I would like to know what type of nouns car exhaust fumes, car accident, school day, air quality... are because, many times, I write such nouns with apostrophes, like car's exhaust fumes, air's quality, and car's engine, then find out they are not written this way.

For example:

(bedroom, football, sunglasses...) = Compound nouns

(high-spirited, long-lasting, never-ending...) = compound adjectives

(car exhaust fumes, car accident, school day, air quality...) = ?

I know my question might sound silly but please bear with me.


You would use the possessive ('s) if you are talking about a specific car and its exhaust fumes, however generally speaking about air pollution uses the noun "car" as an adjective.

car exhaust
history teacher

are examples.


These are called noun adjuncts source source. An adjunct is a word, or phrase, that modifies the meaning, but is not grammatically required to be present. Possesives and adjective are also adjuncts.

There often is a choice between a possesive, an adjective and a noun adjunct. There is a choice between "nation's" "national" and "nation". When an adjective exists, and the meaning is not clearly one of possesion, then it is the adjective that is usually preferred. For example the "national debt" is preferred to "the nation's debt" or "the nation debt". But note we talk about "nation states" and the "nation's favourite poem". The meaning of the adjective may be different from the noun. A "history book" and an "historic book" are different.

The last example may be due to the nation being treated as an intelligent being, that can have "favourites". When the noun means something that is alive or sentient then a descriptive possesive can be used. For example "Cow's milk" or "children's games". When the noun is inanimate, and the sense is not one the noun adjuct may be preferred "a table leg" or "car battery". Abstract nouns are less likely to be used as descriptive possessives, for example the "history book" instead of "history's book".

It is easy to think of exceptions, and established usage is more important than any rules.

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