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Polycyclic aromatic hydrocarbons (PAHs)

Polybrominated diphenyl ethers (PBDE)


Dioxin is the common name used to refer to the chemical 2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD. In addition to dioxin itself there are other compounds, such as the polychlorinated dibenzodioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and some polychlorinated biphenyls (PCBs), that have similar structures and activity as dioxin. These are often commonly referred to as dioxin-like compounds or "dioxins".

Dioxins are chemical contaminants that have no commercial usefulness by themselves. They are formed during combustion processes, such as waste incineration, forest fires and backyard trash burning, and during manufacturing processes such as herbicide manufacture and paper manufacture.

PCBs can be transported by various environmental media, including air and water, spreading contamination to other areas.

Wildlife and people are constantly exposed to dioxins through ingestion of dioxins that are present at low levels as environmental contaminants in food. Although they are at low levels in food , some dioxins are very slowly removed from the body and therefore they accumulate in fat tissue. In laboratory animals, dioxins are highly toxic, cause cancer, and alter reproductive, developmental and immune function.

The way in which dioxin affects cells is similar in some way to the way in which hormones such as estrogen work. Dioxin enters a cell and binds to a protein present in cells known as the Ah receptor. The receptor when bound to dioxin can then bind to DNA and alter the expression of some genes. This can lead to alterations in the level of specific proteins and enzymes in the cell. While it is not known exactly how changes in the levels of these different proteins cause the toxicity of dioxin, it is believed by most scientists that the initial binding of dioxin the Ah receptor is the first step.

(Summary from: US National Institute of Health, National Institute of Environmental Health Sciences (NIEHS). Dioxin Research at the National Institute of Environmental Health Sciences (NIEH). 2/28/2006).


These pesticides affect the nervous system by disrupting the enzyme that regulates acetylcholine, a neurotransmitter. Most organophosphates are insecticides. They were developed during the early 19th century, but their effects on insects, which are similar to their effects on humans, were discovered in 1932. Some are very poisonous (they were used in World War II as nerve agents). However, they usually are not persistent in the environment (1).

(1) US Environmental Protection Agency. Types of Pesticide - Chemical Pesticides.

Summary from: US National Institute of Health, National Institute of Environmental Health Sciences (NIEHS). Dioxin Research at the National Institute of Environmental Health Sciences (NIEH). 2/28/2006).

Polycyclic aromatic hydrocarbons (PAHs)

Polycyclic aromatic hydrocarbons (PAHs) are a family of chemical compounds made up of carbon and hydrogen atoms, with a molecular structure consisting of at least two fused aromatic rings, each with five or six carbon atoms. The PAH family includes about 100 substances, differing in the number and position of their rings.

PAHs are generally formed from the incomplete combustion of organic matter.  Forest fires and volcanoes are a source of PAHs. Today, most of the PAHs present in the environment come from human activities. PAHs can enter the aquatic environment directly, through industrial and municipal effluents, accidental crude oil spills and PAH emissions from creosote treated materials used in water (for example, on pilings). Nearly all types of organic fuel combustion can produce PAHs. The most important sources are the incomplete combustion of fossil fuels from domestic heating systems and transportation (car exhaust fumes), garbage incineration, aluminum reduction smelting for aluminum production, catalytic cracking of crude oil and coal liquefaction and gasification. All these human activities release PAHs into the atmosphere, where they tend to adhere to particles in suspension, some of which will enter the aquatic environment through atmospheric fallout. Owing to their low solubility and high affinity for particulate matter, PAHs are not usually found in water in notable concentrations. Their presence in surface water or groundwater is an indication of a source of pollution.

Animal studies have also shown that PAHs can cause harmful effects on the skin, body fluids, and ability to fight disease after both short- and long-term exposure. But these effects have not been seen in people.**

**Agency for Toxic Substances and Disease Registry (ATSDR). 1995. Toxicological Profile for Polycyclic Aromatic Hydrocarbons (PAHs). Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service.

Polybrominated diphenyl ethers (PBDE)

Polybrominated diphenyl ethers (PBDE) are a class of widely used fire retardants most commonly employed in building materials, textiles, furnishings, electronics, and plastics.  Because they are physically combined (not chemically bound) with materials, PBDEs can migrate out of products over time and into the environment. Environmental release of PBDEs occurs in production waste streams, municipal waste incineration and treatment, and from landfills.  PBDEs accumulate in fat tissue, and have been detected in tissues of humans and both aquatic and terrestrial wildlife.  Because of their tendency to be stored in the tissue of plants and animals, PBDEs are passed through the food chain, putting top predators at greater risk of higher exposures. Monitoring studies indicate that PBDE concentrations in soil, sediment, humans, and wildlife are increasing.  A retrospective study of herring gull eggs collected from the Great Lakes from 1981 to 2000 found that concentrations increased exponentially, with a doubling time of 2-3 years (Norstrom et al 2002).

Research into the effects of wildlife exposure to PBDEs has just begun.   While PBDEs do not appear to be acutely toxic, early studies have found effects to physiological and reproductive parameters in the lab.  More research is required to determine the effects of chronic exposure to environmentally-relevant levels of these contaminants in the field.

Norstrom R.J., M. Simon, J. Moisey, B. Wakeford, and D.V.C. Weseloh. 2002. Georgaphic distribution (2000) and temporal trends (1981-2000) of brominated diphenyl ethers in Great Lakes herring gull eggs. Environmental Science and Technology 36:4783-4789.

Acute Toxicity: The ability of a substance to cause severe biological harm or death soon after a single exposure or dose. Also, any poisonous effect resulting from a single short-term exposure to a toxic substance. EPA — Terms of Environmnt: Glossary, Abbreviations and Acronyms


In the early 1980s, scientists documented that waterfowl in marshy areas of California's San Joaquin Valley were producing offspring that were stillborn or deformed. Scientists determined that selenium (chemical symbol Se) was the cause. Selenium is an essential trace element that occurs naturally in the environment. Although small amounts of selenium are important for both wildlife and people, elevated concentrations can cause selenium toxicosis. Fish and wildlife can be exposed to selenium in water, sediment or food. In parts of the western United States, the soils contain rather high levels of selenium. There, selenium is most commonly found in rocks and soil that originated in the ocean (marine sedimentary deposits).

Parts of the west are highly productive farming areas. However, the long summers and hot, dry climate that makes the west so attractive for farming also demands the extensive use of irrigation. Unfortunately, long-term irrigation flushes out the salt, selenium and other trace elements found in the soil into irrigation return flows. Traditionally, these selenium-tainted agricultural waters have been disposed of in evaporation ponds. However, evaporation ponds are an attractive nuisance to birds that stop to feed on plants and animals found at the ponds. These plants and animals often contain high concentrations of selenium, which the waterbirds then accumulate in their tissue, causing the kind of problems scientists began observing in the Valley in the 1980s.

Agency for Toxic Substances and Disease Registry (ATSDR), ToxFAQs™ for Selenium,


Agency for Toxic Substances and Disease Registry (ASTDR):


* These chemicals are Persistent Organic Pollutants (POPs). POPS are chemical substances that persist in the environment, bioaccumulate through the food web, travel long distances and pose a risk of causing adverse effects to human health and the environment. United Nations Environment Programme: Substance Profiles for the Persistent Organic Polluntants.


Last updated: February 13, 2013