I'm allergic to car ac i m resulted to severe headache why?

Multifactorial: Its hard to say why you may have such a reaction to the car air conditioner. However, a common scenario is that your nasal sinuses become irritated and this leads to the headaches. I interpret these allergic reactions to be runny nose/sinus symptoms. Why is the ac doing this? Perhaps there is something in the air filtration system that leads to the irritation and allergic reaction.