DDT (dichlorodiphenyltrichloroethane) first came into production in 1939 by Dr. Paul Muller. It was widely utilised as a means of controlling common pests such as flies mosquitoes and lice. The use of DDT is now banned in both the United States and Australia.
HCB (hexachlorobenzene) was also used as a form of pest control since the 1940’s. In Australia, it was primarily used as a fungicide for the protection of stored grain.
Dieldrin, a Cyclodiene derivative, is one of the most persistent pesticides known, is used in the control of termites. Dieldrin is also a very stable compound.
The aromatic compounds such as PCBs (polychlorinated biphenyls), DDT and Dieldrin, tend to become bound up in sediments, allowing them to enter the food web. The US and the UK put a ban on the manufacture of PCBs in the late 1970’s, though despite this, PCBs are still reaching the environment. Currently, major sources of these include evaporation and leeching from landfills and incineration of hazardous waste. The harmful nature of organochlorines is due to their persistence, toxicity and their ability to accumulate in living tissues. These properties make organochlorines a very dangerous group of chemicals to which natural systems may be exposed to.
Most organochlorines are extremely stable. Their chlorine-carbon bond is generally very strong and resists being broken down by normal biochemical and physical processes. As the majority of organochlorines are foreign to nature, living organisms have not found many methods to detoxify them. As a result, organochlorines remain in the environment for a very long time. Some may take 100 years to fully break down into harmless chlorides, while some may not degrade at all to any significant extent.
When organochlorines do break down, they often will produce other organochlorines because their chlorine-carbon bond remains intact as part of another compound. These are often more toxic and far more hazardous than the original substance. An example of this occurring in the environment is trichloroethane found in groundwater degrading to highly toxic vinyl chloride.
Most organochlorines dissolve far more readily in fats than they do in water, therefore are inclined to build up in the fatty tissues of living organisms. The concentration of organochlorines through this process of bioaccumulation is massive.
Animals at the top of the food chain are exposed to the greatest concentrations of organochlorines. This process is called biomagnification, since the chemicals that accumulate at one level of the food chain are further concentrated at the next level, and again at the proceeding one after that.
Humans are at the top of their various food chains, and thus are exposed to high levels of organochlorines. For example, populations who eat large amounts of fish and other marine life are particularly at risk. These lipophilic chemicals can also be absorbed into the body via other routes such as absorption through the skin and inhalation.
Organochlorines that are accumulated in fatty tissues can be remobilised into the bloodstream and breast milk. In this way, organochlorines may be passed from one generation to the next.
As few living organisms have developed mechanisms to deal with these chemicals, organochlorines have potentially lethal effects. They have the ability to:
- Cause cancer
- Lead to infertility
- Impair the development of unborn young
- Damage the nervous system, liver, kidneys and other organs.
Ways in which organochlorines are thought to affect organisms are the mimicking of natural hormones, interference with cell function and interference with enzyme activity, all which may lead to hormone imbalances. The toxicity of the individual organochlorine compounds depend not only on the particular chemical concerned, but also on the species, strain, age and condition of the animal involved.
The effects of organochlorines on the endocrine, immune and reproductive systems of both animals and humans represent major interference on a large scale. Since both the contaminants and their toxic effects are passed on from one generation to the next, the impact of this interference may increase over the next century, even if the release of these chemicals were stopped immediately.
The damage to both the environment and humans can not be attributed to one single compound; rather a whole range of organochlorines may be responsible. Organochlorines are most often released as diverse mixtures including many compounds which can not be identified.
If we are to safeguard the health and survival of both humans and animals, action must be taken now to terminate the release of organochlorines and other toxic persistent chemicals into the environment. Otherwise, it may be too late.
References:
Setac, J. (1982). Environmental Toxicology and Chemistry. New York : Pergamon Press.
McDiarmid, C. (2000). A Brief History of Human Applications of Chlorine. Retrieved March 23, 2003, from
Weltman, E. (1995). Generations: Reproductive & Developmental Effects of Organochlorines. Retrieved March 21, 2003, from
Wiesner, D. (1991). Pesticides: the new plague of organochlorines. Retrieved March 20, 2003, from
Fawell, J. (1988). Environmental Toxicology : Organic Pollutants. New York : Halsted Press.
Harris, C. (2001). Chemosphere. Factors affecting the transfer of organochlorine pesticide residues to breast milk, 43 (2), 243-256.
Gabrielsen, G. (2001). Environmental Pollution. Biomagnification of organochlorines, 113 (2), 187-198.