There is no clinical evidence that face masks are efficient in birch pollen-induced asthma. As the use of face masks has become widespread worldwide to limit the spread of SARS-CoV-2, this study aims to demonstrate the clinical efficacy of medical masks in patients allergic to birch in the same way that clinical trial with drugs.

All patients should have positive skin prick tests and specific IgE to birch at screening visit. 24 patients with asthma response during 1-hour baseline exposure to airborne Bet v 1 in ALYATEC environmental exposure chamber.will be randomized into 2 groups: with (n=12) and without (n=12) a KOLMI® surgical face mask (OP-Air), for a 6-hour exposure to birch allergen.

Allergic symptoms will be observed during the 6-hour exposure and compared between the 2 different groups (with and without face mask) to evaluate the efficacy of the face mask.

Main Inclusion Criteria:

• Subjects who signed the written informed consent,
• Subjects covered by health social identification number,
• Subjects able to understand and complete study-related procedures,
• Subjects reported history of symptomatic birch pollen allergen-triggered asthma (GINA1) with associated allergic rhinitis,
• Positive skin prick test (SPT) with cat hair extract with mean wheal diameter ≥ 5 mm compared to negative control (NaCl reaction < 2 mm),
• Positive sIgE tests for birch tree pollen and Bet v 1 (≥0.7 kUa/L),
• Subjects with FEV1 ≥ 70% predicted at screening and before allergen exposure,
• Subjects with at least one drop in FEV1 ≥20% during the 1-hour baseline exposure
• ACT≥ 20,
• Women of childbearing potential with a negative pregnancy test throughout the study period and a highly effective contraception: oral contraceptives, condom with spermicide, intrauterine device, bilateral tubal ligation, vasectomized partner.

Main Exclusion Criteria:

• Allergen immunotherapy to birch pollen allergen for more than 1 month in the 3 years prior to the screening visit,
• Ongoing allergen immunotherapy to another allergen,
• History of anaphylactic reactions to birch allergen exposure or birch allergen immunotherapy,
• History of anaphylactic reactions to another allergen in the last 6 weeks before inclusion,
• Nasal polyposis, deviation of nasal septum or diagnosis of uncontrolled non-allergic rhinitis,
• Subjects allergic to indoor environmental allergens (molds, house dust mite allergens) with obvious exposure to these allergens causing allergic symptoms,