Chemicals are important to modern society but many also pose threats to human health and the environment. Contamination concerns relate to persistence, distribution, and damage to health. The following criteria are used to judge whether substances cause contamination problems.

• Toxicity to humans
• Toxicity on the aquatic environment
• Toxicity on the ecosystem
• Effect on materials and structures used on sites
• Persistence in soil
• Tendency accumulate in living tissues
• Occur in significant concentrations on many sites

The HSE sets Occupational Exposure Limits (OELs) for about 400 hazardous substances in the workplace, now in conjunction with EU standards.

Of 100,000 chemicals registered in Europe, EU has a list of 129 hazardous ones. There is a Directive of Dangerous Substances to coordinate efforts to control chemical contamination.

The UK government has a "Red List" of dangerous substances requiring special discharge consents.

World Health Organisation - Priority List of Toxic chemicals