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Learn about harmful materials and illnesses caused by exposure

*The information on this page was obtained from the National Institute of Environmental Health Sciences, available at: 

Environmental agents that may cause negative health effects:


Dioxins are a class of chemical contaminants that are formed during combustion processes such as waste incineration, forest fires, and backyard trash burning, as well as during some industrial processes such as paper pulp bleaching and herbicide manufacturing. The most toxic chemical in the class is 2,3,7,8-tetrachlorodibenzo-para-dioxin (TCDD). 

Humans are primarily exposed to dioxins by eating food contaminated by these chemicals. Dioxin accumulates in the fatty tissues, where they may persist for months or years. People who have been exposed to high levels of dioxin have developed chloracne, a skin disease marked by severe acne-like pimples. Studies have also shown that chemical workers who are exposed to high levels of dioxins have an increased risk of cancer. Other studies of highly exposed populations show that dioxins can cause reproductive and developmental problems, and an increased risk of heart disease and diabetes. More research is needed to determine the long-term effects of low-level dioxin exposures on cancer risk, immune function, and reproduction and development.

Endocrine Disruptors

Endocrine disruptors are chemicals that may interfere with the body’s endocrine system and produce adverse developmental, reproductive, neurological, and immune effects in both humans and wildlife. A wide range of substances, both natural and man-made, are thought to cause endocrine disruption, including pharmaceuticals, dioxin and dioxin-like compounds, polychlorinated biphenyls, DDT and other pesticides, and plasticizers such as bisphenol A. Endocrine disruptors may be found in many everyday products– including plastic bottles, metal food cans, detergents, flame retardants, food, toys, cosmetics, and pesticides developmental, reproductive, neurological and immune effects. Research shows that endocrine disruptors may pose the greatest risk during prenatal and early postnatal development when organ and neural systems are forming.


Lead is a highly toxic metal found in small amounts in the earth’s crust. Because of its abundance, low cost, and physical properties, lead and lead compounds have been used in a wide variety of products including paint, ceramics, pipes, solders, gasoline, batteries, and cosmetics. Since 1980, federal and state regulatory standards have helped to minimize or eliminate the amount of lead in consumer products and occupational settings. Today, the most common sources of lead exposure in the United States are lead-based paint in older homes, contaminated soil, household dust, drinking water, lead crystal, and lead-glazed pottery. While extreme lead exposure can cause a variety of neurological disorders such as lack of muscular coordination, convulsions and coma, much lower lead levels have been associated with measurable changes in children’s mental development and behavior. These include hyperactivity; deficits in fine motor function, hand-eye coordination, and reaction time; and lowered performance on intelligence tests. Chronic lead exposure in adults can result in increased blood pressure, decreased fertility, cataracts, nerve disorders, muscle and joint pain, and memory or concentration problems.


Mercury, also known as quicksilver, is a naturally-occurring metal that is toxic to living organisms. Metallic or elemental mercury — an odorless, shiny, silver-white liquid — is commonly used in thermometers, barometers and fluorescent light bulbs. Metallic mercury is extremely dangerous with a few drops generating enough fumes to contaminate the air in a room. Furthermore, skin contact with the metal results in the absorption of mercury into the blood stream and potential health problems. 

Mercury may be released naturally into the air from volcanoes and the earth’s crust, but man-made sources include the incineration of waste and coal-burning power plants. Once these industrial activities release mercury into the air, it ultimately falls back to earth, is fixed by plankton into methylmercury and is concentrated up the food chain by the fish that eat them. 

NIEHS, the Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) have studied the physiological effects of methylmercury, a common form of organic mercury (mercury combined with carbon), on humans via fish consumption. Although fish and shellfish have many nutritional benefits, consuming large quantities of fish increases a person’s exposure to mercury. Pregnant women who eat fish high in mercury on a regular basis run the risk of permanently damaging their developing fetuses. Children born to these mothers may exhibit motor difficulties, sensory problems and cognitive deficits.

