In Merriam-Webster's Collegiate Dictionary, Eleventh Edition (2003), the first definition of combustible is "capable of combustion," the first definition of combustion is "an act or instance of burning," the only definition of flammable is "capable of being easily ignited and of burning quickly," and the only definition of flammability is "ability to support combustion; esp : a high capacity for combustion."
On the strength of these definitions, I would have expected the terms combustible and flammable to be largely or entirely interchangeable. But in the course of an (as yet unsuccessful) online search for historical examples in which people mistakenly caused fires because they misunderstood the term inflammable to mean "not capable of bursting into flames," I came across this passage in "Hazard Connotation of Fire Safety Terms," in Proceedings of the Human Factors and Ergonomics Society, 54th Annual Meeting- 2010 (link to .pdf copy of article):
Prior research suggests that people misunderstand the meanings of the words Combustible and Flammable (Main, Frantz, & Rhoades, 1993). According to the National Fire Protection Association (NFPA), Combustible substances are those with a flash point of 100 to 150 degrees Fahrenheit. Substances classified as Flammable have flash point temperatures between 20 and 100 degrees Fahrenheit.
Thus from the standard/regulatory definitions, Flammable presents a greater fire hazard than Combustible. However, research by Main et al. (1993) suggests that people are more likely to believe that Combustible connotes greater hazard than Flammable.
The NFPA's definitions specify that Flammable substances can ignite at lower temperatures than Combustible substances can, and thus are a greater fire hazard. My questions are:
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Do the NFPA's technical definitions of Flammable substances and Combustible substances reflect a fundamental difference in meaning of the everyday words flammable and combustible, or did the NFPA just arbitrarily assign different flash-point levels to the two terms in its standards?
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Do other English-speaking countries share the NFPA's understanding of Flammable and Combustible, or is this a U.S.-only thing?
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In the United States, how widespread is public awareness of the distinction between Flammable and Combustible?
Best Answer
Inspired by a provocative (in a good way) comment by FumbleFingers (above), I did some research into the evolution of the definitions of combustible, flammable, and inflammable in Webster's dictionaries over the past 200 years. The entries for flammable are a bit spotty—absent from the 1806 dictionary, present in the next four editions (1828 through 1890), and then absent from the Collegiate series dictionaries until the Sixth Collegiate (1949). (I expect that flammable appears in the big New International (1909) and Second New International (1934) dictionaries, but those overlap with the Collegiate series, and I find the pared-down listings in the Collegiates a better guide to which words were in widespread, approved use, in the judgment of the Merriam-Webster authorities of the time.) The other two terms are listed in all of the dictionaries I consulted.
Here is a quick summary of how the wording of the relevant definitions has changed over the years:
A Compendious Dictionary of the English Language (1806):
An American Dictionary of the English Language (1828):
An American Dictionary of the English Language (1847):
An American Dictionary of the English Language (1864):
Webster's International Dictionary 1890):
Webster's Collegiate Dictionary, First Edition (1898):
Webster's Collegiate Dictionary, Second Edition (1910):
Webster's Collegiate Dictionary, Third Edition (1916):
Webster's Collegiate Dictionary, Fourth Edition (1931):
Webster's Collegiate Dictionary, Fifth Edition (1936):
Webster's New Collegiate Dictionary, Sixth Edition (1949):
Webster's New Collegiate Dictionary, Seventh Edition (1963):
Webster's New Collegiate Dictionary, Eighth Edition (1973):
Webster's New Collegiate Dictionary, Ninth Edition (1983):
Merriam-Webster's Collegiate Dictionary, Tenth Edition (1993):
Merriam-Webster's Collegiate Dictionary, Eleventh Edition (2003):
...
I notice three especially interesting things about these evolving definitions. First, two very early definitions (those from 1828) support FumbleFingers' emphasis on the relatively easy and rapid burning behavior envisaged by inflammable: Whereas combustible is associated with wood and coal, inflammable is associated with oils and spirits. This important difference emerges despite previous definitions (in the 1806 dictionary) that offered no indication that such distinctions existed. Just as important, the "oils or spirits" example remains associated with inflammable through the most of the next century (all the way through the 1890 Webster's International dictionary).
Second, though the adverb easily persists in the definitions of inflammable of the middle period (from 1890 until 1963), the explicit inclusion of "combustible" as a definition of inflammable and of "inflammable" as a definition of combustible suggests something approaching interchangeability of the terms during this period, at least in general use.
Third, reemphasis of differences between the two terms begins in the Seventh Collegiate (1963), which clears the way for the new era of (relatively) stark contrast by removing "inflammable" as a definition of combustible for the first time since the 1847 dictionary, and by removing "combustible" as a definition of inflammable for the first time since the 1864 dictionary. The vivid addition to the definition of flammable in the Seventh Collegiate ("capable of being easily ignited and of burning with extreme rapidity") makes the revised definition so much more precise than the Sixth Collegiate's definitions of flammable ("Capable of being easily ignited; inflammable") and of inflammable ("Capable of being easily set on fire; combustible") that I can't help wondering whether the altered definitions of 1963 reflect a sudden dramatic shift between 1949 and 1963 in the way most people used those words, or whether Merriam-Webster's lexicographers were trying to establish a definition of flammable that was consistent with the terminology used by fire prevention authorities, regardless of how contemporaneous nonexpert writers understood the terms combustible, flammable, and inflammable.