--for those who speak English as their second language or who just want to improve their writing
(Illustrated version of Quirks of English)
Due to--this phrase may be used incorrectly more often than it is used correctly. Because due to is an adjective phrase, it should modify only nouns, not verbs.
The high pollution index was due to a layer of cold air that trapped a layer of warm, pollutant-filled air close to the ground.
Here, due to modifies pollution index, a noun.
Due to means caused by, which gives us an easy way to test for proper usage. If you substitute caused by for due to, does your sentence still make sense?
The high pollution index was caused by a layer of cold air that trapped a layer of warm, pollutant-filled air close to the ground.
To modify a verb, use because of instead of due to.
Because of this year's poor turnout, the conference will not be held again next year.
Here, because of modifies will not be held, a verb phrase. (Caused by this year's poor turnout, the conference will not be held again next year does not make sense.) Because of is usually the correct choice at the beginning of a sentence.