Abstract
Objective: Heavy metals and metalloids are recognized as environmental neurotoxins capable of altering neuronal signaling, neurodevelopment, and emotional regulation. Growing evidence indicates their involvement in a range of psychiatric and cognitive disorders; however, the strength, consistency, and quality of these findings remain uncertain. This systematic review synthesizes the existing human-based evidence regarding the associations between exposure to heavy metals and metalloids and mental health outcomes.
Methods: A comprehensive literature search was conducted in PubMed and Scopus databases for studies published between January 2013 and March 2023, following the PRISMA 2020 guidelines. Search terms encompassed key metals and metalloids, including lead, mercury, cadmium, arsenic, manganese, iron, copper, aluminium, boron, and silicon, in combination with psychiatric and cognitive outcomes. After the removal of duplicates, 553 unique records were screened, and 45 studies met all inclusion criteria for qualitative synthesis.
Results: The findings revealed substantial variability in the quality and strength of evidence across elements. Lead exhibited the most consistent associations with depression, cognitive impairment, and psychomotor slowing, supported by multiple meta-analyses and human cohort studies. Mercury was moderately associated with anxiety and mood dysregulation, while arsenic exposure correlated with psychosis-related and memory impairments. Cadmium demonstrated limited human evidence, with animal data suggesting anxiety-like and neurodevelopmental effects. Manganese and iron were linked to Parkinsonian and cognitive symptoms in occupational or low-quality studies.
Conclusion: The overall evidence highlights distinct neuropsychiatric profiles and varying levels of methodological rigor across metals. These findings emphasize the need for well-designed longitudinal studies to clarify causal mechanisms and inform preventive and clinical strategies addressing heavy metal neurotoxicity in mental health.

Key words: Heavy Metals, Metalloids, Environmental Exposure, Neurotoxicity, Mental Health