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Heavy metals in food: arsenic, cadmium, lead and mercury
Heavy metals differ considerably in their sources, behaviour, toxicity, and the foods in which they are most commonly found.
Heavy metals differ considerably in their sources
Heavy metals differ considerably in their sources, behaviour, toxicity, and the foods in which they are most commonly found. While arsenic, cadmium, lead, and mercury are all regulated contaminants, each follows different pathways into the food chain and presents distinct health concerns.
This guide explains where these heavy metals originate, which foods are most affected, and why they are monitored as part of food safety programmes. Understanding these differences helps food manufacturers, laboratories, and quality professionals identify potential risks and select appropriate analytical methods for regulatory compliance.
Arsenic in food
Where does arsenic in food come from?
Arsenic is a naturally occurring element found in the Earth’s crust and enters the food chain primarily through soil and groundwater. Concentrations vary considerably between regions depending on local geology. Groundwater contamination is particularly well documented in parts of Bangladesh, West Bengal, and the Mekong Delta, where naturally arsenic-rich sediment layers have resulted in some of the world’s most extensively studied cases of population exposure. In these regions, contamination originates from geological processes rather than industrial pollution.
Arsenic occurs in both inorganic and organic forms, an important distinction for food safety and toxicological assessment. Inorganic arsenic is significantly more toxic than organic arsenic compounds such as arsenobetaine, which is found mainly in seafood and is generally considered to have low toxicity. For this reason, food regulations and analytical methods increasingly focus on measuring inorganic arsenic rather than total arsenic, as total arsenic values alone may overestimate the health risk in foods naturally rich in organic arsenic, such as fish and shellfish.
Which foods contain the most arsenic?
Rice is the food most closely associated with inorganic arsenic contamination. Unlike cereals such as wheat or maize, rice is cultivated under flooded, anaerobic conditions that increase the availability of arsenic in the soil and promote uptake by the plant.
Within the rice category, brown rice generally contains more inorganic arsenic than white rice because arsenic accumulates in the outer bran layer, which is removed during polishing. Rice-based products, including rice cakes, rice crackers, and infant cereals, may also contain relatively higher concentrations because processing removes water and concentrates naturally occurring minerals.
Seafood can contain high total arsenic concentrations, but this is predominantly present as organic arsenic, which is considerably less toxic than inorganic arsenic. Certain algae-derived foods and food supplements, including seaweed and kelp products, can also accumulate arsenic naturally from seawater. In addition, fruit juices, particularly apple juice, have received regulatory attention in some markets following occasional findings of elevated inorganic arsenic linked to soil and groundwater conditions during cultivation.
Health concerns
Long-term exposure to inorganic arsenic has been associated with a wide range of adverse health effects and is supported by extensive epidemiological research in populations exposed to arsenic-contaminated drinking water. The International Agency for Research on Cancer (IARC) classifies inorganic arsenic as a Group 1 carcinogen, with sufficient evidence linking chronic exposure to skin, bladder, and lung cancer.
In addition to its carcinogenic effects, chronic inorganic arsenic exposure has been associated with cardiovascular disease, skin lesions, and developmental effects. Early-life exposure has been linked to adverse birth outcomes and impaired childhood development, making infants and young children particularly vulnerable. This increased sensitivity is reflected in stricter regulatory limits for inorganic arsenic in rice products intended for infants and young children.
Cadmium in food
Where does cadmium in food come from?
Cadmium enters the food chain primarily through soil. Both naturally occurring (geogenic) background levels and human activities contribute to cadmium concentrations in agricultural soils. Certain volcanic and sedimentary soils naturally contain elevated cadmium levels, independent of industrial or agricultural activity. In contrast, phosphate fertilizers are one of the best-documented anthropogenic sources because the phosphate rock used to produce these fertilizers naturally contains cadmium. Repeated application over many years can gradually increase cadmium concentrations in agricultural soils.
The amount of cadmium absorbed by plants depends not only on the concentration in the soil but also on its bioavailability. Soil pH is an important factor, with cadmium uptake generally increasing under acidic conditions. For this reason, liming, which raises soil pH, is sometimes used to reduce cadmium availability in regions with naturally elevated soil concentrations. Mining and ore processing are additional recognized sources of cadmium contamination, as cadmium frequently occurs alongside zinc in mineral deposits and can spread into surrounding agricultural land through tailings, runoff, or airborne dust.
