Rainy days are often a welcome respite for those who have allergies. The humidity in the air weighs down pollen and keeps it from spreading around and causing symptoms.

During heavy rain, however, pollen counts may increase because it can cause pollen grains to split into smaller particles. The smaller particles spread more quickly and can enter the small airways and noses of people who have allergies.

In addition, the rain can also affect another allergy trigger that some sufferers don’t like: mold. “Mold is a major allergen, and it thrives in humid conditions, which happens to be what we get with rainy weather,” says Dr. Katie Marks-Cogan, an allergist in San Francisco.

This can lead to a more severe allergic reaction for those who have allergies to tree, grass or weed pollens. The rain can also cause these pollens to break down into microscopic fragments that may enter the small airways and noses of those who are prone to sneezing or other allergy symptoms due to pollen exposure, she adds.

Rain can also exacerbate sinus pressure and headaches in people who have other allergy symptoms, especially in those with swollen or blocked sinus drains. It’s a problem in many areas of the country, and some experts say that it’s not just rain’s effect on allergies but other factors.