Word groups often come between the subject and the verb. Such word groups, usually modifying the subject, may contain a noun that at first appears to be the subject. By mentally stripping away such modifiers, you can isolate the noun that is in fact the subject.

The subject is levels, not pollution. Strip away the phrase of air pollution to hear the correct verb: levels cause.

The subject is slaughter, not pandas or pelts.

note: Phrases beginning with the prepositions as well as, in addition to, accompanied by, together with, and along with do not make a singular subject plural. The governor as well as his press secretary was on the plane. To emphasize that two people were on the plane, the writer could use and instead: The governor and his press secretary were on the plane.