Foodborne Chemicals: Heavy Metals

Public health agencies and clinicians should educate the public regarding ways to minimize heavy metal exposures. Known exposures should be discussed immediately with a local poison control authority.

Elemental mercury is a well–known toxin. It is used in medical instruments, although less commonly than in the past, and in dental amalgam fillings. Family physicians may encounter patients who have inhaled elemental mercury resulting from spills at home or in schools, although these exposures may not lead to toxicity. Mercury also forms several compounds and is a common environmental pollutant, as a result of industrial processes.

Aside from occupational exposures, the main exposure route is through ingestion of fish, especially those high on the food chain, such as shark, tuna, and swordfish, and certain fish taken from some freshwaters. Dental fillings are not believed to present a public health hazard, although alternatives to amalgam fillings are available. Vaccines may contain thimerosal, a compound that includes mercury. However, the effective amount of mercury from vaccines is not currently considered unsafe, although debate about this will likely continue.

Mercury poisoning may cause digestive, respiratory, and neurologic disorders, and can be lethal. (The phrase “mad as a hatter” refers to neurologic sequelae of mercury exposures in the felt hat industry of the 19th century.) Mercury easily crosses the placenta in pregnant women and may cause birth defects, even in the absence of maternal symptoms.

Mercury toxicity is usually diagnosed by a blood test. Chelation is available for acute mercury toxicity.

Cadmium, a heavy metal common in the environment, can contribute to kidney and bone disease and is considered a probable carcinogen by the federal Environmental Protection Agency (EPA). Cigarettes are a common source of cadmium exposure. Incineration of household waste, particularly batteries, may release cadmium into the atmosphere, and industrial processes such as mining and land applications of sewage sludge can pollute water and air. Phosphate fertilizers commonly contain cadmium, and grain and vegetable crops easily absorb the metal through polluted irrigation waters.

Fish concentrate cadmium in their livers and kidneys, leading to toxicity in populations that commonly consume whole fish, especially fish taken from urban waters. Shellfish also concentrate cadmium.

Cadmium toxicity can be diagnosed through urine and blood tests, although blood generally shows evidence of acute exposures only. For this reason, prevention of excess exposure is of paramount importance. No well–studied and accepted chelating agent is available for cadmium in humans.

Lead is very common in landfills harboring old electronic devices (solder) and cathode ray tubes, in mine runoff areas, and in manufacturing facilities where lead is used in batteries, radiators, lead glazes, and other products. Groundwater and waterways may become contaminated from these sources, and many old houses still use lead pipes to connect to city water mains. In addition, older houses often have lead–based paint, which may flake and be consumed by small children.

Lead may also be present in toy jewelry, household crystal and glazed pottery used for serving foods and beverages, imported Mexican candy, and traditional medicines. For example, some Ayurvedic and Chinese medicines may contain lead, mercury, and arsenic.^1,2 Air emissions, especially in areas where leaded gasoline is still available, may cause surface contamination of crops.

Lead poisoning can lead to nervous system and kidney damage, and can cause several nonspecific symptoms. It may also adversely affect fetal and childhood development, and fertility in men. During pregnancy and lactation, women can mobilize lead stored in bone from past exposures.

Lead poisoning is diagnosed through blood lead concentrations, among other laboratory findings, and chelation agents are available for treatment of patients with high levels or acute symptoms. 

High–calcium diets may protect against lead accumulation by reducing gastrointestinal absorption of this mineral.³ High blood levels of vitamin C are also independently associated with lower prevalence of elevated blood lead concentrations.⁴ However, neither calcium nor vitamin C has yet been found to reduce body lead burden in randomized controlled clinical trials.

Arsenic is present in some pesticides, treated wood, and mining runoff. Exposures may also come from the smelting process. Arsenic is also a component of chicken feed supplements used to treat parasites,⁵ and often reaches drinking water, especially untreated well water. Exposure through water is of particular concern, because the arsenic compounds formed are readily bioavailable.

Long–term exposure is related to hyperkeratosis,⁶ as well as increased risk of skin and other cancers, including lung cancer. Exposure may result from occupational inhalation.

Arsenic poisoning is commonly diagnosed through urine tests, although hair and nail samples can also reveal exposure. Chelation therapy is available.