Heavy Metals and Contaminants

TL;DR. These are the substances in food that nobody put there deliberately. Lead is the one that matters most, because there is no safe level and the damage to children's brains is permanent, and the main sources are old plumbing, old paint, and some imported spices and cosmetics. Mercury concentrates up the food chain, which is why the advice targets a short list of large predatory fish. Cadmium and inorganic arsenic come from soil and are highest in rice, leafy vegetables, and cocoa. Aflatoxin is a potent natural carcinogen from mould. Almost all of this is managed by variety in the diet.

1. Lead

There is no known safe level of lead exposure. The CDC removed the concept of a "level of concern" and now uses a reference value that is periodically revised downward.

What it does: lead substitutes for calcium and zinc in enzymes and in neural signalling. In children it causes irreversible reductions in IQ, attention, and impulse control, with effects demonstrated at blood levels once considered normal. In adults it raises blood pressure and causes kidney damage, and it is associated with cardiovascular mortality.

Historical scale. Tetraethyl lead in petrol was one of the largest mass poisoning events in history. Population blood lead levels fell by roughly 90 percent in countries that removed it, and the "lead-crime hypothesis" proposes that the fall in violent crime from the 1990s tracks the removal of leaded petrol two decades earlier with striking consistency across countries. It remains contested and the correlation is remarkable.

Where lead still comes from:

What to do:

• If your home may have lead pipes: run the tap for 30 seconds to two minutes if water has stood, use only cold water for drinking and cooking (hot dissolves more lead and sits in a tank), and ask your water company to test.
• Never sand or dry-scrape old paint. Renovation of pre-1980 housing is a major exposure route, and it is exactly the situation where children are present.
• Buy spices from reputable sources, and be suspicious of unusually vivid colour.
• Do not use traditional cosmetics containing kohl or surma on children.
• Adequate calcium, iron, and vitamin C reduce lead absorption, which matters because deficient children absorb more.

2. Mercury

Methylmercury is the dietary form: bacteria in water convert inorganic mercury, and it then biomagnifies up the food chain, concentrating roughly tenfold at each step.

Effects: neurotoxic, crosses the placenta, and impairs fetal brain development. The Minamata disaster in Japan, where industrial discharge caused severe congenital neurological disease, established this definitively.

Where it is, and the pattern is simply size and lifespan:

The critical framing: the balance of evidence favours eating fish, including in pregnancy. Large cohort studies, including ALSPAC, found maternal fish consumption associated with better child neurodevelopmental outcomes, and avoiding fish entirely appeared worse than the mercury risk. Choose the right fish; do not avoid fish (Chapter 55).

Dental amalgam contains elemental mercury. Extensive review has found no evidence of harm from existing fillings; the EU restricted new amalgam use in children and pregnant women from 2018 as a precaution and for environmental reasons, and removing sound fillings releases more mercury than leaving them.

Ethylmercury (thimerosal) in vaccines is a different compound, cleared from the body within days rather than accumulating (Chapter 74).

3. Cadmium

Taken up from soil by plants more readily than most metals, with phosphate fertilisers and sewage sludge as significant agricultural sources.

Effects: accumulates in the kidneys over decades with a biological half-life of 10 to 30 years, causing kidney damage and bone demineralisation. The Japanese itai-itai disease outbreak, from cadmium-contaminated rice near a mine, produced severe osteomalacia and is the defining case.

Where it is: shellfish (especially crab brown meat and oysters), offal (kidney and liver), rice, leafy vegetables and root vegetables, cocoa and dark chocolate, sunflower seeds, flaxseed, and tobacco, which is a major source in smokers.

Dark chocolate attracted attention after a 2022 Consumer Reports analysis found cadmium and lead above California's stringent Proposition 65 levels in many products. Those thresholds are far stricter than most food safety limits; the finding is real, the risk at ordinary consumption is modest, and varying your chocolate sources and not eating large amounts daily is a reasonable response.

