Cancer is a disorder of cells in the body which carries a high risk of premature death.  It begins with a group of cells that fail to respond to the normal control mechanism and continue to divide without need.  The new growths are called tumours or neoplasia and may be either "benign" or "malignant".  A "benign" tumour is one that remains localised whereas "malignant" tumours invade neighbouring tissues, enter blood vessels, lymphatic vessels and other spaces and can be carried to other areas of the body to form new tumours called "secondaries" or "metastases".

Cancer may arise from various causes, one of which is the adverse effects of certain substances on the cells of the body either directly or via their metabolites.  Although it is often difficult to prove a causal link between exposure to a certain chemical and subsequent cancer, there is strong evidence to link a number of compounds with cancer.  There is also evidence to suspect that a number of compounds are cancer causing. 

Under the UK Control of Substances Hazardous to Health (COSHH) Regulations, a carcinogenis defined as a substance or preparation which either:

• is classified for labelling purposes as a carcinogenic category 1 or 2 (see below) carrying the risk phrases R45 ‘May cause cancer’, or R49 ‘May cause cancer by inhalation’; or
• would be classified if the European system for classifying substances and preparations dangerous for supply was applied (e.g. for by-products such as hardwood dust)

The Chemicals (Hazard Information & Packaging for Supply) (CHIP) Regulations itself divides carcinogens into three categories:

Includes list of substances classified as Carcinogenic, Mutagenic or Toxic to Reproduction.

List of carcinogens

• OSHA (Occupational Safety & Health Administration)
• Carcinogenic Potency Project
• Oxford University list
• Royal Society of Chemistry