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http://www.ca5.uscourts.gov/opinions%5Cpub%5C93/93-02564.CV0.wpd.pdf

Flammable gas
Flammability is measured by the ISO 10156:1996 test method.

Flammable aerosols
The GHS uses the general definition of an "aerosol", that is an aerosol spray, but also includes similar dispensing systems which produce foams rather than true aerosols. An aerosol dispensing system typically contains a propellant gas under pressure and a liquid phase: some aerosols also contain small solid particles in suspension in the liquid. An aerosol is potentially flammable if it contains a flammable gas, a flammable liquid or a flammable solid. However if the proportion of flammable components is less than 1% and the heat of combustion is less than 20 kJ/g, the aerosol is not classified as flammable (and may be transported under UN transport class 2.2).

The heat of combustion may either be taken from literature values of the standard enthalpy change of combustion (multiplied by a factor of 0.95 to take account of incomplete combustion) or measured experimentally by standard methods such as ASTM D240,ISO/FDIS 13943:1999 or NFPA 30B. The Spray ignition test, the Enclosed space ignition test and the Aerosol foam flammability test are described in the UN Manual of Tests and Criteria.

Flammable liquids
Under the GHS, a flammable liquid is any liquid with a flash point of no more than 93 ºC.

Flammable solids
The GHS and the UN Manual of Tests and Criteria both use the phrases "flammable solid" and "readily combustible solid" in recognition of the fact that many solids with burn without a flame (but while giving off heat and decomposing). The distinction is not important for GHS classification, which is based on the UN Model Regulations.

The Burning rate test is described in the UN Manual of Tests and Criteria.

Self-reactive substances and mixtures
The category of "self-reactive substances or mixtures" is wide, and runs from some extremely shock sensitive explosives to other substances which pose no significant risk in their use of transport. The GHS definition is based on the UN Recommendations for the Transport of Dangerous Goods: the following are not classified under this hazard group, even though they may have similar hazards: After these three groups have been removed, the GHS defines "self-reacting substances and mixtures" as those remaining which
 * Class 1 goods, that is "explosives" under the transport regulations;
 * Organic peroxides, although the categorization scheme for these substances is essentially the same as for other self-reactive substances except for the transport class;
 * Oxidizing agents.

Substances or mixtures for which the heat of decomposition is less than 300 J/g, or for which the self-accelerating decomposition temperature for a 50 kg package is higher than 75 ºC, do not need to be classified under this heading. The heading also includes a "type G", which are substances or mixtures which might otherwise meet the definition of "self-reactive", but which have no significant transport or handling risks in that respect.

Pyrophoric liquids and solids
Pyrophoric substances will catch fire spontaneously on contact with air. The GHS contains a single category, but distinguishes between liquids and solids because of the different transport hazards. Pyrophoric substances may be considered to be the extreme case of self-heating substances (see below).

Self-heating substances and mixtures
Self-heating susbtances and mixtures will gain temperature dangerously when exposed to air and without any external heat source. They may eventually catch fire, but only when large quantities are exposed to air and after a period of hours or days: this is the difference from substances classed as "pyrophoric" (see above). In general, the risk is higher with larger amounts of the substance or mixture and at higher ambient temperatures.

For transport purposes, the threshold level of self-heating is that of charcoal, which will eventually catch fire in air at 50 ºC if packed as a cube of 27 m3 volume (sides of 3 metres): substances which are more hazardous than charcoal must be labelled for transport. The practical tests are carried out on smaller quantities and at higher temperatures.