Smoking roughly doubles body cadmium burden, which is a rarely mentioned harm of tobacco.

4. Inorganic arsenic

Two forms matter and behave completely differently. Organic arsenic in seafood (arsenobetaine) is essentially non-toxic and is excreted. Inorganic arsenic is a Group 1 carcinogen causing skin, lung, and bladder cancer, plus skin lesions and cardiovascular disease.

The largest public health disaster involving it is in Bangladesh and West Bengal, where tube wells drilled from the 1970s to provide microbiologically safe water tapped naturally arsenic-contaminated groundwater, exposing tens of millions of people. The WHO has described it as the largest mass poisoning of a population in history.

In food, rice dominates, because flooded paddy conditions chemically mobilise arsenic and rice takes it up efficiently (Chapter 51).

Practical steps:

• Vary your grains. The simplest and most effective measure.
• Cook rice in excess water (6 to 10 parts to 1) and drain, which removes 40 to 60 percent. Parboiling and draining first removes more.
• Origin matters: basmati from India and Pakistan, and rice from California and parts of Europe, tend to be lower; rice from the southern US and parts of South Asia higher.
• Brown rice contains more than white, because it concentrates in the bran. One of the few cases where refining reduces a harm.
• Infants and young children are the priority group. UK and US guidance advises against rice drinks as a milk substitute for under-fives and recommends varying infant cereals.
• Apple and grape juice are monitored for inorganic arsenic; limits exist and levels are generally low.

5. Aflatoxin and other mycotoxins

Aflatoxin B1 is one of the most potent naturally occurring carcinogens known, produced by Aspergillus flavus and A. parasiticus growing on crops in warm humid conditions. It causes liver cancer, and its effect multiplies with hepatitis B infection, which is why liver cancer rates in parts of sub-Saharan Africa and Southeast Asia are so high.

Where it is: peanuts and groundnuts, maize, tree nuts (especially pistachios and Brazil nuts), dried figs, spices, and, as aflatoxin M1, in the milk of animals fed contaminated feed.

Regulation is strict. EU limits are among the tightest in the world, and border rejections are frequent. Home-produced and informally traded food in high-burden regions is where the exposure concentrates.

Practical steps: buy from reputable sources, store nuts and grains cool and dry, and discard anything shrivelled, discoloured, mouldy, or tasting bitter or musty. Do not cut mould off nuts or grain and eat the rest.

Other mycotoxins: ochratoxin A (cereals, coffee, dried fruit, wine), patulin (apple juice from rotten fruit, which is why juicers should discard bruised fruit), fumonisins (maize), and deoxynivalenol (wheat). Ergot alkaloids in rye caused historical mass poisonings (St Anthony's fire) and are now controlled.

6. Dioxins, PCBs, and persistent organic pollutants

Dioxins are unintentional by-products of combustion and some industrial processes. They are extremely persistent, fat-soluble, and bioaccumulate.

Over 90 percent of human exposure is dietary, and it concentrates in animal fat: fatty fish, meat, dairy, and eggs.

PCBs were used in electrical equipment until banned in the 1970s and 80s, and persist in the environment and food chain.

Effects: classified as carcinogens, with additional effects on the immune, reproductive, and endocrine systems. The good news is that levels in food have fallen substantially in most industrialised countries since emission controls, by a factor of several since the 1980s.

Practical steps: trim visible fat, vary protein sources, and follow local fish advisories for recreationally caught freshwater fish, which is where the highest exposures now occur in wealthy countries.

7. Other contaminants worth naming

8. What actually reduces your exposure

Ranked by effect:

1. Vary your diet. This is the single most effective measure and it applies to every contaminant in this chapter. Contaminants concentrate in specific foods from specific places; eating a range dilutes any one exposure.
2. Fix your water if you have old plumbing, and never use hot tap water for cooking or drinking.
3. Do not smoke. It is a major source of cadmium, lead, and polonium-210.
4. Choose the right fish rather than avoiding fish.
5. Vary your grains and cook rice in excess water.
6. Buy spices, nuts, and supplements from reputable sources, and reject anything musty, discoloured, or suspiciously bright.
7. Store food dry and cool to prevent mould growth.
8. Wash produce, mostly for microbial reasons.
9. Trim visible fat for persistent organic pollutants.
10. Be careful during renovation of old housing, particularly with children in the home.

