when an arborvitae is goin brown

[janetbeautiful]

I've just moved to a house with about 20 emerald green arborvitaes'. about 4 are slowly turning brown. I don't know how old they are or how to maintain them (they are about 8 ft. tall and bushy - not pruned) In my past experience with these types, once they start going brown they die. so how do I save them? Thanks!!!!!!!!!!!!!!!!