No one knows since there has been no canon description. Currently the only information about the Breen comes from the Typhon Pact series of novels which states that the Breen are actually 4 species that united and wear the helmets so that everyone appears equal and is treated equally. It is considered taboo to walk around in public without the helmet and being fully clothed. The only time the Breen ever take off their helmet is when they are at home as it is considered an intimate act to reveal your face or species to anyone.
Edit: actually it's to my understanding that the breen home world is so cold they need those suits and helmets to survive as well our climates are too warm. The suits are actually environment suits I'm sure of that . (14 October 2015)
FYI, you can sign your post with four ~ signs, and it will print your username and the UTC time. Actually, according to Weyoun, it actually had a quite temporate climate. As mentioned above, the books expand on thisby explaining that the suits are a cultural thing. That's not to say they do not have environmental functions. One of the subspecies mentioned in the books is from an extremely cold climate and has a mostly liquid form that will evaporate without the suit.-Cpthunt (talk) 17:09, October 16, 2015 (UTC)