It has been observed that the linear relationship between the logarithm of bioconcentration factor (log BCF) of highly hydrophobic chemicals and their log K<SUB>ow</SUB> breaks when log K<SUB>ow</SUB> becomes greater than 6.0. Consequently, super hydrophobic chemicals were not thought to cause baseline toxicity as a single compound. Researchers often call this phenomenon as ""hydrophobicity cutoff"" meaning that bioconcentration or corresponding baseline toxicity has a certain cutoff at high log K<SUB>ow</SUB> value of hydrophobic organic pollutants. The underlying assumption is that the increased molecular size with increasing hydrophobicity prohibits highly hydrophobic compounds from crossing biological membranes. However, there are debates among scientists about mechanisms and at which log K<SUB>ow</SUB> this phenomenon occurs. This paper reviews three hypotheses to explain observed ""cutoff"": steric effects, kinetic or physiological limitations. and chemical activity cutoff. Although the critical molecular size that makes biological membranes not permeable to hydrophobic organic chemicals is uncertain, size effects in combination with kinetic limitation would explain observed non-linearity between log BCF and log K<SUB>ow</SUB>. Chemical activity of hydrophobic chemicals generally decreases with increasing melting point at their aqueous solubility. Thus, there may be a chemical activity cutoff of baseline toxicity if there is a critical chemical activity over which baseline effects can be observed.
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