8 different hazard classes largely based on the United Nations Dangerous Goods System. Additions and changes have been necessary since the scope of the GHS includes all target audiences. Explosives, which are assigned to one of six subcategories depending on the type of hazard they present, as used in the UN Dangerous Goods System. Gases are category 1 flammable if they start to flame in a range in air at 20 °C (68 °F) and a standard pressure of 101.3 kPa. Category 2 is Non flammable and non toxic gases, and category 3 is toxic gases. Substances and mixtures of this hazard class are assigned to one of two hazard categories on the basis of the outcome of the test or calculation method. A flammable liquid is a liquid with a flash point of not more than 93 °C (199.4 °F). Substances and mixtures of this hazard class are assigned to one of four hazard categories on the basis of the flash point and boiling point. A pyrophoric liquid is a liquid that, even in small quantities, is liable to ignite within five minutes after coming into contact with air. Substances and mixtures of this hazard class are assigned to a single hazard category on the basis of the outcome of the UN Test N.3. A flammable solid is one that is readily combustible or may cause or contribute to fire through friction. Readily combustible solids are powdered, granular, or pasty substances which are dangerous if they can be easily ignited by brief contact with an ignition source, such as a burning match, and if the flame spreads rapidly. it is further divided into • flammable solids, • polymerizing substances self-reactive substances, are thermally unstable solids liable to undergo a strongly exothermic thermal decomposition even without participation of oxygen (air), other than materials classified as explosive, organic peroxides or as oxidizing. pyrophoric substance more colloquially described as spontaneously combusting substances are those solids or liquids that even in small quantities are liable to ignite within five minutes after coming into contact with air. Substances and mixtures of this hazard class are assigned to a single hazard category on the basis of the outcome of the UN Test N.2. Self-heating substances A self-heating solids or liquids, other than a pyrophoric substance, is one which, by reaction with air and without energy supply, are liable to self-heat. Substances and mixtures of this