On hot humid days allergies become worse due to nasal passages swelling. Humidity can also trigger congestion and pressure on the sinus cavities. Allergy reactions on a humid day sometimes include headaches, watery eyes, sneezing, runny nose, and itchy throat. These severe allergy reactions can often prevent free breathing and extreme respiratory discomfort.
When there is too much humidity you have too much dampness in the air. Which then gives the chance for mold spores to grow. Many people are allergic to the mold spores. Which then would cause your lungs to tighten or the sinuses to swell. Its basicly the particles in the air which then cause you the distress.
When relative humidity is high, this increases certain irritants in the air. With things like pollen count, mold, and bacteria increase, making our allergies worse.