Heavy metals pose a significant threat to human health in the wake of rapid economic and agricultural growth since the beginning of the twenty-first century. This development has led to climate change and environmental pollution, exposing humanity to the dangers of heavy metals [39], [7]. Environmental contamination by heavy metals is a severe problem for human health [53].

Many metal(loid)s are substances with a greater density than water, such as lead (Pb), arsenic (As), mercury (Hg), zinc (Zn), nickel (Ni), chromium (Cr), and cadmium (Cd), Their density is correlated with their toxicity, and they can be poisonous even at low concentrations [23]. Heavy metals have been accumulating in the soil due to mining, irrigation of crops with wastewater, leaching from landfills, application of organic and inorganic fertilizers, and wastes [36]. The release of heavy metals into the air, water, and soil is a consequence of these activities, threatening ecosystems, and the health of exposed populations [26], [52].

The body absorbs these metals through various pathways, including the cutaneous, respiratory, and gastrointestinal tracts [16]. Heavy metals can also react with specific substances, such as oxygen and chloride, to contaminate the body [16]. Excessive heavy metal dosage in the body could result in cellular structural alterations directly by affecting organic particles or indirectly by generating reactive oxygen species, causing a wide range of diseases, including malignancies, nervous diseases, endocrine disorders, and impotence [18].

The human biomonitoring (HBM) technique is utilized for analyzing and monitoring cumulative chemical exposures in the public by detecting specific metabolites, mainly in the urine [3], [55]. HBM is widely used in environmental epidemiology investigations to determine the link between chemical exposure and health effects [42].

By utilizing statistics obtained from a suitable sample of the population, the Reference Value (RV) can be determined as a high-margin estimate of the baseline exposure to an environmental chemical compound in human biological fluids, such as blood or urine, over a specific timeframe for an individual or a group of individuals [32], [42]. This method proves valuable in assessing a range of chemicals within the human body [41]. However, the comprehensive biomonitoring of these metal(loids) in Iran has not been conducted on a national scale, and currently, there is no Reference Value (RV) established for biomonitoring within the healthy adult population in Iran.

The objective of this study, as the first national HBM survey, was to investigate the levels of Al, As, Hg, Pb, Cr, Cd, and Ni, their geographical variations, and determine the reference using urine samples of healthy Iranian adults.