Heavy Metals —
Where They Hide

Lead was used so extensively throughout the 20th century — in paint, petrol, plumbing and ceramics — that its residue is now present virtually everywhere. Despite the phasing out of leaded petrol, the contamination it left in roadside soil persists. Homes built before 1970 frequently contain lead-based paint, and as it ages and flakes it generates dust that settles on food preparation surfaces and is absorbed by young children during normal hand-to-mouth behaviour.

Tap water is a particularly under-recognised source. In older UK properties, water flowing through lead pipes picks up dissolved lead — the level at your tap depends on the age of the plumbing in your street and building, not the quality of the treatment works. Soft water and slightly acidic water dissolve lead from pipes more readily than hard water.

Lead stored in bone matrix is not inert. It releases back into circulation during periods of high bone turnover — during pregnancy, lactation, menopause and calcium deficiency — exposing a developing foetus or nursing infant to a mother's lifetime accumulated lead burden. There is no known safe level of lead exposure.

Mercury exists in two principal dietary forms. methylmercury — the organic, fat-soluble form — is produced by bacteria in aquatic sediments and concentrates through biomagnification in the tissue of large predatory fish. A single large tuna may have consumed thousands of smaller fish; the mercury absorbed by each accumulates in the tuna's muscle tissue and is then consumed intact by the person eating it.

inorganic mercury from amalgam fillings is a distinct and debated exposure. Mercury vapour is released continuously during chewing. The significance of this source varies enormously between individuals — those with many fillings, who grind their teeth, or who eat large quantities of hot food will have higher exposure than the figures that appear in average dietary surveys.

Mercury's most damaging target is the nervous system. It crosses the blood-brain barrier readily and accumulates in neurons, disrupting neurotransmitter function, mitochondrial energy production and myelin integrity. Minamata disease demonstrated the catastrophic end of this spectrum. Chronic low-level exposure from regular large-fish consumption sits at the other end — subtler, harder to attribute, and far more widespread.

Tobacco smoke delivers cadmium directly to the lungs with near-complete absorption — a single cigarette contains 1–2 µg of cadmium, and smokers typically carry a body burden twice that of non-smokers. For non-smokers, diet is the primary route: cadmium enters crop plants from soil, where it has accumulated from phosphate fertilisers, industrial deposition and sewage sludge over decades.

The foods carrying the highest cadmium loads are not intuitive. Dark chocolate and cocoa — absorbed through the cacao plant's remarkable efficiency at drawing metals from soil — can be significant contributors in people who consume them regularly. Leafy greens and root vegetables grown in contaminated or heavily fertilised soil carry variable but sometimes substantial loads. Offal from animals that have concentrated cadmium via their own lifetimes of exposure can be unexpectedly high.

The kidney is the primary accumulation site, and cadmium damage there is insidious — renal tubular dysfunction typically precedes any measurable symptom by years. itai-itai disease sits at the severe end of what chronic cadmium toxicity looks like. Subclinical renal impairment from a lifetime of moderate exposure is a more likely outcome for the general population, and largely preventable through adequate zinc status and reduced exposure.

Arsenic exists in two fundamentally different dietary forms. inorganic arsenic — the toxic, carcinogenic form — is found in contaminated groundwater and concentrated in rice arsenic. organic arsenic from seafood is substantially less harmful and rapidly excreted. The critical question is always the inorganic fraction.

Rice is the most important single source of inorganic arsenic for most people in the UK. It is grown in flooded paddy fields where anaerobic soil conditions mobilise arsenic into the water that the plant absorbs. The concern is highest for infants and young children eating rice-based foods, whose smaller bodies receive proportionally higher doses. Rice milk in particular was found to contain arsenic at levels high enough for the Food Standards Agency to advise against it for children under five.

Practical exposure reduction for rice is straightforward: rinse thoroughly before cooking, cook in a large volume of water (5–10:1 water:rice ratio) and drain — this process can reduce arsenic content by 40–70%. Varying cereal intake beyond rice — using oats, quinoa, millet and barley as alternatives — reduces chronic daily arsenic accumulation significantly.