Breakdown of a substance catalyzed by enzymes in vitro or in vivo. In ecotoxicology, it is almost en-
tirely due to microbial or fungal activity.
Note 1:    Biodegradation  may  be  classified  for  purposes  of  hazard assessment into  three  cate-
gories:

1. Primary. Alteration of the chemical structure of a substance resulting in loss of a specific property of that substance.
2. Environmentally acceptable. Biodegradation to such an extent as to remove unde-sirable properties of the compound. This often corresponds to primary biodegradation but depends on the circumstances under which the products are discharged into the environment.
3. Ultimate. Complete breakdown of a compound to either fully oxidized or reduced simple molecules (such as carbon dioxide, methane, nitrate, ammonium, and (or) water).

Note 2:    The products of biodegradation can be more harmful than the substance that was de-
graded.