Inflammability

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Inflammability is the ease with which a substance will ignite, causing fire or combustion. Materials that will ignite at temperatures commonly encountered are considered inflammable, with various specific definitions giving a temperature requirement. The flash point is the important characteristic. A volatile substance may have sufficient vapor pressure to form flammable (or even explosive) mixtures with air in temperatures as low as −10 °C (14 °F), so that ignition can occur even without direct contact. Flash points below 37.8 °C (100 °F) are regulated in the United States by OSHA as potential workplace hazards. Examples of flammable liquids are gasoline, ethanol, and acetone. Diesel fuel is in one of the less heavily regulated flammability categories, and biodiesel is considered nonflammable or noninflammable with a flash point usually over 150 °C (300 °F) even though biodiesel will combust inside a diesel engine.

[edit] Linguistic ambiguity

Flammable is of relatively recent origin (from Latin flammare "to set on fire" + -able) and many unfamiliar with English semantics and grammar use it in place of the older inflammable (Latin inflammare + -able) where inflammare is from in- + flammare and the prefix in- has the same meaning as the preposition "in" (inside), so it literally means "to cause to burn" (cognate with inflammatory and inflammation). The problem is the Latin prefix in- also means "not", so inflammable may also be parsed as "not flammable" (non-flammable) like for example invisible (in- + visible). In Romance languages, the word inflammable is still used with the original Latin meaning (French: inflammable, Spanish: inflamable, Italian: infiammabile, Portuguese: inflamável) and the opposite is built with in- "not": French for non-flammable is ininflammable (in- "not" + inflammable "flammable").

The Elements of Style ("Strunk and White"), on the other hand, says:

Flammable. An oddity, chiefly useful in saving lives. The common word meaning "combustible" is inflammable. But some less educated people are thrown off by the in- and think inflammable means "not combustible." For this reason, trucks carrying gasoline or explosives are now marked FLAMMABLE. Unless you are operating such a truck and hence are concerned with the safety of children and illiterates, use inflammable.