How does air pressure affect my sinuses?

Sinuses are made of delicate bony passages lined with an even more delicate nasal tissue. Normally the passages should be open and air can travel in and out of the sinuses from higher to lower pressure. Air inside the sinuses, inside the nose and outside of the nose should all be at about the same pressure. The problem comes when one or more of the passages swells and becomes blocked. High pressure can usually get out of a sinus, but low pressure can further swell the nasal lining which can cause pain, increased mucus production, possibly bleeding and predispose you to a sinus infection. This is why you should never scuba-dive with sinus congestion and it is wise to take precautions when flying if you feel your sinuses aren't working correctly.