Undesirable substance" more commonly known as contaminants – are defined in legislation as any substance or product, with the exception of pathogenic agents, which is present in and/or on the product intended for animal feed and which presents a potential danger to animal or human health or to the environment or could adversely affect livestock production. The range of substances covered by the Directive comprises arsenic, lead, mercury, dioxin and certain mustards.
Generally, these undesirable substances are either naturally-occurring environmental contaminants that are present at low levels in feed and food products, particularly vegetable crops drawing nutrients directly from the soil, or are process contaminants that may be introduced into the feeding stuff either during or as a consequence of its treatment, manufacture and storage.