A toxic heavy metal is a confusing term for a metal-like element that can be harmful to health. Not all heavy metals are toxic, and some toxic metals are not heavy. Examples of toxic elements include cadmium, lead, and mercury, which are listed by the World Health Organization as 10 major chemicals of public concern. Other examples are chromium, nickel, antimony, arsenic, bismuth, thallium, and tin.
These toxic elements occur naturally in the Earth. Human activities cause them to become more concentrated, and they can enter plants, animals, and humans through breathing, eating, or direct contact. Once inside the body, they attach to and disrupt important parts of cells. People have known about the harmful effects of arsenic, mercury, and lead for a long time, but organized studies on the toxicity of heavy metals began in 1868. In humans, heavy metal poisoning is often treated with special medicines called chelating agents. Some elements considered toxic heavy metals are needed in very small amounts for human health.
Controversial terminology
The International Union of Pure and Applied Chemistry (IUPAC), which sets rules for naming, says the term "heavy metals" is unclear and can be misleading. The IUPAC report discusses the effects on health and laws of calling "heavy metals" toxins when there is no scientific proof linking them to harm. The word "heavy," which refers to density, has little effect on living things, and pure metals are usually not the active parts in biological processes. Many studies have agreed with this view. The most popular toxicology textbook, Casarett and Doull's Toxicology, uses the term "toxic metal" instead of "heavy metals." However, many scientific and related articles still use "heavy metal" to describe harmful substances.
Major and minor metal toxins
Metals that can cause harm in different ways include arsenic (As), beryllium (Be), cadmium (Cd), chromium (Cr), lead (Pb), mercury (Hg), and nickel (Ni).
Some elements are important for the health of animals and plants but can be harmful if present in large amounts or in certain forms. These include cobalt (Co), copper (Cu), iron (Fe), magnesium (Mg), manganese (Mn), molybdenum (Mo), selenium (Se), and zinc (Zn).
Contamination sources
Toxic metals are naturally present in the Earth. They can gather in larger amounts because of human actions or natural processes, such as when metals build up in peat soils that are later drained for farming. Common sources of these metals include fertilizers, old water pipes, tiny plastic pieces in oceans, and landfills with broken electronics. Arsenic may be used in some dyes. Rat poison stored in places where grain and mash are kept may also be a source of arsenic.
The areas affected by these metals can be very large. For example, up to one-sixth of China’s farmland may have heavy metal contamination.
Lead is the most common heavy metal contaminant. It was widely used in gasoline as a chemical called tetraethyl lead, (CH₃CH₂)₄Pb, from the 1930s to the 1970s. In areas where people live and work, lead levels in water and soil are estimated to be two to three times higher than they were before industrial times. Even though leaded gasoline was mostly stopped being used in North America by 1996, soil near roads built before that time still has high levels of lead. Lead from chemicals like lead(II) azide or lead styphnate used in guns can build up at shooting ranges. This pollutes the area and puts people who work at the ranges at risk of lead poisoning.
Entry routes
Toxic metals can enter the bodies of plants, animals, and humans through breathing in air, eating food, or handling objects. Jobs like welding, galvanizing, brazing, and soldering expose workers to harmful fumes that can be inhaled and cause a condition called metal fume fever. Cars and trucks release many harmful metals into the air, including arsenic, cadmium, cobalt, nickel, lead, antimony, vanadium, zinc, platinum, palladium, and rhodium. Water sources such as groundwater, lakes, streams, and rivers can become polluted when toxic metals from industrial or household waste seep into them. Acid rain can make this problem worse by releasing metals that are trapped in soil. Toxic metals can move through soil more easily when there are large spaces in the soil (called macropores) or when dissolved organic compounds are present. Plants absorb toxic metals from the water they take in. Animals eat these plants, and people get most of their toxic metal exposure by eating plants and animal-based foods. Contact with skin, such as touching soil or metal-containing toys and jewelry, is another way toxic metals can enter the body. Because toxic metals are hard for the body to break down, they can build up in the bodies of living things over time.
Detrimental effects
Toxic metals can attach to important parts of cells, such as structural proteins, enzymes, and nucleic acids, and stop them from working properly. The symptoms and effects depend on the type of metal or metal compound and how much someone is exposed to. In general, being exposed to toxic heavy metals over a long time can cause cancer, problems with the brain and nerves, and issues with the heart and blood vessels. In humans, common signs of exposure to these metals, including chromium and arsenic, are listed in the table.
History
The harmful effects of arsenic, mercury, and lead were known to people in ancient times. However, serious studies about how harmful heavy metals are started in 1868. In that year, Wanklyn and Chapman talked about the dangers of heavy metals like arsenic, lead, copper, zinc, iron, and manganese in drinking water. They said there was not enough research and asked for more data to be collected. In 1884, Blake suggested that the toxicity of a metal might be connected to its atomic weight. The following sections give a short history of the "classical" toxic heavy metals (arsenic, mercury, and lead) and some more recent examples (chromium and cadmium).
Arsenic was known in ancient times as realgar (As₄S₄) and orpiment (As₂S₃). Strabo, a Greek geographer and historian who lived from about 64 BCE to 24 CE, wrote that only slaves worked in realgar and orpiment mines because the fumes from the ores were deadly. In 1900, arsenic-contaminated beer poisoned over 6,000 people in Manchester, England, and is believed to have killed at least 70 people. Clare Luce, an American ambassador to Italy from 1953 to 1956, suffered from arsenic poisoning. The source was found to be flaking arsenic-laden paint on the ceiling of her bedroom. She may have also eaten food contaminated by arsenic from flaking paint in the embassy dining room. As of 2014, groundwater polluted with arsenic was still harming millions of people in Asia.
