Chemical classes are groupings that relate chemicals by similar features. Chemicals can be classified by their structure (e.g., hydrocarbons), uses (e.g., pesticides), physical properties (e.g., volatile organic compounds [VOCs]), radiological properties (e.g., radioactive materials), or other factors. The chemical classes identified below are ones used by the Agency for Toxic Substances and Disease Registry (ATSDR) to address hazardous substances.