Birch tree species are common throughout temperate North American, European, and Asian areas. Birch pollen is one of the most common allergens, usually in areas where exposure to high levels of birch pollen is common. The allergen may cause atopic dermatitis, contact urticaria (hives), atopic eczema, asthma, wheezing, allergic conjunctivitis (pinkeye), eye redness, oral-pharyngeal itching, or rhinoconjunctivitis (inflammation of the lining of the nose and the mucous membrane that covers the front of the eye and lines the eyelids). There is insufficient evidence in humans to support the use of birch for any indication. One study shows that birch bark ointment may be beneficial for actinic keratosis (precancerous condition of thick, scaly patches of skin).