The first emperor of unified China, Qin Shi Huang, is reported to have died after eating mercury pills meant to give him eternal life. The phrase "mad as a hatter" likely refers to mercury poisoning among milliners (workers who made hats), known as "mad hatter disease," because mercury-based compounds were used in felt hat production during the 18th and 19th centuries. Historically, gold amalgam (a mix of gold and mercury) was widely used in gilding (covering surfaces with gold), causing many worker deaths. It is estimated that 60 workers died during the gilding of the main dome of Saint Isaac's Cathedral. In the 1950s, outbreaks of methylmercury poisoning occurred in Japan due to mercury released into rivers and coastal waters by industry. The most famous cases were in Minamata and Niigata. In Minamata alone, over 600 people died from what became known as Minamata disease. More than 21,000 people filed claims with the Japanese government, and nearly 3,000 were officially diagnosed with the disease. In 22 cases, pregnant women who ate contaminated fish showed no or mild symptoms but gave birth to babies with severe developmental disabilities. Since the Industrial Revolution, mercury levels in many near-surface seawaters have tripled, especially near Iceland and Antarctica.
The harmful effects of lead were known to ancient people. In the 2nd century BCE, the Greek botanist Nicander described the stomach pain and muscle weakness caused by lead poisoning. Dioscorides, a Greek physician who lived in the 1st century CE, wrote that lead "makes the mind give way." Lead was used widely in Roman aqueducts from about 500 BCE to 300 CE. Julius Caesar's engineer, Vitruvius, wrote that water from earthenware pipes was healthier than water from lead pipes because lead made the water harmful by producing white lead, which is bad for the body. During the Mongol period in China (1271–1368 CE), lead pollution from silver smelting in the Yunnan region was nearly four times higher than modern mining pollution. In the 17th and 18th centuries, people in Devon, England, suffered from a condition called "Devon colic," which was caused by drinking cider contaminated with lead. In 2013, the World Health Organization estimated that lead poisoning caused 143,000 deaths and contributed to 600,000 new cases of children with intellectual disabilities each year. In Flint, Michigan, lead contamination in drinking water became a problem in 2014. The cause was corrosion in lead and iron pipes used to deliver water. In 2015, lead levels in drinking water in northeastern Tasmania, Australia, reached over 50 times the national safety limit. The cause was a combination of old and damaged water infrastructure, including lead pipes and household plumbing.
Chromium(III) compounds and chromium metal are not considered harmful, but chromium(VI) has been known to cause cancer and other health problems since at least the late 19th century. In 1890, Newman noted that workers at a chromate dye company had a higher risk of cancer. Skin irritation from chromate was reported in aircraft workers during World War II. In 1963, 60 automobile factory workers in England developed skin conditions, including redness and wet sores, after sanding chromate-based paint on car bodies. In 2011, chromium was released from the Newcastle Orica explosives plant in Australia. Up to 20 workers and 70 nearby homes were exposed. The town was informed three days after the release, leading to public criticism of Orica for downplaying the risk and the government for its slow response.
Cadmium exposure became a problem in the early 20th century. In 1910, the Mitsui Mining & Smelting Company in Japan began releasing cadmium into the Jinzū River as a byproduct of mining. People who lived nearby later ate rice grown in water contaminated with cadmium and suffered from weak bones and kidney failure. At the time, the cause of these symptoms was unclear, with suggestions of a local disease, bacteria, or lead poisoning. In 1955, cadmium was identified as the likely cause, and in 1961, it was directly linked to mining operations. In 2010, cadmium was found in Walmart-exclusive Miley Cyrus jewelry. Walmart continued selling the jewelry until May 2010, when testing confirmed the presence of cadmium.
It is estimated that about 2.3% of the U.S. population has high levels of cadmium in their urine. Cadmium toxicity is usually higher in smokers than in non-smokers. This is because cadmium is used in industrial and agricultural equipment that produces and harvests goods. Cadmium exposure has increased rapidly over the past century.
Remediation
In humans, heavy metal poisoning is usually treated with special chemicals called chelating agents. These chemicals, such as CaNa₂ EDTA, change heavy metals into forms that do not react with the body and can be removed through urine or feces. While chelating agents help remove harmful metals, they can also take away some metals that are needed for the body to function properly. For this reason, doctors sometimes give vitamin and mineral supplements at the same time.
Soil that is polluted with heavy metals can be cleaned using several methods: isolation, immobilization, toxicity reduction, physical separation, or extraction. Isolation uses barriers, such as covers or underground walls, to contain the polluted soil. Immobilization changes the soil’s properties to make it harder for heavy metals to move through the soil. Toxicity reduction uses chemical or biological methods to change heavy metals into less harmful forms. Physical separation removes the polluted soil and uses machines to separate the metals from the soil. Extraction uses chemicals, heat, or electricity to remove metals from the soil. The method chosen depends on the type of metal and the conditions of the polluted area.
Although heavy metals are naturally present in Earth’s crust, most pollution and human exposure come from human activities. These include mining, which harms air quality, smelting and industrial processes that pollute air and water, and the use of agricultural equipment that contaminates soil. Some natural events, such as weathering and volcanic eruptions, also contribute to heavy metal pollution.
Benefits
Some elements that are usually considered toxic heavy metals are important for human health when present in small amounts. These elements include vanadium, manganese, iron, cobalt, copper, zinc, selenium, strontium, and molybdenum. Not having enough of these essential metals may increase the risk of heavy metal poisoning.
Selenium is the most dangerous among the essential heavy metals for mammals. Selenium is usually removed from the body, but it becomes harmful when the amount taken in is more than the body can remove.