I have that reaction to the exhaust that airplanes introduce to the cabin when reversing out of the gate before takeoff. It boggles my mind why all outside air

I have that reaction to the exhaust that airplanes introduce to the cabin when reversing out of the gate before takeoff. It boggles my mind why all outside air isn't shut off when reversing. I get an instant headache and have difficulty breathing, and I'm generally not sensitive to much of anything.