|Organochlorines:||DDT, DDE, Chlordane, Aldrin, Dieldrin, Endrin, Heptachlor epoxide, PCBs, Dioxin|
|Inorganics:||arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium, zinc|
|Other contaminants:||Carbamates, Hexachlorobenzene, Hydrocarbons, Organophosphate, Chlorophenols|
|Organochlorines are compounds that contain carbon, chlorine, and hydrogen.
Their chlorine-carbon bonds are very strong which means that they do not
break down easily. They are highly insoluble in water, but are attracted
Since they resist metabolism and are readily stored in fatty tissue of any animal ingesting them, they accumulate in animals in higher trophic levels. This may occur when birds eat fish that have been exposed to the contaminant. It may also affect humans if they drink milk of a dairy cow that has ingested the chemical because the chemical is excreted in its milk fat. This is called biological magnification.
The intentional production and the unintentional release have elevated levels of these chemicals in the biosphere far beyond natural background levels.
|DDT and related organochlorines: (return to top)|
DDT (originally named dichloro diphenyl trichoroethane) was first synthesized in 1873 by a German graduate student, but it was forgotten about until 1938 when Dr. Paul Muller rediscovered it while searching for a long-lasting insecticide against the clothes moth. It was extremely effective against flies and mosquitoes and was used extensively in North America as a domestic and agricultural pesticide until it was banned by the EPA on January 1, 1973 due to its long residual life and its accumulation in food chains. The terms DDT or DDTs are often used to refer to a family of isomers ( p,pí-DDT and o,pí-DDT) and their breakdown products (p,pí-DDE, o,pí-DDE, p,pí-DDD, and p,pí-DDD). DDT data are often expressed as the sum of these six components.
|Relatives of DDT (return
Dichlordiphenyldichloroethylene (DDE) is a product of the breakdown of DDT in an organism's body. It is produced in most animals when the body tries to rid itself of DDT.
One major problem related to contaminants identified in fish-eating birds is the thinning of eggshells. This is caused by the presence of DDE in female birds. Eggshells are made of calcium carbonate which is made from calcium and carbon dioxide in the bird's body. This reaction takes a specific enzyme which is inhibited by the DDE. Since the eggshell does not contain as much calcium carbonate as it should it is thinner than normal and more prone to breakage during incubation by the adult.
Dicofol: Dicofol is an organochlorine miticide used on a wide variety of fruit, vegetable, ornamental and field crops. It is manufactured from DDT, however, modern manufacturing processes can produce technical grade dicofol which contains less than 0.1% DDT. Dicofol is moderately persistent in soil, with a half-life of 60 days. It is susceptible to chemical breakdown in moist soil and is subject to degradation by UV light. It is practically insoluble in water and adsorbs very strongly to soil particles. It is therefore nearly immobile in soils and unlikely to infiltrate groundwater
Three other relatives of DDT are methoxychlor, ethylan, and chlorobenzilate.These are all pesticdes which stick strongly to soil particles. They do not dissolve easily in water nor evaporate easily into the air. They may take several months to break down.
|Other organochlorines: (return
Cyclodienes: Cyclodienes were developed after World War II. Generally they are persistent insecticides and are stable in soil. They have been used in greatest quantity as soil insecticides for the control of termites and soil-borne insects.
Chlordane is a cyclodiene that was used extensively in home and agricultural applications. Chlordane is a term that represents a group of a large number (140) of individual compounds. Some of them are: cis-chlordane, trans-chlordane, cis-nonachlor, trans-nonachlor, and oxychlordane. Chlordane data are usually expressed as the sum of several of the most abundant and persistent components and metabolites of the technical chlordane mixture. Like DDT, chlordane compounds are very persistent in the environment, resistant to metabolism, have a strong affinity for lipid, and biomagnify in aquatic food webs.
