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.

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.

DDD (1,1-dichloro-2,2-bis(p-chlorophenyl)ethane)- DDD is produced by metabolism of DDT in some organisms.

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.

endrin- (return to top)
Endrin is more acutely toxic than the rest of the cyclodiene group, but is less persistent in the environment 

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.
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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.

There are three main groups of organophosphates:
Aliphatic are simple phosphoric acid derivatives bearing short carbon chains.  Some of these are malathion, trichlorfon, monocrotophos, dimethoate, dicrotophos, oxydemetonmethyl, disulfoton, dichlorvos, mevinphos, methamidophos, and acephate.

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.

Hydrocarbons (return to top)
 Hydrocarbons are compounds that contain only carbon and hydrogen.  They are classified as aliphatic or aromatic.  Aliphatic hydrocarbons are not linked together to form a ring.  Compounds include the alkanes, alkenes, and alkynes, and substances derived from  them by replacing one or more hydrogen atoms by atoms of other elements or groups of atoms.  Aromatic hydrocarbons have elements linked together in rings.

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.  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.

Inorganics: (return to top)