Heavy metals are a long-lasting environmental contaminant that can pose serious public health risks if not contained, because they can bioaccumulate in living organisms and persist in the environment. They occur naturally but human behavior, e.g., industrial and agricultural practices and the urban lifestyle, has greatly increased their abundance in the environment. While some heavy metals are essential for biological processes in trace amounts, many are toxic even at low exposure levels, affecting multiple organ systems and potentially causing long-term health consequences.

What is a heavy metal?

Heavy metal is a broad term that describes a group of naturally occurring metallic elements of relatively high atomic weights (in the range of 63.5–200.6 g mol⁻¹) and density (>5 g/cm³) compared to water. Some heavy metals, such as iron, zinc, copper, and manganese, are essential for human, plant and microbial life at low concentrations, but the same metals can cause toxicity at higher concentrations. While other heavy metals, such as arsenic, cadmium, lead, thallium, and mercury, serve no biological function, they can unfortunately enter the human body due to their presence in the environment. Similarly to essential metals, they induce toxicity once specific concentrations are reached (1).

No universal classification system exists to define and differentiate heavy metals, but they do share several properties as a group, for example:

How heavy metals behave in the environment

• Heavy metals are non-biodegradable and persist long-term in the environment.
• They bioaccumulate in organisms and persist in food chains.
• Collectively, they can be transported via air, water, and soil, albeit with varying mobility.
• Certain heavy metals readily form complex compounds with organic matter. Some of these complexes are intentionally used by humans or have important physiological functions, while others pose a risk to human and animal health. For example, iron (Fe) forms a complex with the chelating agent ethylenediaminetetraacetic acid to yield Fe-EDTA, which is widely used in agriculture and industry as an iron supplement. On the other hand, lead in contaminated soil can form a complex with citric acid secreted by plant roots and microorganisms to form lead-citrate. This complex is easily taken up by plant roots, where it can enter the food chain and eventually the human body, leading to the toxicities associated with other forms of lead, such as neurological damage, developmental problems, kidney damage, cardiovascular issues and cognitive impairment.

Speciation, toxicity and bioavailability

• Most heavy metals undergo redox cycling between different oxidation states. Reactive oxygen species (ROS) are generated during the process, which can affect the biophysical properties and toxicity of the metal by impacting how it interacts with biological materials. For example, copper (Cu) can cycle between Cu+ and Cu2+, producing hydroxyl radicals which are highly reactive and can damage almost all types of macromolecules including carbohydrates, nucleic acids (via mutagenesis), lipids (via peroxidation) and proteins (by misfolding).
• Besides the example above, redox cycling can yield a range of other ROS and lead to oxidation states that can impact heavy metal toxicity by influencing any of the following: how easily the metal can cross cell membranes, how the metals are metabolized and where they are likely to accumulate in the body, the binding behavior of proteins including enzymes, the ability of the metal to cross the blood-brain barrier (highly relevant for mercury (Hg) when it exists as methylmercury (CH3Hg+), and others.
• Some heavy metals are amenable to biotransformation into alternative chemical or oxidative states through the enzymatic action of certain microbes. While this doesn’t eliminate the heavy metal, it can influence toxicity, bioavailability and persistence in a positive or negative manner. Many efforts have been undertaken to exploit microorganisms for biotransformation in a process known as bioremediation. This is a process that has been extensively studied for environmental cleanup applications, though its effectiveness varies depending on the specific metal and environmental conditions (see Ref. 2 for a review on this topic).

Understanding these chemical and environmental properties helps explain why heavy metals pose such significant public health challenges. Their persistence, bioaccumulation, and complex interactions with biological systems make them particularly concerning for human health, especially in vulnerable populations.

Lead and other heavy metals: A major public health concern

According to sources cited by the National Organization for Rare Disorders (NORD), 8,884 single exposures to heavy metals were reported in the United States in 2021. Of those exposures, 2,787 were reported in in children younger than the age of 6 and 4,014 were reported in individuals older than 19 years.