Trans-Nonachlor is one of the major constituents of the insecticide chlordane. It was used extensively prior to 1983. Its use was canceled after 1988 due to concern about the risk of cancer. Trans-nonachlor is the most bioaccumulative of the chlordanes.
|Aldrin and Dieldrin: (return
Aldrin and dieldrin are insecticides with similar structures. Sunlight and bacteria change aldrin to dieldrin so we mostly find dieldrin in the environment. From 1950-1970, aldrin and dieldrin were popular pesticides for crops like corn and cotton. They bind tightly to soil and slowly evaporate to the air. Dieldrin is stored in the fat of a body and leaves the body very slowly. Because of concerns about damage to the environment and the potential harm to human health, EPA banned all uses of aldrin and dieldrin in 1974 except to control termites. In 1987, EPA banned all uses.
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Heptachlor epoxide- (return to top)
Heptachlor epoxide is the biological conversion product of Heptachlor which is an insecticide used in the control of termites and it has been used in the cotton industry. The epoxide is more toxic than the parent heptachlor pesticide.
Action: All organochlorine insecticides are nerve poisons but
DDT has a different action than the chlorinated cyclodienes. DDT acts on
the sodium channels in the nervous system so that the passage of an 'action
potential' along the nerve is disrupted. It causes uncontrolled repetitive
spontaneous discharges along the nerve. Uncoordinated muscle tremors and
twitches are characteristic symptoms. The chlorinated cyclodienes
act on the GABA receptors which function as a channel for chloride ions
through the nerve membranes. They bind to the GABA receptors and reduce
the flow of chloride ions. Typical symptoms include convulsions.
PCBs (return to top)
PCBs (polychlorinated biphenyls) were first introduced into commerce in 1929 and became widely used in electrical transformers, cosmetics, varnishes, inks, carbonless copy paper, pesticides and for general weatherproofing and fire-resistant coatings to wood and plastic. Because PCBs degrade very slowly in the environment and build up in the food chain. They have been banned from further production in many countries. There are 209 different PCB compounds called congeners. A congener may have between 1 and 10 chlorine atoms, which may be located at various positions on the PCB molecule.
ortho-PCBs, are congeners
of PCBs that have one or more chlorines in ortho-position (positions 2
or 6). Non-ortho PCBs can assume a completely flat (planar) conformation,
which is close to that of dioxins. Chlorines in para-positions seem
to be more easily biodegraded, thus ortho-congeners are present in the
environment at higher concentrations than non-ortho congeners.
Dioxin is the popular name for a class of organochlorines known as polychlorinated dibenzo-p-dioxins (PCDDs) or dibenxofurans (PCDF). The entire "dioxin" family consists of 75 different dioxins and 135 different furans.
Dioxins and furans are by-products of the production of certain chemicals (such as those used to produce some pesticides and wood preservatives), of the chlorine bleaching process used in some pulp and paper mills, and of the incomplete combustion of materials that contain both chlorine atoms and organic matter. Although they are most often associated with industrial activities, some natural occurrences such as forest fires are believed to make a small contribution to the presence of dioxins and furans in the environment.
The dioxin 2,3,7,8-tetrachloro-p-dibenzo-dioxin (2,3,7,8-TCDD) is the most toxic of the dioxins and furans. The toxicity of mixtures of dioxins and furans is usually expressed in terms of this substance, that is, in 2,3,7,8- TCDD toxic equivalents ( TCDD EQs or TEQs) which is the concentration of 2,3,7,8 TCDD that would be expected to produce the same type and degree of response as the chemicals involved. In egg tissue extracts this is measured using the H411E-extract bioassay.
Chlorophenols are synthetic organic compounds which have fungicidal and bactericidal properties.They have primarily been used for long-term wood preservation and for short-term wood protection to control sap stain and mold on
freshly cut lumber. Two of the main members of this family used for wood preservation are pentachlorophenol (PCP) and tetrachlorophenol (TCP).
Organophosphates (OPs) is a generic term to include all the insecticides containing phosphorus. They are often called organic phosphates, phosphorus insecticides, nerve gas relatives, phosphates, phosphate insecticides, and phosphorus esters or phosphoric acid esters. They are all derived from phosphoric acid and are generally the most toxic of all pesticides to vertebrate animals. They are chemically unstable so they have virtually replaced the persistent organochlorine compounds. They exert their toxic action by inhibiting the cholinesterase enzymes of the nervous system which results in the accumulation of acetylcholine. This interferes with neuromuscular junctions producing rapid twitching of the voluntary muscles and eventually paralysis.
