Chemicals in our environment [Archives:2007/1025/Community]
By: Mohammed Alhifi
Environmental Sciences,
University of Sana'a,
Education Faculty of Arhab
[email protected]
One of the consequences of technological progress and industrial revolution has been the release of a large number of chemicals into the environment although considerable research efforts have been made into a variety of alternative pest control methods in recent years. Use of chemical pesticides is still a mainstay in modern agriculture and public health programmers.
At the same time anxiety over chemical pollution has made the study of hazardous effects of pesticides as one of the principle areas of research. In particular organochlorine insecticides have been extensively investigated and today several of them have been suspended from general application in the U.S. and other countries. Nevertheless, its felt for the time being that pesticides are a necessary evil and, in absence of any feasible substitute, their use will continue to combat the vast range of vectors of diseases.
With the slow development of civilization, so man has gradually realized the extent to which pests harm his crops, annoy him and transmit diseases to both human and domestic animals. The use of chemicals to kill pests is not a new concept. As early as the late 16th century arsenic could be used to kill insects and the Chinese used arsenic sulphide as an insecticide.
The use of arsenical compounds has continued, and during the early part of the 20th century, large quantities of such compounds as lead arsenate were used to control insect pests. Another arsenical compound Paris green (Copper Aceto-arsenite) was extensively applied to pools of standing water in the tropics, in attempts to control malaria-transmitting mosquitoes. It was not realized at the time how persistent arsenical pesticides were, although it is now known that they can persist in soil for 40 years, and many orchard soils still contain large amount of these chemicals.
For instance, in a recent survey of arsenic residues in arable soils in Canada, residues of arsenic ranging from 11-121 parts per million were reported. Although it was known that organochlorine were very persistent, up till the early 1950s there was little anxiety as to possible long-term ecological hazards caused by their use. There was some evidence that large residues in soil could be pytotoxic, small quantities of some were reported from plant and animal tissues and in cows' milk and there were some instances of fish being killed when water was sprayed in anti-malaria and other pest campaigns, but unavoidable hazards and of little concern.
As pesticides pass in most cases directly or indirectly into food products. For many pesticides legal limits (tolerance doses) are imposed regarding the residues that may be left in foodstuffs. By “tolerance dose” it is meant the quantity of a substance that may be absorbed by one person from his daily diet in the course of a lifetime without coming to any harm as far as can be judged from present scientific knowledge.
The level at which a pesticide dose not make damage to the biological system is expressed as milligrams per kilogram of body weight. In ordinary circumstances, one hundredth part of this dose is then prescribed as the safe limit for human being.
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