2025.11.25
Heavy Metals
Heavy metals refer to metals (such as lead) or metalloids (such as arsenic) that, when compared at equal amounts, are heavier than other metals. They naturally exist in small quantities in the Earth’s crust and cannot be destroyed or eliminated. The increase in industrial activities in recent decades has caused higher contamination of water, soil, and air with heavy metals, resulting in the contamination of agricultural products. In addition, processed foods may also become contaminated with heavy metals during production and processing.
Some heavy metals, such as iron, manganese, copper, cobalt, and zinc, are harmless to the body in small amounts but toxic at high levels. However, others such as mercury, lead, and cadmium are toxic and dangerous even at low concentrations.
Humans are exposed to heavy metals through the consumption of contaminated food and water, inhalation, and dermal absorption. Since the rate of absorption and accumulation of heavy metals is greater than their excretion, they gradually accumulate in organs such as the liver, kidneys, bones, and brain, and can cause harmful effects, including impaired kidney function, headaches, nausea and migraines, nervous system disorders, Alzheimer’s disease, skin lesions, osteoporosis, gastrointestinal inflammation, autism, and cancer.
Entry of Heavy Metals into Food
- Entry Through Soil
In addition to natural factors such as naturally occurring heavy metals in the Earth’s crust, volcanic eruptions, and forest fires, human activities such as agricultural and industrial operations can increase heavy metal concentrations in soil.
Agricultural activities and the use of fertilizers, pesticides, and irrigation with wastewater are sources of heavy metal entry into soil. For example, phosphate fertilizers contain the heavy metal cadmium because this metal naturally exists as an impurity in phosphate rock. Agricultural pesticides, such as the fungicide “Anthracol,” contain heavy metals including zinc, cadmium, and lead; or herbicides such as “Molinate or Ordram,” which contain heavy metals including zinc, cobalt, and cadmium. The use of such fertilizers and pesticides leads to the introduction of heavy metals into soil.
Industrial activities such as mining operations and ash from burning fossil fuels like coal can also introduce heavy metals into soil.
The growth of plants and crops in soil contaminated with heavy metals leads to absorption through the roots, transport via xylem vessels to different parts of the plant, and their accumulation in agricultural products.
- Entry Through Water
Heavy metals are naturally found in small amounts in water, but due to human activities, their concentrations have increased. Various water sources such as groundwater and municipal water may contain different levels of lead, arsenic, cadmium, mercury, chromium, and nickel.
Discharge of wastewater from agricultural lands and factory waste—such as from textile, dye, pharmaceutical, and ceramic industries—and acid rain can contaminate water, resulting in contamination of fish and aquatic organisms. Furthermore, using contaminated water for irrigation introduces heavy metals into fruits and vegetables.
The presence of heavy metals such as zinc and lead in soldered joints, galvanized pipes, copper pipes, and brass fittings can transfer heavy metals into drinking water. Using such water for washing raw ingredients or preparing baby formula can lead to the ingestion of heavy metals.
- Entry Through Air
Cadmium, lead, and mercury are common air pollutants primarily released due to industrial activities. Millions of tons of heavy metals enter the air every year, negatively impacting the environment and human health. Burning fossil fuels and fuel used in power plants (such as mazut and coal), along with industrial operations, release heavy metals into the air.
Heavy metals in the air can settle on parts of crops that grow above the soil, particularly on leaves. Additionally, heavy metals present in polluted air may contaminate water and soil through rainfall, ultimately entering the food supply.
- Entry During Food Production, Cooking, and Packaging
Since cookware is used daily and directly contacts food, if these containers contain heavy metals, they may transfer them into food.
In aluminum and ceramic pots especially at high temperatures, with extended cooking durations, and when acidic foods such as vinegar or vegetable oils are used heavy metals such as lead and cadmium may leach into food. Moreover, if colored ceramic mugs (large cups) are not coated with food-grade paints (paints that do not transfer harmful chemicals to foods), they may contaminate tea or coffee with heavy metals.
During production and processing in factories, the use of old or low-quality equipment can contaminate food with heavy metals. In general, high-quality stainless steel is suitable for machinery and equipment used in food production. Water used for washing or as an ingredient in food products is another source of contamination. It is also worth noting that heavy metals present in agricultural products are not removed by washing, processing, or cooking.
