Some of the substances that make up Earth’s crust are elements, substances that cannot be naturally broken down into simpler substances. A few of these elements are poisonous even if present in a low concentration. These are known as heavy metals. Examples of heavy metals include mercury, cadmium, arsenic, chromium, thallium, and lead.
A school of bluefin tuna snagged during a mattanza (a mass tuna catch) that periodically occurs off the coast of Sicily, Italy.
The heavy metal-water connection
There is a connection between heavy metals and water. Because heavy metals are part of Earth’s crust, they can be worn away by the action of weather. When they are worn off of rock, they can collect in surface or groundwater (fresh water in rock and soil layer beneath Earth’s surface). Depending on the chemistry of the water, the metals might stay in the water, or come out of the water and gather on an available surface such as plants. Heavy metals can, therefore, enter peoples’ bodies via drinking water and food.
Heavy metals get into fresh and salt water when water or other fluids are added to the water body. Examples include industrial waste, sewage (even treated sewage can contain high levels of heavy metals), run-off (water that flows over surfaces) after a rainfall and from mining operations.
Microorganisms, plants, and animals depend on water for life. Heavy metals can bind to the surface of microorganisms and may even penetrate to the inside of the cell. Inside the microorganism, the heavy metals can be chemically changed as the microorganism uses chemical reactions to digest food. A well-known example is the ability of some bacteria to change mercury to a modified form called methylmercury. Methylmercury can be absorbed much more easily than mercury into the bodies of insects and other small organisms. When these small organisms are eaten by bigger living organisms such as fish, the heavy metals enter the fish. But, instead of staying in the fish for only a short time, the metals can remain in the fish for extended periods. As the fish eats more of the smaller organisms, the amount of heavy metals increases. As bigger organisms eat the smaller organisms (making up the food chain), the heavy metals build-up in concentration in the larger living things. This increase in concentration of substances over time and in bigger living organisms is called bioaccumulation. For example, at the top of the ocean food chain, fish such as tuna can contain significant quantities of mercury.
The hazards of eating mercury-contaminated fish came to public attention in 1956, when 121 people around Minamata Bay in Japan were poisoned after eating fish that had been contaminated by mercury spilled from a nearby industry. Forty-six of the 121 people died. The symptoms of lack of coordination, paralysis, difficulty swallowing, convulsions, and brain damage became known as Minamata Disease.
After shrimp, tuna is the most popular seafood in the United States. The presence of mercury in tuna has lead to a great deal of concern about the safety of canned tuna, which is used to make a popular lunchtime sandwich for school-aged children. Calculations have shown that a 45-pound child who eats one 6-ounce can of chunk white tuna a week takes in an amount of mercury that exceeds the dose recommended as safe by the United States Environmental Protection Agency. Nevertheless, as of 2004, no firm rules have yet been put into place about how much tuna people should eat until scientists confirm the amounts and sources of mercury that are harmful to humans.
The effects of heavy metals
Bioaccumulation of heavy metals is dangerous to human health. Lead, cadmium, cobalt, nickel, and mercury can affect the formation of blood cells. The build-up of heavy metals can cause malfunctions in the liver, kidneys, the circulatory system (responsible for the circulation of blood throughout the body), and the movement of nerve signals. Some heavy metals may also play a role in the development of various cancers.
Lead The lead atom (smallest component of an element having the chemical properties of the element) is similar in size and shape to the calcium atom. Lead can substitute in the body for calcium, particularly in bone. In children, where bones are still developing and the child is not taking in the required amount of calcium, the lead can become stored in the bone. If the lead gets out of the bone, as can happen when a child gets a suitable amount of calcium, the free lead can cause damage to nerves and to the brain.
Mercury Mercury, especially the form methylmercury, is very dangerous to people. Methylmercury is produced by microorganisms that live in the water. A lengthy exposure to mercury (such as can occur when mercury-containing fish are eaten) can damage the liver and cause brain damage. If a pregnant women takes in too much mercury it can cause birth defects in her child.
Effect of bioaccumulation on water organisms
Many organisms that live in fresh and salt water are harmed when heavy metals accumulate inside them. Shellfish do not have any mechanism to prevent bioaccumulation. This actually makes the shellfish a good indicator to scientists that a problem of heavy metal accumulation may exist in a certain area. If shellfish are found with unacceptably high levels of heavy metal, scientists are alerted to the possibility of contamination in organisms throughout the body of water.
The problem of bioaccumulation of heavy metals is proving difficult to solve. Much work still needs to be done by scientists to develop reliable methods of detecting the presence of unacceptable levels of heavy metals, and in protecting waterways from exposure to heavy metal-containing pollution.
WORDS TO KNOW
Bioaccumulation: The buildup of a substance in a living thing to a concentration that is greater than the concentration of the substance in the natural environment.
Heavy metal: A group of metal elements that are poisonous even when present in low concentrations.