Why does my fire alarm keep ringing?!?

This might be a question asked more than once...but it's seriously bugging me. The fire alarm keeps beeping for no particular reason at all! It'd be fine one moment and then a while later it starts beeping like crazy! My brother thinks it's because of carbon monoxide? He says it's an odorless, invisible gas and that it might be setting the alarms off. Not only one of them is going off, all of them are. We have about...6 or 7 in our house; one in every bedroom (5 rooms in total), one in the hallway and another in the living room. They all go off, sometimes at different times, sometimes together. It really hurts my ears...is it possible that they're all being faulty? Sorry that this was kind of long...