Heavy metals present in dust settling on utensils and surfaces that come into contact with food can also contaminate food.
Packaging made from recycled paper such as disposable cups and containers, pizza boxes, sandwich wrappers, and chemical inks used in printed packaging can transfer heavy metals into food. Heavy metals such as nickel, lead, and chromium from green inks; cadmium, zinc, and lead from white inks; copper and cobalt from blue inks; lead, cadmium, and copper from red inks; cadmium and zinc from yellow inks; and cadmium and lead from black inks may migrate into food.
Furthermore, the use of lead-based solder for sealing the outer seams of metal cans in canned foods can result in the transfer of heavy metals into food. A study on pregnant Mexican women found that more than 30% had high levels of lead in their bodies, where canned foods were the main source of exposure. Additionally, if tin coating is used in the production of cans, this heavy metal may leach into the contents and alter the taste of the food.
Foods Prone to Heavy Metal Contamination
- Seafood
Heavy metals such as mercury, lead, cadmium, and arsenic contaminate fish and seafood, with the highest concentration belonging to mercury.
Heavy metals enter the bodies of fish through contaminated water or through the food chain by consuming contaminated organisms. Generally, almost all fish contain mercury, which typically accumulates more in the internal organs than in the muscle (flesh) and skin. The accumulation of heavy metals is lower in smaller fish such as pomfret, trout, salmon, sole, and shrimp, but higher in larger, predatory fish that feed on smaller fish—such as tuna and swordfish, which are also used for canned fish.
- Rice
Rice contamination with heavy metals is important because rice is consumed daily, especially in Iran. According to the U.S. Food and Drug Administration (FDA), rice and rice-based foods contain more arsenic compared to other foods.
Sources of arsenic contamination in rice include:contaminated paddy soils, irrigation with wastewater-polluted water, the use of arsenic-based insecticides. Due to its structure and water-flooded cultivation method, rice absorbs heavy metals especially arsenic ten times more than other crops. Brown rice contains more arsenic than white rice because its outer bran layer, which contains the highest level of arsenic, remains intact. Reports also indicate cadmium accumulation in rice.
- Leafy Vegetables
Leafy vegetables such as parsley, lettuce, spinach, and watercress are more likely to be contaminated due to absorbing heavy metals from air and their greater proximity to soil and water. These vegetables are more commonly contaminated with cadmium, chromium, and lead.
The accumulation of heavy metals varies across different plant parts. For example, Cadmium, due to its high mobility, is absorbed through roots and transported to leaves, where it accumulates. It may also accumulate due to pesticide use. Lead has low mobility from roots to aerial plant parts. Thus, contamination in vegetables like lettuce often results from contact with soil particles containing lead and exposure to lead-contaminated air.
Younger, smaller plants usually contain lower heavy metal concentrations because exposure increases over time. Larger leaf surface area also increases heavy metal absorption, so broad-leaf vegetables are more contaminated than narrow-leaf plants.
Research shows that greenhouse vegetables often contain more heavy metals than field-grown vegetables due to excessive fertilizer and pesticide use and dense greenhouse cultivation.
Most Common Heavy Metals in Food and Their Risks
According to the FDA, the most common heavy metals in food are arsenic, lead, cadmium, and mercury.
Arsenic
- History
During the Middle Ages, arsenic was known as a substance used for murder and suicide. It was called the “poison of kings” during the Middle Ages and Renaissance. Arsenic was involved in several notorious poisonings the most famous being, according to some evidence, the death of Napoleon Bonaparte in 1851. Because adding arsenic to food or drink did not change its taste or smell, it remained a common poison until the mid-1800s.
- Sources
Arsenic naturally exists in the environment. Events such as volcanic eruptions can bring arsenic from underground to the surface. Industrial activities in paint production, wood preservatives, glass manufacturing, and paper production also introduce arsenic into the food chain. The use of arsenic-containing pesticides (such as lead arsenate, copper arsenate, etc.) can increase contamination.
According to the FDA, rice may contain more arsenic than other foods. Besides rice and its derivatives, seafood and edible mushrooms may also contain high levels of arsenic.
