Heavy Metals

Overview

Heavy Metals are a group of metallic elements characterized by relatively high atomic weight and density, generally above 5 g/cm³, that can exert toxic effects on biological systems at low concentrations. The term is not strictly defined in chemistry but is widely used in toxicology, environmental health, and regulatory science to describe metals and metalloids of significant health concern. These include both elements essential in trace amounts, such as zinc, copper, and iron, as well as non-essential and highly toxic elements such as lead, cadmium, mercury, and arsenic.

In the context of human and environmental health, the designation “heavy metals” most commonly refers to a subset of toxicologically significant elements: lead (Pb), cadmium (Cd), arsenic (As), mercury (Hg), nickel (Ni), tin (Sn), aluminum (Al), and chromium (Cr, particularly hexavalent Cr VI). These metals are of primary concern because they persist in the environment, bioaccumulate in plants and animals, and interfere with critical cellular functions when absorbed by humans. Unlike organic contaminants, heavy metals are not degradable and thus remain in ecosystems indefinitely, cycling through soil, water, air, and food chains.

The toxicological behavior of heavy metals stems from their chemical reactivity and ability to bind to proteins, enzymes, and nucleic acids. They often substitute for essential metals in biological systems, a process termed mismetallation, which disrupts enzymatic activity, mitochondrial function, and cellular signaling. Many also generate reactive oxygen species, leading to oxidative stress, DNA damage, and chronic inflammation. For example, lead interferes with calcium-mediated processes and neurodevelopment; cadmium disrupts kidney function and bone metabolism; arsenic impairs DNA repair and is a known carcinogen; and mercury binds to thiol groups, damaging neural tissue. Even aluminum, historically considered less harmful, has been implicated in neurotoxicity and is now under closer scrutiny.

Exposure routes include contaminated food, water, and air, as well as occupational and consumer product pathways. Dietary intake is a major source, with staples such as rice, cereals, seafood, and leafy vegetables often carrying elevated levels due to contaminated soil or irrigation water. Industrial activities such as mining, smelting, and fossil fuel combustion release heavy metals into the environment, while consumer goods ranging from cosmetics to cookware may contribute additional exposures.

From a regulatory standpoint, heavy metals are strictly monitored because of their persistence, bioaccumulation, and cumulative health risks. Agencies such as the FDA, EFSA, WHO, and Codex Alimentarius set maximum allowable limits for these metals in food and water, while occupational standards aim to protect workers from hazardous exposures. However, permissible thresholds vary globally, and recent scientific evidence has driven calls for stricter limits, particularly for products consumed by infants and children, who are far more vulnerable to long-term neurological and developmental harm.

In summary, “heavy metals” refers to a group of high-density elements with toxicological significance that accumulate in ecosystems and human tissues, disrupting biological processes and contributing to chronic disease. The eight metals prioritized by the Heavy Metal Tested and Certified (HMTC) program—lead, cadmium, arsenic, mercury, nickel, tin, aluminum, and chromium—represent the most relevant threats to food safety, consumer product safety, and public health.

Research Feed

September 23, 2025

What was studied? This study investigated the effects of prenatal heavy metal exposure and infant neurodevelopment, considering the adverse effects of multiple heavy metals—cadmium (Cd), nickel (Ni), mercury (Hg), and lead (Pb). Heavy metal levels were measured in maternal urine samples collected at the 12th week of gestation, while infant neurodevelopment was assessed at 40 days using the Bayley Scales of Infant and Toddler Development. The study applied multiple statistical approaches, including Generalized Additive Models (GAM), Multivariable Linear Regression (MLR) with restricted cubic splines (RCS), Bayesian Kernel Machine Regression (BKMR), and Weighted Quantile Sum (WQS) regression, to evaluate both individual and joint effects of these metals on early neurodevelopment. Who was studied? The study examined 400 mother-infant pairs recruited from a community-based birth cohort in Tarragona, Spain, between 2013 and 2017. Mothers were recruited during their initial prenatal visits, and urine samples were analyzed for metal concentrations using ICP-MS/MS with creatinine adjustment. Infants were assessed at 40 days old by trained psychologists, focusing on cognitive, language, and motor domains. The mothers had a mean age of 30.9 years, with most belonging to a low- or middle-socioeconomic class, and nearly 70% reported never smoking. Infants were almost evenly split between male and female, with 74.5% breastfed. Most important findings Cadmium was consistently associated with adverse neurodevelopmental outcomes. GAM and MLR analyses confirmed a negative linear association between Cd exposure and both cognitive and expressive language scores (β = −1.47 and β = −0.32, respectively, both statistically significant). Pb demonstrated a non-linear, inverted U-shaped relationship with language development, indicating risk at both low and high exposure levels. WQS regression revealed that mixtures of heavy metals were significantly associated with impaired expressive language development (β = −0.26, 95% CI = −0.44, −0.07), with Cd and Ni identified as the main contributors. BKMR analyses supported an overall negative trend for metal mixtures, though not statistically significant. Mercury exposure showed no consistent associations. Key implications The study highlights that prenatal heavy metal exposure and infant neurodevelopment are particularly negatively impacted by cadmium and nickel exposure, with expressive language being the most vulnerable domain. The findings underscore the limitations of focusing on single-metal exposures, as real-world scenarios typically involve complex mixtures. Importantly for a certification program such as Heavy Metal Tested and Certified (HMTC), the evidence supports the inclusion of cadmium and nickel within the Infant and Child Foods Standards alongside lead and mercury as priority metals for regulatory thresholds, given their demonstrable neurodevelopmental risks even at low levels of prenatal exposure. These results emphasize the urgency of establishing stricter heavy metal limits in foods consumed by pregnant women, since dietary intake is a major source of exposure. For industry, compliance with reduced heavy metal thresholds is not only protective of infant health but also scientifically justified by evidence linking prenatal exposure to cognitive and language deficits in early life. For regulators, the study validates the need for mixture-based risk assessment approaches, moving beyond single-metal evaluations to capture the cumulative effects on vulnerable populations. Citation Kou X, Palleja-Millan M, Canals J, Rivera Moreno V, Renzetti S, Arija V. Effects of prenatal exposure to multiple heavy metals on infant neurodevelopment: A multi-statistical approach. Environmental Pollution. 2025;367:125647. doi:10.1016/j.envpol.2025.125647.