Which foods contain the most cadmium?
Cocoa and chocolate are among the best-known examples of cadmium accumulation in food. Cacao grown in parts of Ecuador, Peru, and the Dominican Republic often shows elevated cadmium concentrations because the trees are cultivated in naturally cadmium-rich volcanic soils. Cadmium levels can increase further during cocoa processing, as fermentation, drying, and the production of cocoa liquor, cocoa powder, and cocoa butter progressively concentrate the mineral content. This is one reason why cocoa powder is subject to a higher EU maximum level than milk chocolate.
Cadmium is also found in leafy vegetables, root vegetables, tubers, and certain grains. Leafy vegetables generally accumulate more cadmium than fruiting vegetables because of differences in plant uptake and their larger surface area. Bivalve molluscs and crustaceans can also contain elevated cadmium concentrations, as their filter-feeding behaviour allows them to accumulate metals from surrounding water and sediments. In addition, botanical food supplements sourced from regions with naturally elevated soil cadmium may contain higher concentrations than the corresponding whole foods because supplements are often produced as concentrated extracts.
Health concerns
Cadmium is primarily associated with kidney toxicity following long-term dietary exposure. Once absorbed, it accumulates in the renal cortex, where it has an estimated biological half-life of 10 to 30 years. Sustained exposure can lead to tubular dysfunction, which is commonly detected by increased urinary excretion of low-molecular-weight proteins such as beta-2-microglobulin, a recognised biomarker of early cadmium-related kidney damage.
Cadmium is also classified as a Group 1 carcinogen by the International Agency for Research on Cancer (IARC). Although this classification is based primarily on evidence from occupational exposure, dietary intake contributes to the body’s cumulative cadmium burden over a lifetime. Because cadmium is eliminated very slowly, chronic low-level exposure through food is considered the primary pathway of concern, rather than isolated episodes of acute exposure.
Lead in food
Where does lead in food come from?
Lead can enter the food chain through several pathways, including contaminated soil, atmospheric deposition, drinking water infrastructure, and food contact materials. Historically, leaded petrol was one of the largest sources of atmospheric lead emissions. Although its use has been phased out globally, lead deposited decades ago remains in soils near former high-traffic areas and can still contribute to contamination of crops grown in these locations.
Lead can also enter food through ageing water distribution systems. Older plumbing components and lead-based solder may release lead into drinking water and water used during food processing. Food contact materials represent another potential source. Historically, lead-based solder was used in canned food seams, while ceramic glazes often contained lead and cadmium compounds as colourants and stabilisers. Migration of lead from certain imported or artisanal ceramic products into acidic foods remains a recognised contamination pathway.
Which foods contain the most lead?
Spices, particularly turmeric and paprika, are among the food categories most closely associated with lead contamination. While some contamination originates from soil and atmospheric deposition, an additional and well-documented source is the deliberate addition of lead chromate to turmeric to enhance its yellow colour. This practice is considered food fraud rather than environmental contamination and has resulted in numerous RASFF notifications involving imported spices.
Lead can also be found in fruits and vegetables, especially leafy vegetables grown near busy roads, industrial sites, or historically contaminated land. Because uptake differs between crops, the European Union sets separate maximum levels for leafy vegetables, root vegetables, tubers, and fruiting vegetables.
Other closely monitored categories include drinking water, infant foods and infant formula, and, to a lesser extent, certain cereal products. These foods receive particular regulatory attention because current scientific evidence indicates that no safe threshold for lead exposure has been established.
Health concerns
Lead is a well-established neurotoxicant, with the greatest concern relating to cognitive development in infants and children. Numerous studies have linked lead exposure to reduced IQ and impaired neurological development, even at exposure levels previously considered acceptable. In its 2010 scientific opinion, the European Food Safety Authority (EFSA) concluded that no threshold could be identified below which lead exposure poses no risk to neurodevelopment. This finding underpins the precautionary ALARA (As Low As Reasonably Achievable) approach used to minimise lead concentrations in food.
