Some words are understood, but not written. Think of the sentence this way: So you think that by staying at home you are safe from all the terrible kinds of pollution (that is) present outdoors, such as in or near factories, roads, and garbage dumps? So "present" is actually an adjective.

"Present" here means "existing"

....kinds of pollution (that are) present outdoors...... be present = 存在

In this sentence, present means existing and is an adjective describing the noun outdoors. So in more words the sentence could be written as "....kinds of pollution that exist outdoors.....". The word present can be a noun or an adjective, and when it is, its two syllables are pres-ent. It can also be a verb in which case its two syllables are pre-sent. The pronounciations are slightly different.