Many allergens can be avoided, but cigarette smoke and fragrances, which can bring on severe asthma attacks or cause anaphylactic reactions in sensitive people, are much harder to avoid because they're in the air. During the holiday season it's even more difficult for sensitive people to avoid because people smoke at parties and indoors because of winter weather and people use fragrances in an effort to make themselves or their locations seem attractive for special occasions.
Probably the most dangerous thing to use is an "odor eliminator," which keeps people from smelling the odors actually in the air but do nothing to prevent an allergic reaction to them.
Please be considerate and avoid smoking, wearing fragrance, or using odor eliminators in public places. By doing so you'll allow many people to stay healthy and might even save someone's life.