In addition to its effects on the nervous system, lead compounds are classified by the International Agency for Research on Cancer (IARC) as Group 2A carcinogens, meaning they are probably carcinogenic to humans. Chronic lead exposure has also been associated with cardiovascular and kidney effects in adults. However, the potential impact on brain development in children remains the primary driver of food safety regulations and exposure reduction strategies.
Mercury in food
Where does mercury in food come from?
Mercury enters aquatic environments from both natural geological sources and human activities. Today, coal combustion remains one of the largest anthropogenic sources of atmospheric mercury emissions. After being deposited into rivers, lakes, and coastal waters, inorganic mercury is converted by microorganisms into methylmercury. This transformation is central to food safety because methylmercury is the form that bioaccumulates in aquatic organisms and poses the greatest toxicological risk to humans.
Artisanal and small-scale gold mining is another well-documented source of mercury contamination. Mercury is used to extract gold from ore, releasing it into surrounding soils and waterways where it can accumulate in fish consumed by local communities, particularly in parts of South America, Africa, and Southeast Asia. Industrial wastewater discharge and historical industrial activity near rivers, lakes, and coastal waters can also contribute to localised mercury contamination in aquatic ecosystems used for fishing or aquaculture.
Which foods contain the most mercury?
Fish and seafood are by far the most important dietary sources of mercury. Methylmercury accumulates within individual fish over time and biomagnifies through the aquatic food chain. As a result, predatory species occupying higher trophic levels, including tuna, swordfish, shark, and marlin, generally contain much higher mercury concentrations than smaller, shorter-lived fish.
Freshwater fish caught in waterways affected by historical or ongoing mercury contamination, including areas influenced by artisanal gold mining, may also contain elevated mercury concentrations. Outside fish and seafood, mercury levels in food are generally low, making seafood consumption patterns the primary determinant of dietary mercury exposure.
Health concerns
Methylmercury is a well-established developmental neurotoxicant. It readily crosses both the blood-brain barrier and the placental barrier, making the developing foetal nervous system particularly sensitive to its effects. Maternal exposure during pregnancy, even at levels that produce no noticeable symptoms in the mother, can impair foetal brain development. This is why many health authorities recommend limiting the consumption of high-mercury predatory fish during pregnancy.
Unlike arsenic, cadmium, and lead, mercury and methylmercury are not classified by the International Agency for Research on Cancer (IARC) in the same way as these other heavy metals, as their primary health concern is neurotoxicity rather than carcinogenicity. In adults, prolonged exposure to high methylmercury concentrations has also been associated with neurological symptoms affecting coordination, sensation, and motor function. However, these effects are generally linked to exposure levels well above those expected from a varied and balanced diet.
Heavy metal | Main food sources | Main health concern |
Arsenic | Rice, rice products | Cancer, developmental effects |
Cadmium | Cocoa, vegetables, shellfish | Kidney damage |
Lead | Spices, vegetables, drinking water | Neurodevelopment |
Mercury | Fish and seafood | Neurotoxicity |
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FAQ
What is the difference between arsenic, cadmium, lead, and mercury in food?
Arsenic, cadmium, lead, and mercury are all regulated heavy metals, but they differ in their sources, toxicity, and food distribution. Arsenic is primarily associated with rice, cadmium with cocoa and vegetables, lead with spices and vegetables, and mercury with fish and seafood.
Why does rice contain more arsenic than wheat or maize?
Rice is grown under flooded, anaerobic conditions that increase the bioavailability of arsenic. Wheat and maize are typically grown in aerobic soils, where arsenic is less readily available for plant uptake.
Which form of arsenic is most toxic?
Inorganic arsenic is significantly more toxic than most organic arsenic compounds. As a result, food safety regulations increasingly focus on inorganic arsenic rather than total arsenic.
Which foods contain the most cadmium?
Cadmium is most commonly associated with cocoa, chocolate, leafy vegetables, root vegetables, grains, shellfish, and certain botanical food supplements.
Why is cadmium considered a long-term health concern?
Cadmium has an estimated biological half-life of 10 to 30 years. Because it is eliminated very slowly, dietary exposure can lead to accumulation over a lifetime.
Which foods are most commonly associated with lead contamination?
Lead is most commonly associated with spices, leafy vegetables, fruits and vegetables grown on contaminated soils, drinking water, infant foods, and certain cereal products.
