Flammable vs Inflammable vs Combustible

If you work with fuels, oils, or chemicals on daily basis, you’ve likely come across these three terms before! “Flammable”, “inflammable”, and “combustible” are similar terms that only slightly vary in definition, but most people don’t know what these slight differences are! In fact, many people fall under the misconception that “flammable” and “inflammable” are antonyms with opposing definitions, while in fact the opposite is true! Flammable and inflammable are far closer to meaning the same thing than they are to having opposite definitions. Take a look below at our in-depth look at these three terms and the definitional distinctions between them.


Flammable, from the Latin flammare (to catch fire), can be used to refer to all substances that are able to set alight when a source of ignition is introduced, without the need for any other substances involved. This means that, even though most people don’t use the term to refer to them, materials like wood, paper, dried leaves, and fabric fibres are all, technically, “flammable substances”. Flammable fuels or chemicals, which are more commonly referred to in this manner, are those which give off fumes which can be ignited.


Inflammable is a term that is often misunderstood, as many believe the term means non-flammable. Rather, the opposite is true. “Inflammable” entered the English lexicon at a different time to “flammable”, coming from the Latin inflammare (to cause to catch fire). Some treat “inflammable” as a synonym to “flammable”, which is closer but also not fully accurate. While both terms do mean a substance that can be ignited, “inflammable” refers to substances that are capable of igniting without the need for an external source of ignition, such as unstable chemical compounds.


Combustible finds its roots in the Latin combuere (to burn up and consume), and again has a definition that is similar to both flammable and inflammable, but with some subtle differences. “Combustible” substances are flammable, but only when the substance is heated above room temperature. This is all down to the “flash point” of the fuel, which is the temperature at which a chemical will be evaporating enough fumes to facilitate ignition. “Flammable” liquids have a flash point below room temperature, so do not need heating to ignore. “Combustible” liquids have a flash point above room temperature, so will not ignite without being heated first.

If you work with kerosene, red diesel, or any other types of fuel oils, knowing the difference between flammable, inflammable, and combustible is absolutely vital! For more industry tips and information, check back with our blog next week!