User Guide to Thresholds and Classification - Environmental ...

327User Guide for Thresholds and ClassificationsQSAR prediction that a substance is not rapidly degradable is considered a better justification for aclassification than application of a default classification, when no useful degradation data are available.VolatilisationChemicals may be removed from some aquatic environments by volatilisation. The intrinsic potential forvolatilisation is determined by the Henry‘s Law constant (H) of the substance. Volatilisation from the aquaticenvironment is highly dependent on the environmental conditions of the specific water body in question, suchas the water depth, the gas exchange coefficients (depending on wind speed and water flow) andstratification of the water body. Because volatilisation only represents removal of a chemical from waterphase, the Henry‘s Law constant cannot be used for assessment of degradation in relation to aquatic hazardclassification of substances. Substances that are gases at ambient temperature may however for examplebe considered further in this regard (see also Pedersen et al, 1995).No degradation data availableWhen no useful data on degradability are available – either experimentally determined or estimated data –the substance should be regarded as not rapidly degradable.General interpretation problemsComplex substancesThe harmonised criteria for classification of chemicals as hazardous for the aquatic environment focus onsingle substances. Certain types of intrinsically complex substance are multi-component substances. Theyare typically of natural origin and need occasionally to be considered. This may be the case for chemicalsthat are produced or extracted from mineral oil or plant material. Such complex chemicals are normallyconsidered as single substances in a regulatory context. In most cases they are defined as a homologousseries of substances within a certain range of carbon chain length and/or degree of substitution. When this isthe case, no major difference in degradability is foreseen and the degree of degradability can be establishedfrom tests of the complex chemical. One exception would be when a borderline degradation is foundbecause in this case some of the individual substances may be rapidly degradable and other may be notrapidly degradable. This requires a more detailed assessment of the degradability of the individualcomponents in the complex substance. When not-rapidly-degradable components constitute a significantpart of the complex substance (for example, more than 20%, or for a hazardous component, an even lowercontent), the substance should be regarded as not rapidly degradable.Availability of the substanceDegradation of organic substances in the environment takes place mostly in the aquatic compartments or inaquatic phases in soil or sediment. Hydrolysis, of course, requires the presence of water. The activity ofmicro-organisms depends on the presence of water. Moreover, biodegradation requires that the microorganismsare directly in contact with the substance. Dissolution of the substance in the water phase thatsurrounds the micro-organisms is therefore the most direct way for contact between the bacteria and fungiand the substrate.January 2012 EPA0109