See the above question as to bedbugs. Get a Doctor's opinion as to the cause of your problem. Also get witnesses or other proof that if you stay out of the unit, the problem ceases.
If you can do these things, then you have to give the landlord written notice of the problem and an reasonable opportunity to correct it. If he does not or can not fix it, then you can give him written notice that you are considering yourself "constructively evicted" and are breaking the lease and will no longer be responsible for the unpaid rent.