FLAMMABLE VS. INFLAMMABLE - Actually, the in- in inflammable was derived from the Latin preposition meaning en- (like enflamed), not the Latin prefix meaning -un. It's not like everyone knew the derivation of the word, so the change probably made sense. However, confusion persists today regarding which word to use. Flammable is the preferred modern term for a material that catches fire readily. Inflammable means the same thing. If a material won't burn easily, you could say it is not flammable or non-flammable.

.................................................................................................................................................................

Flammable vs. Inflammable

What Is the Difference Between Flammable and Inflammable?

by Anne Marie Helmenstine, Ph.D. 

Flammable and inflammable are two words that cause confusion.

You can tell both words pertain to flames, but it's difficult to know whether they mean the same thing or are opposites.

Flammable and inflammable mean exactly the same thing: a substance burns easily or readily catches fire.

Why are there two different words?

According to Merriam-Webster's Dictionary of English Usage, back in the 1920s, the National Fire Protection Association urged people to start using the word "flammable" instead of "inflammable" (which is the original word) because they were concerned some people might think inflammable meant not-flammable.

Actually, the in - in inflammable was derived from the Latin preposition meaning en- (like enflamed), not the Latin prefix meaning -un.

It's not like everyone knew the derivation of the word, so the change probably made sense. However, confusion persists today regarding which word to use.

Flammable is the preferred modern term for a material that catches fire readily.

Inflammable means the same thing. If a material won't burn easily, you could say it is not flammable or non-flammable.

Examples of flammable materials include wood, kerosene, and alcohol.

Examples of nonflammable materials include helium, glass, and steel.

While it may surprise you, another example of a non-flammable substance is oxygen -- which, as an oxidizer, is instead combustible.

Anne Marie Helmenstine, Ph.D.

Ph.D. in biomedical sciences from the University of Tennessee at Knoxville - Oak Ridge National Laboratory.

Science educator with experience teaching chemistry, biology, astronomy, and physics at the high school, college, and graduate levels.

ThoughtCo and About Education chemistry expert since 2001.

Widely-published graphic artist, responsible for printable periodic tables and other illustrations used in science.

Experience

Anne Helmenstine, Ph.D. has covered chemistry for ThoughtCo and About Education since 2001, and other sciences since 2013. She taught chemistry, biology, astronomy, and physics at the high school, college, and graduate levels. She has worked as a research scientist and also abstracting and indexing diverse scientific literature for the Department of Energy.

In addition to her work as a science writer, Dr. Helmenstine currently serves as a scientific consultant, specializing in problems requiring an interdisciplinary approach. Previously, she worked as a research scientist and college professor. 

Education

Dr. Helmenstine holds a Ph.D. in biomedical sciences from the University of Tennessee at Knoxville and a B.A. in physics and mathematics with a minor in chemistry from Hastings College. In her doctoral work, Dr. Helmenstine developed ultra-sensitive chemical detection and medical diagnostic tests.

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/difference-between-flammable-and-inflammable-607314