Under the microscope the lung is inflamed with two marker cells, the eosinophil and the mast cells. Both belong to the white blood. One has granules that stain pink named eosinophils after the Dawn Goddess Eos. There are groups of asthmatic patients. Their asthma is episodic and comes on when pollens or molds from wet leaves are airborne.
Asthma developing late in life tends to be nonallergic without positive skin tests.
Exposure to industrial pollutants like sulfur dioxide during thermal inversions makes all kinds of asthma worse but possibly can cause asthma newly in people without lung disease. 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.