food, drinking water, air pollution, and endogenous mobilization of metals stored in maternal tissues (particularly bone and adipose tissue). Significant geographical variability was observed, with higher concentrations reported in industrialized and mining regions. Infants represent a highly vulnerable population due to immature detoxification systems, increased gastrointestinal absorption, and ongoing neurodevelopment, which may amplify toxic effects even at low exposure levels. Conclusions: Although human milk remains the gold standard for infant nutrition, the presence of heavy metals highlights the need for continuous environmental monitoring and preventive strategies aimed at reducing maternal exposure. The benefits of breastfeeding clearly outweigh the potential risks; however, minimizing environmental contamination remains a critical public health priority. Future research should focus on standardizing analytical methods, improving biomonitoring strategies, and better characterizing long-term health outcomes associated with early-life exposure to toxic metals.