- Harmful Effects of Arsenic on the Human Body
According to the International Agency for Research on Cancer (IARC), arsenic exposure increases the risk of cancers of the prostate, liver, blood, and skin. Arsenic consumption is also associated with cardiovascular disease and diabetes.
During pregnancy, arsenic can cross the placenta, negatively affecting the fetus and potentially causing fetal death. The FDA prioritizes monitoring arsenic levels in children’s foods because children have lower body weight and consume food more frequently than adults.
Lead
- History
Lead toxicity was described and documented as early as 2000 BC. During the Roman era, gout (an inflammatory joint disease) was identified among the Roman upper class, and some believe excessive lead consumption was responsible. Lead was known as the “father of metals” and widely used in Roman life pipes, utensils, bullets, cosmetics (such as kohl), pigments, and even contraceptives. Over time, with rising mysterious illnesses, some Romans suspected a link between lead and disease.
- Sources
Lead is introduced into the environment naturally and through industrial activities. Lead may unintentionally enter foods through manufacturing processes. For example, according to the FDA, plumbing systems containing lead can contaminate water used in food production.
Industrial uses of lead include battery manufacturing, ammunition, solder, and printing inks. The primary heavy metal released from ceramic glazes is lead especially when exposed to acidic foods such as vinegar and fruit juices. Studies show that blue-carbon, turquoise, and aqua-colored glazes often release excessive lead.
High-risk occupations include auto repair workers, battery manufacturers, construction workers, glass workers, and painters.
The European Union reports that foods such as grains, vegetables especially leafy vegetables potatoes, and tap water are major contributors to dietary lead exposure.
- Harmful Effects of Lead on the Human Body
Lead is particularly harmful to vulnerable groups, including infants, children, pregnant women, fetuses, and people with chronic illnesses.
Children are often exposed through ingestion of contaminated soil, dust, or lead-based paints. Maternal exposure is dangerous because lead easily crosses the placental barrier and is found in breast milk. A blue line on the gums (Burton’s line) may indicate lead poisoning.
Lead absorption is 10% in adults and 40% in children. High levels of lead severely impact health and child development especially the brain and nervous system causing learning disabilities, behavioral issues, and reduced IQ. In adults, long-term exposure damages kidneys, the nervous system, and increases blood pressure. Lead mimics calcium, so in calcium deficiency, the body may absorb and store lead in bones.
Cadmium
- History
Cadmium is a chemical element discovered in 1817 by a German chemist. In 1912, cadmium poisoning became a known issue in Japan when cadmium released from mining operations in Toyama Prefecture contaminated river water used for irrigating rice fields. The rice absorbed the cadmium, and the population became exposed through consumption of contaminated rice. This illness, which occurred after World War II, became known as Itai-Itai disease, with symptoms including joint and spinal pain, weakened and softened bones, and severe bone fragility.
The first report linking cadmium exposure to cancer in humans was published in 1967, showing increased risk of prostate cancer in workers occupationally exposed to cadmium. Cadmium exposure is also associated with gastrointestinal cancers, breast cancer, lung cancer, and kidney cancer.
- Sources
Cadmium can naturally exist in soil. It may also enter the environment through volcanic eruptions, forest fires, and industrial activities such as the production of batteries, plastics, and solar panels.
Consumption of contaminated food is the primary source of human cadmium exposure. Smokers are exposed both through food and cigarette smoke; smoking a pack of cigarettes exposes a person to approximately 2–4 micrograms of cadmium or more.
Studies show that cadmium concentrations are higher in plant-based foods such as whole grains, grain sprouts, oilseeds and non-oilseeds, fruits, and vegetables compared to animal-based foods. Therefore, vegetarians may be more exposed.
- Harmful Effects of Cadmium on the Human Body
Cadmium can cause kidney failure. It also contributes to osteoporosis and is classified as a carcinogen for humans (lung, breast, bladder, and endometrial cancers). Infants, pregnant women, and breastfeeding mothers are more vulnerable to cadmium toxicity.
Cadmium is excreted only in small amounts through the digestive system and has a long biological half-life (approximately 10–30 years). Therefore, cadmium is one of the most persistent heavy metals in the body, accumulating significantly over time due to slow excretion.
Mercury
- History
Mercury is another chemical element that is far more toxic than cadmium and lead. Over 2,000 years ago, Spaniards observed that slaves working in mercury mines gradually became ill and died.
