The National Allergy Bureau reports pollen and mold spore counts to the public. The NAB divides allergen counts into four categories: tree pollens, grass pollens, weed pollens and mold spores. These allergens cause seasonal allergic rhinitis, or hay fever, for more than 35 million people in the U.S., according to the NAB. Counts are reported as grains per cubic meter of air. Linking pollen counts to allergy symptoms is complex and problematic, but many patients develop symptoms when counts range from 20 to 100 grains per cubic meter, according to the NAB.

For tree pollen, the moderate level (YELLOW) ranges from 15 to 89 grains per cubic meter. Hay fever in the early spring is often caused by pollens from trees such as oak, elm, birch, maple and walnut, according to the NAB. Learn more at the NAB Web site.

SOURCE: National Allergy Bureau, a division of the American Academy of Allergy, Asthma & Immunology