The dry air is good for preventing dust mites and molds, but it also irritates nasal passages and can make allergies worse. My sinus allergies have gotten much worse since moving here. This is why a humidifier is prescribed for many people with allergy and sinus problems.
From webmd: "Keeping indoor humidity low helps control dust mites, which thrive in moist, warm air. But too-dry air can irritate nasal passages and make allergy symptoms worse. Strike a balance by keeping the humidity in your home between 30%-50%."
Nasal Allergy Relief Products Slideshow: Air Purifiers, Bedding, and More