Two air pollutants could contribute to increases in allergy incidence worldwide by chemically modifying airborne protein allergens. Previous studies have found a link between worsening air pollution and increases in the prevalence of allergies, but little is known about possible molecular mechanisms responsible for this correlation. Ulrich Pöschl, Christopher Kampf, Manabu Shiraiwa, and colleagues at Max Planck Institute for Chemistry, in Mainz, Germany, studied how the air pollutants ozone and nitrogen dioxide modify the protein Bet v 1, a birch pollen allergen. They found that ozone oxidizes tyrosine residues in the protein to form reactive oxygen intermediates. These species can then either react with NO2 to produce nitrated tyrosine or dimerize with other oxidized intermediates. These types of modifications could make proteins more allergenic by changing their epitopes, the antigenic structures recognized by antibodies, or by forming new epitopes, for example.