There are three main groups of organophosphates:
Phenyl derivatives contain a benzene ring with one of the ring hydrogens displaced by attachment to phosphorus and others are frequently displaced by other groups. These are generally more stable than the aliphatic OPs so their residues are longer lasting. Some of these are parathion, stirofos, profenophos, sulprofos, and isofenphos.
Heterocyclic derivatives contain ring structures which are composed of different or unlike atoms. One of more of the carbon atoms is displaced by oxygen, nitrogen, or sulfur and their rings may have three, five, or six atoms. Some of these are diazinon, azinphosmethyl, chlorpyrifos, methidathion, phosmet, and dialifor. These are complex molecules and generally have longer lasting residues than many of the aliphatic or phenyl OPs. They also have many breakdown products which makes it difficult to measure their residues in the laboratory.
The carbamate are derivatives of carbamic acid. They also inhibit the enzyme cholinesterase. These insecticides have very low mammalian oral and dermal toxicity and they can be used for a broad spectrum of insect control.
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.
Inorganics: (return to top)
|Arsenic||A relatively common element that occurs in air, water, soil, and tissues. Large quantities are released each year as a result of agricultural and industrial activities. Large quantities may be hazardous to fish and wildlife. It is present in water contaminated by mine tailings, smelter wastes, and natural mineralizations. (return to top)|
|Cadmium||A heavy metal released into the environment in a variety of ways such as smelting, burning coal and oils, and from wear of vehicle tires. Effects of cadmium poisoning include bone damage, suppressed egg production, eggshell thinning in birds and kidney damage. (return to top)|
|Chromium||At high concentrations chromium is a mutagen and a carcinogen. It is introduced into the environment from sewage and solid wastes. (return to top)|
|Copper||Copper is an essential element for many animals and is a component of many metalloenzymes and respiratory pigments. It is one of the the most common contaminants found in urban runoff. It is generally more toxic to aquatic organisms than to birds or mammals. (return to top)|
|Lead||All measured effects of lead are adverse. It reaches the aquatic environemnt through industrial and municipal discharges, atmospheric deposition, weathering processes and highway runoff. It usually inhibits the formation of heme, and adversely affects blood chemistry. It way also cause neurological effects such as degenerative lesions present in the meylin of brachial nerves. (return to top)|
|Mercury||Mercury is a metal that can exist in a range of organic and inorganic forms with varying degrees of toxicity. It is a mutagen, teratrogen, and a carcinorgen and causes embryocidal, cytochemical, and histopathological effects. In aquatic organisms it adversely affects reproduction, growth, behavior, osmoregulation, and oxygen exchange. It may also cause kidney lesions, neurological damage, and reduced food intake leading to weight loss, progressive weakness in wings and legs and an inability to coordinate muscle movements in marine birds. It can enter the environment through industrial and domestic coal combustion, waste incineration, gold mining, and the dumping of sewage sludge. (return to top)|
|Nickel||Nickel enters the environment as a contaminant through mining and smelting operations, combustion of fossil fuels, and other industrial processes. It has been shown that when injected in chicken eggs the nickel produced embyronic malformations and death. (return to top)|
|Selenium||Selenium has the ability to bioaccumulate in aquatic food chains and thereby contaminate the diet and induce reproductive effects in fish and birds. One source is the weathering of rocks. Two major human related causes are the processing and combustion of fossil fuels and the irrigation of seleniferous solids or with selenium contaminated water. Sensitivity to selenium vaires from one class of organisms to another. (return to top)|
|Zinc||Zinc is required for normal growth and development in animals. Excess zinc may suppress or inhibit tumor proliferation and can be teratogenic to frog and fish embyros. (return to top)|
The structural formulas on this page came from the UNEP Chemical site for Persistant Organic Pollutants