Flammable, Inflammable, and Nonflammable: How to Choose the Right Word

The first two mean exactly the same thing

flammable and inflammable

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The adjectives "flammable" and "inflammable" mean exactly the same thing: easily set on fire and capable of burning quickly. Metaphorically speaking, "inflammable" also can mean easily angered or excited. Of the two terms, the older word for something capable of burning is "inflammable," but early in the 19th century the word "flammable" was coined as a synonym for "inflammable." The adjective "nonflammable" means not easily set on fire.

How to Use Inflammable

Despite beginning with "in-," "inflammable" does mean "burnable," and it has since at least 1605, according to the Oxford English Dictionary The prefix "in-" can make a word negative, as in "incapable," "inflexible," and "incompetent, but it also can add emphasis, as in "invaluable," "inflame," and "intense." The prefix also can mean "within," as in "incoming," "inbreeding," and "infighting."

The "in-" of "inflammable," called an intensive or an intensifier, is of the emphatic type. But people came to believe that the prefix was confusing, which could be dangerous in emergency signage, so "inflammable" is falling out of use.

How to Use Flammable

"Flammable," the new kid on the block, didn't appear in print until more than 300 years later. In the 1920s the National Fire Protection Association began using "flammable" instead of "inflammable," which it thought was confusing because of the negative-sounding beginning of the word. Insurance companies and fire-safety advocates soon agreed.

In 1959, the British Standards Institution announced: 'In order to avoid any possible ambiguity, it is the Institution's policy to encourage the use of the term 'flammable' and 'nonflammable' rather than 'inflammable' and 'nonflammable.' "

So, which word should a careful writer use? According to "Origins of the Specious: Myths and Misconceptions of the English Language," by Patricia T. O'Conner and Stewart Kellerman:

"Well, history may be on the side of 'inflammable,' but common sense wins here. If you want to be sure you're understood—say, the next time you see a smoker about to light up near a gas pump—go with 'flammable.' "

But "inflammable" hasn't disappeared. According to Merriam-Webster's Dictionary of English Usage, "Our files show that both forms continue to be used. 'Flammable' seems to be less common in British English than it is in American English and 'inflammable' correspondingly more common. 'Flammable' is used literally; figurative use belongs to 'inflammable.' "

How to Use Nonflammable

For a time, a substance that couldn't easily catch fire was referred to as being "noninflammable." "Nonflammable" began to replace that term as "flammable" become more prominent.


Here are some sample sentences illustrating the difference between the three words:

  • Flammable (or inflammable) or combustible liquids should not be stored in stairways or in areas used for exits.
  • For several decades, foam has been used as the primary agent for fighting fires involving flammable (or inflammable) liquids.
  • The Douglas fir and the giant sequoia of western North America have developed thick, nonflammable bark to insulate the living tissue from the heat of the flames.
  • The most common nonflammable gas of all the compressed gases is oxygen.
  • When you are working with lasers, make sure that there are no flammable (or inflammable) materials in the immediate vicinity.