3. What are the responses of a system when exposed to poisons?The human body needs very small quantities of some chemicals that are poisonous in large doses. This applies, for example, to some metals, such as copper, magnesium and manganese, which pose a problem in places of work. The adverse effect is strongly related to the dose. The ultimate effect is death. Usually the effects of toxic chemicals are less severe, from altered food consumption to serious health problems.
3.1 Human bodyThe effects may be immediate or delayed, and they may be reversible or irreversible toxic effects (see Part 1., Introduction to Safety in the Use of Chemicals).Local/systemic toxicity There are two main ways in which chemicals may exert their effects. Local effects occur at the area of the body which has been in contact with the chemical. Examples are injuries from acids or lung injuries from inhaled reactive gases. Systemic effects occur after the chemical has been absorbed and distributed from the entry point to other parts of the body. Most substances produce systemic effects, but some substances may cause both types of effects. An example is tetraethyl lead, which is a gasoline additive and produces skin effects at the contact site. It is absorbed and transported into the body causing typical effects on the central nervous system and on other organs. Target organs The degree of the toxic effect is not the same in all organs. Usually there are one or two organs which show the major toxic effect. These are referred as target organs of toxicity of the particular substance. The central nervous system is the target organ of toxicity most frequently involved in systemic effects. The blood circulation system, liver, kidneys, lungs and skin follow in frequency of systemic effects. Muscle and bones are the target organs for a few substances. The male and female reproduction systems are vulnerable to many substances.
An allergic reaction, or sensitization as it is also called, may appear after repeated contact to a substance. Once the sensitization has bee produced, even very low doses can provoke a reaction. The different allergies are numerous, varying from minor skin irritation to very severe or even fatal reactions. The pattern of sensitization varies according to the species. In humans, the skin and the eyes are the most common areas of allergic response, whereas, for example, in the guinea pigs reactions are more common in the respiratory system. Interactions The effect of simultaneous exposure to two or more substances may differ from a simple additive effect (1+1=2). Organophosphate pesticides, such as dialiphos, naled and parathion, are examples of chemicals where the combined effect is the sum of the effects observed when the chemicals act individually. The effect can be more than the sum of the individual effects of two chemicals (e.g., 1+1=4). An example of an increase in risk is with asbestos fibres and cigarette smoking. They act together: the risk of developing lung cancer after exposure to asbestos fibres is forty times greater for a smoker than for a non-smoker. Another pair of the chemicals where the combined risk is greater than a mere additive effect are the solvents, trichloroethylene and styrene. The adverse effects of two substances may counteract one another (1+1=0). This effect is used to find an antidote to a poison. In other cases, a substance may not cause harm on its own but may make the effect of another chemical much worse (0+1=3). For example, two commonly used solvents isopropanol and carbon tetrachloride have this kind of joint effect. Isopropanol, at concentrations which are not harmful to the liver, increases the liver damage caused by carbon tetrachloride. In some cases, when the exposure to a substance is repeated the body may decrease its sensitivity to the substance, i.e. it increases its tolerance to it. 3.2 The environmentThe environment has a certain capacity to biodegrade toxic substances. However, some substances are resistant to decomposing processes. The adverse effects increase with the concentration of these substances and their accumulation in foodchains.In the natural environment, large numbers of potentially toxic substances are present. In some cases, when the substance is on its own it would cause no harm but it may interact with other toxic substances or under specific conditions it may be concentrated or transformed to a more dangerous compound. An example of an air pollution reaction is the production of photochemical smog. Chlorinated hydrocarbons such as DDT and dieldrin have similar chemical and biological effects. When present together they lead to more serious effects than when acting separately. To assess the effects of toxic substances in the environment some indicators of ecotoxicity are used. In laboratory, fish and insects called Daphnia (water-flea) are used to test acute toxic effects in the aquatic environment. Green algal species are also used in the assessment of water pollution. ANNEX 1. Factors that may influence the human
reproduction system and the effects
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BIBLIOGRAPHYARBETSMILJÖ (Working Environment/ The Work Environment Association), Your body at work, 2nd Ed., Stockholm 1987CLAYTON G.D. and CLAYTON F.E., ed., Patty's Industrial Hygiene and Toxicology, 3rd Revised Ed., John Wiley & Sons Inc., USA 1978 East African Newsletter on Occupational Health and Safety, Supplement 2/ 1989, Institute of Occupational Health, Finland 1989 ILO, International Labour Organisation, Encyclopedia of Occupational Health and Safety, Vol I - III, Geneva 1983 IPCS, International Programme on Chemical Safety, Chemical Safety, Fundamentals of Applied Toxicology, Training Module No 1, Geneva 1992 IPCS, International Programme on Chemical Safety, How to use the IPCS Health and Safety Guides KLAASSEN C.D., AMDUR M.O. and DOULL J., ed., Casarett and Doull's Toxicology, The Basic Science of Poisons, 3rd Ed., Macmillan Publishing Company, New York 1986 NIOSH, RTECS Database, CCINFO Disc, Canadian Centre for Occupational Health and Safety, Hamilton 1993 SAX N.I. and LEWIS R.J.Sr, Dangerous Properties of Industrial Materials, VII ed., Nostrand Reinhold Co., New York 1988 SEILER H.G. and SIGEL H., Handbook of Toxicity of Inorganic Compounds, Marcel Dekker, Inc., New York 1988 |