Chemical hazards may be described under three broad headings – flammability, reactivity and health.
Flammable substances are those that readily catch fire and burn in air. A flammable liquid does not itself burn; it is the vapours from the liquid that burn. For a liquid, the flash point, auto-ignition temperature, explosive limits, vapour density and ability to accumulate an electrostatic charge are important factors in determining the degree of fire hazard.
Reactive chemical hazards invariably involve the release of energy (heat) in relatively high quantities or at a rapid rate. If the heat evolved in a reaction is not dissipated, the reaction rate can increase until an explosion results.
Some chemicals decompose rapidly when heated. Light or mechanical shock can also initiate explosive reactions. Some compounds are inherently unstable and can detonate under certain conditions of pressure and temperature, while others react violently with water or when exposed to air.
Contact with many chemicals can result in adverse health effects. The nature and magnitude of toxic effects will depend on many factors including the nature of the substance, route of exposure, magnitude of the dose, duration of exposure, and individual susceptibility.