Smoking does the same thing. Baker's asthma caused by reaction to wheat flour is one of a family of occupational asthmas including woodworkers, photographers, metal workers, solderers, farmers, plasticiser's, and printers asthma. A weekend free of asthma suggests occupational asthma. This is less obvious in hobby workers asthma. The viruses probably loosen the tight junction between lung tube lining cells and allow allergens to trigger bronchial muscle spasm.
Exercise can start an attack of asthma.
Dry cold air, in cross-country skiing, is much more provocative than warm wet air from indoor swimming. Remember asthma blocks lung tubes and the air sacs are normal. About one-half of the cases develop before age 10 and another third before 40. Most deaths in asthma occur over the age of 55, but life expectancy is normal. It is the only chronic disease, apart from AIDS and tuberculosis, that is on the rise throughout the world.