9. Keeping it in proportion

Three things are true at once, and the discussion usually holds only one:

These are real hazards with real evidence. Lead genuinely damages children's brains, arsenic genuinely causes cancer, and aflatoxin genuinely causes liver cancer at scale.

Regulated food supplies have driven exposures down enormously. Leaded petrol is gone, dioxin levels have fallen several-fold, and monitoring programmes catch violations continuously.

The dietary risks are usually small compared with the alternatives. Avoiding fish to avoid mercury is worse than eating the right fish. Avoiding vegetables to avoid cadmium would be absurd. The framework in Chapter 87 applies throughout: hazard is not risk, and the counterfactual matters.

The exposures with the largest population effects are not dietary at all. They are lead in housing and water, arsenic in groundwater, cadmium from smoking, and air pollution (Chapter 95).

10. The bottom line

• Lead has no safe level and permanently damages children's neurodevelopment. The main routes are old plumbing, old paint during renovation, and adulterated imported spices and cosmetics. Use cold water only, run the tap, and never dry-sand old paint.
• Mercury biomagnifies up the food chain, so the advice targets large predatory fish. Eat the right fish rather than avoiding fish, including in pregnancy.
• Cadmium accumulates in kidneys over decades and is highest in shellfish, offal, rice, leafy vegetables, cocoa, and tobacco. Smoking roughly doubles the body burden.
• Inorganic arsenic concentrates in rice. Vary your grains, cook rice in excess water and drain, and do not give rice drinks to young children.
• Aflatoxin is one of the most potent natural carcinogens known. Discard mouldy, shrivelled, or bitter nuts and grain rather than salvaging them.
• Variety in the diet is the single most effective protection against every item in this chapter.

Sources and notes

Lead toxicity, the absence of a safe threshold, and the CDC's replacement of a "level of concern" with a periodically revised reference value follow CDC and WHO documentation. Population blood lead declines after leaded petrol removal follow NHANES data. The lead-crime hypothesis follows Nevin's and Reyes' analyses, with its ongoing methodological critiques. Lead sources, including old plumbing, pre-1978 US and pre-1992 UK paint, and lead-chromate-adulterated turmeric (Forsyth et al., Environmental Research, 2019), follow ATSDR and public health agency guidance. Flint follows Hanna-Attisha et al., American Journal of Public Health, 2016. Methylmercury biomagnification, the Minamata record, and fish advisories follow WHO, the FDA/EPA joint advice, and the FSA. The finding that maternal fish avoidance was worse than the mercury risk is Hibbeln et al., The Lancet, 2007, from ALSPAC. Cadmium accumulation, itai-itai disease, and dietary sources follow EFSA's 2009 opinion and WHO; the dark chocolate analysis is Consumer Reports, 2022, assessed against California Proposition 65 levels. Inorganic arsenic, the Bangladesh groundwater catastrophe, rice uptake, and cooking-method reduction follow WHO, EFSA's 2009 opinion, and Meharg's work. Aflatoxin, its multiplication with hepatitis B, and regulatory limits follow IARC Monograph 100F and EU Regulation 1881/2006. Dioxins and PCBs follow WHO fact sheets and EFSA's 2018 opinion, including the documented decline in food levels. The 2008 melamine infant formula adulteration follows WHO reporting.

Open questions. The lead-crime hypothesis remains an ecological correlation that cannot be tested experimentally. How much cadmium in cocoa comes from soil versus processing, and whether it is reducible, is not resolved.

👉 Next: plastics, packaging, and cookware.