To educate the public on mercury’s toxic affects, the FDA and EPA released a consumer advisory in 2004 that recommends that young children, women who are pregnant or who may become pregnant and nursing mothers should avoid fish that contain high levels of methylmercury, such as Shark, Swordfish, King Mackerel and Tilefish. Instead, they should eat up to 12 ounces (two average meals) a week of a variety of fish and shellfish that contain lower concentrations of methylmercury. Five of the most common "low-mercury" fish are shrimp, canned light tuna, salmon, pollock and catfish.


Molds are microscopic organisms that play an important role in the breakdown of plant and animal matter. Outdoors, molds can be found in shady, damp areas or places where leaves or other vegetation is decomposing. Indoor molds can grow on virtually any surface, as long as moisture, oxygen, and organic material are present. When molds are disturbed, they release tiny cells called spores into the surrounding air. Exposure to these spores can produce symptoms such as nasal and sinus congestion, eye irritation and blurred vision, sore throat, chronic cough, and skin rash. After contact with certain molds, Individuals with chronic respiratory disease may have difficulty breathing and people who are immunocompromised may be at increased risk for lung infection. A study conducted by NIEHS-funded scientists shows that mold exposure during the first year of life may increase the risk of childhood asthma.


Ozone is a highly reactive form of oxygen. In the upper atmosphere, ozone forms a protective layer that shields us from the sun’s ultraviolet rays. At ground level, ozone is a harmful air pollutant and a primary constituent of urban smog. Ozone is produced when air pollutants from automobile emissions and manufacturing operations interact with sunlight. Long-term exposure to high concentrations of ozone can cause a significant reduction in lung function, inflammation of the airways, and respiratory distress. People with lung diseases are particularly vulnerable to the respiratory effects of ozone. Results from an NIEHS-funded study showed that children who played three or more outdoor sports in areas with high ozone concentrations were more than three times as likely to develop asthma as children who did not engage in sports activities.


A pesticide is any substance used to kill, repel, or control certain forms of plant or animal life that are considered to be pests. Pesticides include herbicides for destroying weeds and other unwanted vegetation, insecticides for controlling a wide variety of insects, fungicides used to prevent the growth of molds and mildew, disinfectants for preventing the spread of bacteria, and compounds used to control mice and rats. Because of the widespread use of agricultural chemicals in food production, people are exposed to low levels of pesticide residues through their diets. Scientists do not yet have a clear understanding of the health effects of these pesticide residues. Results from the Agricultural Health Study, an ongoing study of pesticide exposures in farm families, show that farmers who used agricultural insecticides experienced an increase in headaches, fatigue, insomnia, dizziness, hand tremors, and other neurological symptoms. Evidence suggests that children are particularly susceptible to adverse effects from exposure to pesticides, including neurodevelopmental effects. People may also be exposed to pesticides used in a variety of settings including homes, schools, hospitals, and workplaces.

Diseases and conditions that may be linked to exposure to environmental agents:


Asthma is an inflammatory disease of the lung. This inflammatory process can occur along the entire airway from the nose to the lung. Once the airway becomes swollen and inflamed it becomes narrower, allowing less air through to the lung tissue and causing symptoms such as wheezing, coughing, chest tightness, and trouble breathing. Once considered a minor ailment affecting only a small portion of the population, asthma is now the most common chronic disorder of childhood, and affects an estimated 6.2 million children under the age of 18. The fact that asthma runs in families suggests that genetic factors play an important role in the development of the disease, however, environmental factors also contribute to the disease process. Asthma can be triggered by a wide range of substances called allergens.


Autism is a spectrum of disorders that causes severe impairment in thinking, feeling, language, and the ability to relate to others. People with autism spectrum disorders show deficits in social interaction, verbal and nonverbal communication, and unusual responses to sensory experiences. Autism spectrum disorders are usually first diagnosed in early childhood. Although recent studies indicate that the number of autism cases is increasing dramatically each year, the causes of this disorder are not well understood. Twin and family studies suggest an underlying genetic vulnerability to autism, and a growing area of research indicates that the disease may be caused by an interaction between genetic and environmental factors. One hypothesis is that the disease may be triggered during early fetal development, and that environmental exposures during pregnancy could cause or contribute to the disorder. 

