IF the respiratory issues were due to some thing the landlord did wrong, like not correcting a mold condition in your unit or the HVAC ducts, then you may have grounds to not pay the rent if you had to move out for health reasons: if the landlord fails to correct a condition (after having notice of it) that impairs health or the ability to safely use a rental unit, that is a violation of the "implied warranty of habitabilty." A violation of this warranty can allow the tenant to treat the lease as terminated because he/she was "constructively evicted" by unhealthy conditions. So if the landlord caused the respiratory issues, you may be able to raise the landlord's violation of the  implied warranty of habitability as a defense.But if the landlord had nothing to do with the problem--for example, you were affected by fumes or dust from a nearby construction site--you would still be liable for the rent, even if your health was impaired. Your health issues are your health issues: the landlord is not responsible for things not under its control, and you cannot terminate the lease if the landlord was not at fault.