What to check when whole family gets headaches?

We are all getting headaches and I didnt think of something in the home until my youngest one started complaining last week. We have several combo fire/carbon monoxide detectors in the house. I have replaced all batteries to be sure they are working properly. The alarm is not going off (other then when i put the batteries in and also when i test it) so i am uming its not carbon monoxide. We live in a duplex and have one family attached to us. What else can I check and how? Thanks!