The toxicity of even simple phenols varies considerably -about 50-fold for the few compounds given as examples in Table 1. Further, the toxicity values in Table 1 are expressed as ranges in a few cases. This is done to firmly impress upon the reader the essential fact that there is no single toxicity value for a phenol, or indeed, for any other compound. The toxicity of a chemical is a function of the biological response to the chemical. Thus, toxicity depends upon the chemical, the duration of exposure to the chemical, the mode of exposure, the test conditions, the type of organism exposed to the chemical, the age, health, and gender of the test organism, and the property that is measured as an indicator of the biological response. For these reasons, substantial variation is the rule rather than the exception. Toxicity values of course vary from species to species, for the same chemical - but they also vary from test to test, for the same species, exposed in the same manner, in tests of the same duration.
As a gross generalization, the toxicity of simple phenols decreases in the order:
phenol > ^-cresol > o-cresol > rn-cresol > catechol
These types of phenols do not bioaccumulate to any great extent, and when diluted tend to biodegrade to nontoxic products fairly rapidly (half-lives typically range from days to weeks).
In general, the toxicity of substituted phenols tends to increase with the bulk of the phenolic molecule (i.e., the extent of branching and the number of substitutions). Strongly electronegative atoms in the substituent positions increases a phenol's toxicity.
Halogenated phenols tend to be more toxic and less biodegradable than their nonhalogenated counterparts, and the toxicity of phenols and their resistance to biodegradation increase as the degree of halogenation increases. Pentachlorophenol (PCP), for example, is more toxic and resistant to biodegradation than phenol or o-chlorophenol.
The toxicity of man-made phenols to aquatic organisms is of particular interest because most phenols are at least moderately soluble in water (i.e., >1 ppm). Further, many phenols are used in very large quantities for diverse purposes. Thus, fugitive phenolics are inevitable and these can be moved about readily by water. In Canada, the level of hydroquinone and resorcinol deemed protective of freshwater systems is <2 and <12.5 mgl_1, respectively. These two types of phenols can create unpleasant tastes in fish and shellfish, even at low concentrations. A level of 50 mgl_1 is considered to be protective of freshwater systems for most other types of phenols.
The more highly halogenated phenols pose special environmental problems: many are persistent and tend to bioaccumulate. Several types of man-made phenols also have endocrine-disrupting properties and are biologically active at low concentrations.
Quantitative structure-activity relationship (QSAR) modeling has been used to predict the toxicity of phenols, with reasonable accuracy.
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