Biodegradation occurs mainly through microbial enzyme attack, and enzyme-catalysed reactions are known to depend on hydrophobic, electronic and steric effects. However, most QSBR studies involve correlation with a single parameter, but there is no consistency regarding the class of parameter. This suggests that biodegradation occurs through a range of different mechanisms. A few QSBR studies have reported the need to include more than one class of parameter in correlations.