Breast Cancer

Breast cancer is the most common non-skin cancer among women in the United States, and the leading cause of cancer death for U.S. women aged 20–59. It is the leading cause of cancer death for women overall worldwide. Although scientists have identified many risk factors that increase a woman’s chances of developing breast cancer, they do not yet know how these risk factors work together to cause normal cells to become cancerous. Most experts agree that breast cancer is caused by a combination of genetic, hormonal, and environmental factors. NIEHS-funded scientists have demonstrated that night-time exposure to artificial light can stimulate the growth of human breast tumors in mice by suppressing the levels of a key hormone called melatonin. These results might explain why female night shift workers have higher rates of breast cancer than the general population, and may offer an explanation for the rise in breast cancer incidence in industrialized countries. 


Cancer arises when cells are unable to repair DNA damage and experience abnormal cell growth and division. The process known as metastasis occurs when cancer cells travel to other parts of the body via the bloodstream and replace normal tissue. According to the American Cancer Society, this chronic disease is the second leading cause of death in the U.S. with half of all men and one-third of all women developing some form of cancer during their lifetimes.

Although cancer is responsible for 23% of all deaths in the U.S., millions of Americans have recovered from the disease. People may reduce their chances of getting cancer by employing prevention methods such as having regular screenings and living a healthy lifestyle.

Lung Diseases

Lung disease refers to any disease or disorder in which the lungs do not function properly. Lung disease is the third leading killer in the United States, responsible for one in seven deaths, and is the leading cause of death among infants under the age of one. Some lung diseases, like asthma and emphysema, involve a narrowing or blockage of the airways resulting in poor air flow. Others, including pulmonary fibrosis, pneumonia and lung cancer, are caused by a loss of elasticity in the lungs that produces a decrease in the total volume of air that the lungs are able to hold. Research has shown that long-term exposure to air pollutants can reduce lung growth and development and increase the risk of developing asthma, emphysema, and other respiratory diseases. Results from the NIEHS-supported Harvard Six Cities Study, the largest available database on the health effects of outdoor and indoor air pollution, show a strong association between exposure to ozone, fine particles and sulfur dioxide, and an increase in respiratory symptoms, reduced lung capacity, and risk of early death.


Lupus is a chronic disease that causes the immune system to attack the body’s own tissues and organs, resulting in inflammation of the skin, joints, blood, and kidneys. Common symptoms of lupus include joint and muscle pain, fatigue, low-grade fever, skin rashes, chest pain, unusual hair loss, anemia, and sensitivity to sunlight. While genetic susceptibility plays an important role in the development of lupus, little is known about the contribution of environmental factors to the disease process. Preliminary results from the Carolina Lupus Study, the first population-based study to examine the influence of hormonal and occupational exposures on systemic lupus, show that workers who were exposed to crystalline silica dust, a mineral found in rocks and soil, had a two- to four-fold increased risk for developing the disease.

Parkinson’s Disease

Parkinson’s disease is a chronic neurodegenerative disease, the second most prevalent such disorder after Alzheimer’s disease. It is a slowly progressive disease caused by the death of small clusters of cells in the midbrain that results in reduction of a critical neurotransmitter called dopamine, the chemical messenger responsible for coordinated muscle movement. While research shows that genetic factors play a significant role in one type of Parkinson’s called early-onset in which symptoms begin before the age of 50, genetic predisposition is not considered a contributing factor when symptoms develop later in life. Researchers are testing the hypothesis that Parkinson’s may be triggered by contact with pesticides and other chemical pollutants.

Reproductive Health

Reproductive health refers to the diseases, disorders and conditions that affect the functioning of the male and female reproductive systems during all stages of life. Disorders of reproduction include birth defects, developmental disorders, low birth weight, preterm birth, reduced fertility, impotence, and menstrual disorders. Research has shown that exposure to environmental pollutants may pose the greatest threat to reproductive health. Exposure to lead is associated with reduced fertility in both men and women, while mercury exposure has been linked to birth defects and neurological disorders. A growing body of evidence suggests that exposure to endocrine disruptors, chemicals that appear to disrupt hormonal activity in humans and animals, may contribute to problems with fertility, pregnancy, and other aspects of reproduction.

Visit the National Institute for Environmental Health Sciences at for more information about the health effects of environmental agents.