a 0, will not burn; 1, must be preheated to burn; 2, ignites when moderately heated; 3, ignites at normal temperature; 4, extremely flammable.
SOURCE: Adapted from NFPA (1991b), pp. 325M-11 to 94.
and methyl alcohol (11.1 °C). The degree of hazard associated with a flammable liquid also depends on other properties, such as its ignition point and boiling point. Commercially obtained chemicals are now clearly labeled as to flammability and flash point. Consider the example of acetone given in section 3.C.1.3.1. At ambient pressure and temperature, an acetone spill can produce a concentration as high as 23.7% acetone in air. Acetone is not particularly toxic. However, with a flash point of -18 °C and upper and lower flammable limits of 2.6% and 12.8% acetone in air, respectively (see Table 3.7), it is clear that an acetone spill produces an extreme fire hazard. Thus the major hazard given for acetone in the LCSS is flammability.
The ignition temperature (autoignition temperature) of a substance, whether solid, liquid, or gaseous, is the minimum temperature required to initiate or cause self-sustained combustion independent of the heat source. The lower the ignition temperature, the greater the potential for a fire started by typical laboratory equipment. A spark is not necessary for ignition when the flammable vapor reaches its autoignition temperature. For instance, carbon disulfide has an ignition temperature of 90 °C, and it can be set off by a steam line or a glowing light bulb. Diethyl ether has an ignition temperature of 160 °C and can be ignited by the surface of a hot plate.
3.D.1.2.3Limits of Flammability
Each flammable gas and liquid (as a vapor) has two fairly definite limits of flammability defining the range of concentrations in mixtures with air that will propagate a flame and cause an explosion. At the low extreme, the mixture is oxygen rich but contains insufficient fuel. The lower flammable limit (lower explosive limit (LEL)) is the minimum concentration (percent by volume) of the fuel (vapor) in air at which a flame is propagated when an ignition source is present. The upper flammable limit (upper explosive limit (UEL)) is the maximum concentration (percent by volume) of the vapor in air above which a flame is not propagated. The flammable range (explosive range) consists of all concentrations between the LEL and the UEL. This range becomes wider with increasing temperature and