Also known as HAZMATs, hazardous materials are substances which pose some sort of danger to people or to the environment if handled incorrectly. They fall into several general categories:

• Toxic or poisonous materials are chemically dangerous to people or other organisms. Cyanide, mercury, and PCBs are examples of materials which are hazardous due to their toxicity.
• Flammable (or, properly, inflammable) materials are ones which can easily be ignited or burst into flame, and thus pose a fire hazard. Most of these are fuels, such as gasoline or propane. Materials which can burn, but which are not particularly volatile -- such as wood -- are not generally considered hazardously flammable.
• Explosive materials are substances which can explode if handled improperly. Nitroglycerin is one such. Also, some flammable materials can be explosive under specific circumstances.
• Radioactive materials emit various kinds of radiation. Hazardous radiation can cause radiation sickness, cancer, or birth defects. Uranium and plutonium are radioactive HAZMATs.
• Corrosive materials are strong acids or bases which can cause damage to other materials (often including skin and flesh) with which they come into contact. Hydrofluoric acid and lye are corrosive.
• Biohazardous materials are hazardous because of the germs they may carry -- bacteria, viruses, or other microbes which can cause people or animals to become ill. Medical waste, such as blood and sharps, are considered biohazardous.

Hazardous materials are sometimes described with more specific warnings: a particular poison may be labeled not only as toxic, but also as carcinogenic, for instance. There are also various other sorts of chemical hazards not included in the categories above: metallic sodium, for instance, reacts violently with water; pure oxygen is not itself flammable, but dramatically hastens the combustion of other substances.