The term heavy metal refers to any metallic chemical element that has a relatively high density and is toxic or poisonous at low concentrations. Examples of heavy metals that are harmful to humans include mercury, lead, and arsenic. Chronic exposure to these metals can have serious health consequences. Humans are exposed to heavy metals through inhalation of air pollutants, consumption of contaminated drinking water, exposure to contaminated soils or industrial waste, or consumption of contaminated food. Food sources such as vegetables, grains, fruits, fish and shellfish can become contaminated by accumulating metals from surrounding soil and water. Heavy metal exposure causes serious health effects, including reduced growth and development, cancer, organ damage, nervous system damage, and in extreme cases, death. Exposure to some metals, such as mercury and lead, may also cause development of autoimmunity, in which a person's immune system attacks its own cells. This can lead to joint diseases such as rheumatoid arthritis, and diseases of the kidneys, circulatory system, and nervous system.
Metals are particularly toxic to the sensitive, rapidly developing systems of fetuses, infants, and young children. Some metals, such as lead and mercury, easily cross the placenta and damage the fetal brain. Childhood exposure to some metals can result in learning difficulties, memory impairment, damage to the nervous system, and behavioral problems such as aggressiveness and hyperactivity. At higher doses, heavy metals can cause irreversible brain damage. Children may receive higher doses of metals from food than adults, since they consume more food for their body weight than adults.
Mercury is a toxic heavy metal and a persistent environmental pollutant. Exposure to mercury is associated with serious adverse health and developmental effects, especially in pregnant women, developing fetuses, and young children. Industrial activity results in releases of millions of pounds of mercury into the environment each year, primarily in the form of air emissions from coal-fired power plants. Mercury also is released into the environment by municipal and medical waste incineration, mining, and smelting. Once in the environment, elemental mercury can be transformed by microorganisms to organic forms, most notably methylmercury. Methylmercury is of particular concern because it accumulates in plants, animals, fish, and the human body, and it is more toxic at low doses than other forms of mercury. The health risks associated with mercury are damage to the nervous system and deformities in infants exposed to mercury in the womb. At levels well below World Health Organization limits, it has been shown to affect unborn fetuses and their embryonic nervous systems, leading to learning difficulties, poor memory and shortened attention spans. Low-level exposures also adversely affect male fertility.
Lead is a soft, heavy, blue-gray metal that occurs naturally in the earth's crust. It has spread throughout the environment and into our homes and workplaces through the burning of fossil fuels, mining, and manufacturing. Lead affects almost every organ system in the human body. The central nervous system is particularly vulnerable in infants and children under age six. The effects are the same whether it is breathed or swallowed. Large amounts of lead exposure may lead to blood anemia, severe stomachache, muscle weakness, and brain damage. Lower levels of exposure, may affect a child's mental and physical growth leading to learning disabilities and seizures. Children can be exposed to lead by drinking contaminated water, eating lead-based paint chips, chewing on objects painted with lead-based paint or swallowing house dust or soil containing lead.
Nearly half of a million children living in the United States have blood lead levels high enough to cause irreversible damage to their health. During the last twenty years the U.S. has made significant efforts and enacted legislation to remove lead from gasoline, paints, and many other products. Lead cannot be destroyed or eradicated, thus lead from past medical and scientific products and from old paints and discarded batteries remains in the environment.
Arsenic is a steely grey metal that is widely distributed in the Earth's crust and found naturally in the environment. It cannot be destroyed or eradicated. Because it occurs naturally in the environment it is possible to be exposed to it through air, water and soil contact. There are two different types of arsenic, organic and inorganic. Organic arsenic compounds consist of arsenic combined with oxygen and hydrogen. There is very little information available on the effects of organic arsenic compounds in humans. Inorganic arsenic compounds consist of arsenic combined with oxygen, chlorine and sulfur. The Department of Health and Human Services (DHHS), The International Agency for Research on Cancer (IARC) and EPA have classified inorganic arsenic as a known human carcinogen. Several studies have shown that inorganic arsenic can cause lung, bladder, liver, kidney, prostate and skin cancer. Emerging science also shows that inorganic arsenic may harm pregnant women and their fetuses. Arsenic has been shown to cross the placenta to the fetus and has been found in breast milk. Chronic exposure to arsenic has been shown to affect child development, lowering their IQ scores.