Immediate contact allergy to birch leaves and sap.

Fresh sap and crushed leaf of birch (Betula verrucosa) were tested with the scratch chamber method in 117 atopic persons, 74 of whom were allergic and 43 nonallergic to birch pollen, and in 33 control patients. Immediate positive reactions to birch sap were seen in 39% and to leaf in 28% of the patients allergic to birch pollen, in one of the birch pollen-negative patients, but in none of the control patients. Birch leaves may cause contact urticaria in the Finnish sauna, where bath whisks made of birch sprigs are traditionally used.