in oxygen-rich atmospheres and also changes depending on the presence of other components. The limitations of the flammability range, however, provide little margin of safety from the practical point of view because, when a solvent is spilled in the presence of an energy source, the LEL is reached very quickly and a fire or explosion will ensue before the UEL can be reached.
Several systems are in use for classifying the flammability of materials. Some (e.g., Class I—flammable liquid, etc., see Chapter 4) apply to storage or transportation considerations. Another (Class A, B, C—paper, liquid, electrical fire) concerns the type of fire extinguisher to be used (see Chapter 6, section 6.F.2 on emergency equipment). To assess risk quickly, the most direct indicator is the NFPA (National Fire Protection Association) system, which classifies flammables according to the severity of the fire hazard with numbers 0 to 4 in order of increasing hazard: 0, will not burn; 1, must be preheated to burn; 2, ignites when moderately heated; 3, ignites at normal temperature; 4, extremely flammable (Figure 3.1). Substances rated 3 or 4 under this system require particularly careful handling and storage in the laboratory. Some vendors include the NFPA hazard diamond on the labels of chemicals. The Fire Protection Guide on Hazardous Materials (NFPA, 1991) is a comprehensive listing of flammability data and ratings.
The NFPA fire hazard ratings, flash points, boiling points, ignition temperatures, and flammability limits of a number of common laboratory chemicals are given