Of all the known heavy metals, lead, mercury, cadmium, arsenic are among the most important with respect to human and animal health since they are not needed in the body and are toxic at very low concentrations. Children are most susceptible to heavy metal poisoning because they absorb metals more readily that adults. For example, children in can absorb up to 50 % of ingested lead compared to 10% in adults (3).

Lead is the most common toxic metal in the United States, and a major concern for human health with widespread use that dates back to several thousand years BC. In the last century alone, lead was heavily used in paint production, as a gasoline additive, as a solder in electronics and plumbing (this use is largely phased out) and is still widely used in the production of car batteries and radiation shields today. It is estimated that children in approximately 4 million households are exposed to lead at home in the United States (3).

Among the most dangerous properties of lead are its major deleterious impact on fetal development, its ability to readily traverse the blood-brain barrier and placenta, bioaccumulation in bone where it can compete with calcium, leading to weakening of the bones and forms a reservoir in the bone that can be released back into the bloodstream. There is no concentration at which lead is safe in the blood. Lead poisoning causes neurological damage, particularly in children, affecting cognitive development and behavior.

In addition to the public health consequences of heavy metal exposure, the economic impact is tremendous. This includes loss of productivity in the workforce, the cost of finding suitable, safer alternatives to heavy metals in industry, the direct healthcare costs to treat individuals affected by heavy metals as well as the cost of mitigating and cleaning up areas affected by contamination.

Sources and routes of heavy metal exposure

The main source of heavy metal poisoning is occupational, i.e., exposure in workplaces where heavy metals are present, e.g., within metallurgy (the study of metals). Other sources include inappropriate consumption of food supplements which can lead to overexposure over time, certain traditional medical practices (e.g., Ayurveda) that involve the use of plants, animals and metals, and direct contact with industrial waste or contaminated water and food sources, in particular shellfish which is prone to bioaccumulation of methylmercury through pollution runoff.

Although lead is no longer used in paint production following a ban on residential use in the United States in 1978, houses built before this time are likely to have some lead-based paint. As the paint peels and cracks over time, it releases paint chips and dust, which can easily accumulate in windows, doors, floors, porches, stairways, and cabinets. Another major source of lead exposure is lead leeching from pipes. Other sources of heavy metal exposure include firing ranges, battery manufacturing, dental fillings in countries where the mercury alloy amalgam is still allowed, coal combustion and exhaust emissions and various other industrial practices.

Health impacts and symptoms of heavy metal toxicity

Given the numerous exposure pathways, it’s important to understand the wide-ranging health effects that heavy metals can have on the human body. Beyond the toxic effects of lead summarized above, heavy metal exposure in concentrations above a safe threshold (which differs for every heavy metal) can cause the following:

• Nervous system damage: Lead and mercury can cause permanent brain damage, cognitive decline, memory problems, behavioral changes, tremors, and in severe cases, seizures or coma.
• Kidney and liver dysfunction: Heavy metal accumulation can result in chronic kidney disease, reduced filtration capacity, liver cirrhosis, and potential organ failure.
• Cardiovascular effects: Exposure to lead, cadmium, arsenic, and mercury causes high blood pressure, damaged blood vessels, irregular heart rhythms and increases the risk of life-long heart disease.
• Reproductive and developmental issues: Lead, cadmium, mercury and others have been linked with birth defects, miscarriages, reduced fertility, and developmental delays in children, particularly when exposure occurs during pregnancy.
• Cancer: Arsenic, cadmium, chromium, lead, mercury and nickel are known carcinogens and can increase the risks of developing lung, skin, bladder, and kidney cancers.
• Immune dysregulation: Exposure to lead and mercury in particular can have an immunosuppressive effect and trigger auto-immunity.

Stay tuned for our next article to learn about when and how to test for heavy metal exposure!

References

1. Fisher RM, Gupta V. Heavy Metals. 2024 Feb 27. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan–.
2. Pande V, Pandey SC, Sati D, Bhatt P, Samant M. Microbial Interventions in Bioremediation of Heavy Metal Contaminants in Agroecosystem. Front Microbiol. 2022 May 6;13:824084.
3. Heavy Metal Poisoning. National Organization for Rare Disorders. Accessed 11 December 2024.