In the early 1950s in Japan, abnormal animal behaviors were observed for example, fish spinning at the water’s surface, birds falling while flying, excessive drooling, convulsions, and severe spinning movements in cats. These cats could not walk straight and often fell dead on the ground or into the sea; locals referred to this phenomenon as the “dancing cat.”
The cause was the discharge of mercury-containing chemical waste into Minamata Bay by a fertilizer factory, leading to what became known as Minamata disease.
- Sources
The two major sources of mercury exposure for humans are consumption of fish and dental filling materials (amalgam). According to the World Health Organization (WHO), mercury has destructive effects on aquatic life and reaches humans through seafood consumption. About 50% of dental amalgam is mercury. The U.S. FDA recommends that individuals with mercury sensitivity or allergies, kidney or neurological disorders, as well as children, pregnant women, and women planning pregnancy, use non-amalgam fillings such as composite resins and glass ionomer cements (GIC).
- Harmful Effects of Mercury on the Human Body
Mercury affects the nervous system, causing sensory disturbances, impaired muscle coordination, speech problems, hearing issues, and tremors.
If a pregnant woman consumes seafood with high levels of mercury or methylmercury, the fetus becomes exposed, which may result in infants with cerebral palsy, blindness, and intellectual disabilities. Mercury exposure may also cause: cancers of the lungs, skin and intestines, brain tumors, cardiovascular problems such as hypertension and increased risk of heart attack, negative effects on the reproductive and nervous systems.
Routes of Heavy Metal Entry into the Body
These metals enter the body through inhalation, consumption of contaminated food and water and dermal absorption. Daily food intake accounts for 80–90% of exposure to metals such as arsenic, chromium, cadmium, and lead.
After food enters the digestive system, the acidic environment of the stomach converts heavy metals into ionic forms, which are absorbed through the gastrointestinal wall especially the duodenum and enter the bloodstream.
Some heavy metals are excreted through urine and feces, but because accumulation is faster than excretion, they may be stored in organs such as the liver, kidneys, bones, and brain.
Maximum Residue Limits (MRL) of Heavy Metals
The maximum amount of heavy metals present in human and animal food that does not cause harmful health effects in either the short or long term is referred to as the Maximum Residue Limit (MRL) of heavy metals. Below are the links to the MRL values for each food category according to European, Codex, and Iranian standards.
Link to the European Union heavy metal MRLs:
https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex:32006R1881
Link to the Codex heavy metal MRLs:
https://www.fao.org/fao-who-codexalimentarius/thematic-areas/contaminants/en/
Link to the Iranian national standard for heavy metal MRLs:
Measurement of Heavy Metals in Food Laboratories
One of the most widely used instruments for heavy metal analysis is Atomic Absorption Spectroscopy (AA), which measures these metals in units of ppb (parts per billion) and ppm (parts per million).
Heavy Metal Testing in the Human Body
Commonly analyzed heavy metals include lead, mercury, arsenic, and cadmium. Samples are usually taken from blood, urine, nails, or hair. The type of sample depends on the metal being measured and whether exposure was short- or long-term. Blood and urine tests are the most common.
Dietary Recommendations to Reduce Heavy Metal Risks
- Foods such as tomatoes, onions, garlic, grapes, etc., may help eliminate heavy metals by binding to them and increasing excretion.
- Zinc, calcium, and iron supplementation in deficient individuals helps increase excretion of cadmium and lead.
- Vitamin C reduces blood lead levels.
- Vitamin B1 (thiamine) in grains, fruits, and fresh vegetables reduces lead in the liver, kidneys, bones, and blood.
- Vitamin B6 in nuts, seeds, and avocados decreases lead accumulation in tissues.
- Vitamin E protects against mercury toxicity in the liver and other tissues.
- To reduce lead exposure wash hands regularly, especially children’s hands before eating. And also avoid ceramic ware with lead-based glaze. Remove or clean shoes before entering the home.
- To reduce mercury exposure Avoid large fish such as shark, swordfish, and king mackerel, as predatory deep-sea fish contain more mercury.
- To reduce arsenic exposure Limit consumption of rice and rice-based foods to lower arsenic intake.