Toxicity
From Environment & Energy Wiki
The following information is an edited excerpt from Wikipedia [1]
Toxicity is the degree to which something is able to produce illness or damage to an exposed organism. Toxicity can refer to the effect on a whole organism, such as a human or a bacterium or a plant, or to a substructure, such as a cell (cytotoxicity) or an organ (organotoxicity such as the liver (hepatotoxicity). In the context of EEWiki, it will refer uniquely to the effects on man and laboratory animals.
In the science of toxicology, toxicity is the degree of impact of an external substance or condition and its deleterious effects on living things: organisms, organ systems, individual organs, tissues, cells, subcellular units is the subject of study. A central concept of toxicology is that effects are dose-dependent; even water – generally not considered to be toxic – can lead to water intoxication when taken in large enough doses, whereas for even a very toxic substance such as snake venom, there is a dose below which there is no detectable toxic effect.
Toxicity is the ability of a chemical or physical agent to induce detrimental temporary or permanent tissue change or to detrimentally interfere with normal biochemical processing.
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Types of toxicity
In this context, chemicals in the air, water or soil may be considered. These include inorganic substances such as lead, hydrofluoric acid, and chlorine gas and organic compounds such as methanol, tetrachloromethane etc.
Toxicity can be measured by the effects on the target (organism, organ, tissue or cell). Because individuals typically have different levels of response to the same dose of a toxin, a population-level measure of toxicity is often used which relates the probability of an outcome for a given individual in a population. When such data does not exist, estimates are made by comparison to known similar toxic things, or to similar exposures in similar organisms. Then "safety factors" are added to account for uncertainties in data and evaluation processes. For example, if a dose of toxin is safe for a laboratory rat, one might assume that one tenth that dose would be safe for a human, allowing a safety factor of 10 to allow for interspecies differences between two mammals; if the data are from fish, one might use a factor of 100 to account for the greater difference between two chordate classes (fish and mammals). Similarly, an extra protection factor may be used for individuals believed to be more susceptible to toxic effects such as in pregnancy or with certain diseases. Or, a newly synthesized and previously unstudied chemical that is believed to be very similar in effect to another compound could be assigned an additional protection factor of 10 to account for possible differences in effects that are probably much smaller. Obviously, this approach is very approximate; but such protection factors are deliberately very conservative and the method has been found to be useful in a wide variety of applications.
The uptake of toxic substances can also vary widely across mammals, not only because of different sensitivities within a given metabolic path, but also because of different metabolic paths. This is illustrated, as an example, by rodents developing cancers when exposed to certain halocarbon solvents, such as trichloroethene (trichloroethylene) which, although somewhat toxic to humans at high doses, are not considered carcinogenic.
Assessing all aspects of the toxicity of cancer-causing agents involves additional issues, since it is not certain if there is a minimal effective dose for carcinogens, or whether the risk is just too small to see. In addition, it is possible that a single cell transformed into a cancer cell is all it takes to develop the full effect (the "one hit" theory).
It is more difficult to assess the toxicity of chemical mixtures than of single, pure chemicals because each component display its own toxicity and components may interact to produce enhanced or diminished effects. Common mixtures include fuels, cigarette smoke and industrial waste.
The determination of the toxicity of a substance to a species, including humans, may also be affected by confounding factors, such as genetic make-up (e.g., some races are more susceptible to certain toxins than others), life-styles (e.g., someone who is a heavy smoker may be more - or less - affected by a toxin than a non-smoker), local pollution etc.
Factors influencing toxicity
The toxicity of a substance can be affected by many different factors, such as the pathway of administration (whether the toxin is applied to the skin, ingested, inhaled or injected), the time of exposure (a brief encounter or long term), the number of exposures (a single dose or multiple doses over time), the physical form of the toxin (solid, liquid, gas), the genetic make-up of an individual, an individual's overall health, and many others. Several of the terms used to describe these factors have been included here.
- acute exposure
- a single exposure to a toxic substance which may result in severe biological harm or death; acute exposures are usually characterised as lasting no longer than a day.
- chronic exposure
- continuous exposure to a toxin over an extended period of time, often measured in months or years can cause irreversible side effects.
