Toxicology is defined as "the study of the adverse effects of chemical, physical or biological agents on living organisms and the ecosystem" and is based on the 16th century principle that any substance can be toxic if consumed in sufficient quantity. In a general sense, the toxicity of a substance could be defined as the capacity to cause injury to a living organism.  A highly toxic substance will damage an organism if administered in very small amounts; a substance of low toxicity will not produce an effect unless the amount is very large. Thus, toxicity cannot be defined without reference to the quantity of a substance administered or absorbed, the way in which this quantity is administered (e.g. inhalation, ingestion, injection) and distributed in time (e.g. single dose, repeated doses), the type and severity of injury, and the time needed to produce that injury. The vital factor that determines the amount of harm that a chemical compound produces is the quantity of the compound that comes in contact with a biologic system. This quantity of the compound is commonly called the "dose.". If a sufficient dose is taken into the body or comes in contact with a biologic mechanism, a harmful effect will be the consequence in the sense that the ability of that biologic mechanism to carry on a function is destroyed or seriously impaired. As the dose is increased from minimal to maximal levels, there is no sudden appearance of undesirable effects from any chemical agent. Rather, the response, whether it be beneficial or harmful, is a graded response and is related to progressive changes in dose. One of the most fundamental observations which may be made with respect to any biologic effect of a chemical agent is the relationship between the dose (or concentration) and the response that is obtained. Thus, toxicology has developed into the study of the quantitative effects of chemicals on biologic tissue. Its focus is on the harmful actions of chemicals on biologic tissue, but in the quest for information regarding the harmful actions of chemicals, the toxicologist also acquires information which is relevant to the degree of safeness of the compound. The word "toxic" may be considered synonymous with harmful in regard to the effects of chemicals. Many chemicals are so nonselective in their action on tissues or cells that they may be said to exert an undesirable or harmful effect on all living matter. Furthermore, such chemicals may be effective in rather small concentrations. In contrast to this, a given chemical may be sufficiently selective in its ability to produce harm that it acts only on specific cells. A chemical may be harmful to essential systems in several species of organisms, but capable of exerting its harmful effect only in a few of these species because of protective devices present in the resistant species. When a chemical is said to be toxic, the average person interprets this to mean that it would have a harmful or undesirable effect on humans. This may not be true when the toxicologist uses the word "toxic" and "toxicity," because it is evident that what may be considered harmful to one biologic specimen may be relatively harmless to another specimen; in fact, a chemical that is toxic to some organisms may be desirable as far as man is concerned. For example, a chemical could be harmful or even lethal to the mosquito, but relatively harmless, and therefore indirectly beneficial and desirable, to mankind. For this reason, man can make use of chemicals to his advantage solely because they may be toxic or harmful to some biologic mechanism. Therefore, if the term toxic or toxicity is used, it is necessary to identify the biologic mechanism on which the harmful effect is produced. Toxicity is a relative property of a chemical and may be directly or indirectly desirable or undesirable as far as man is concerned, but toxicity always refers to a harmful effect on some biologic mechanism.  Today,...
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