Asthma and bronchitis are the result of "hyper-responsive" (over-reactive) airways. The airways can react to a number of stimuli, such as:
Inhaled debris or irritants - dust from cat litter, cigarette smoke, perfume or hairspray, carpet fresheners, and perfumes in laundry detergent
Pollens or mold
Infectious agents - viruses and bacteria
Parasites - heartworms, lungworms
Coughing and respiratory distress are the most commonly reported signs with obstructive lung disease. Coughing is a significant finding since there are few causes of cough in the cat. Often the cats assume a squatting position with the neck extending during these coughing episodes. This may make it difficult to differentiate from retching or vomiting. Wheezing may be heard. Siamese cats appear to be more susceptible.
Below are excellent sources for additional information.
Fritz the Brave -Fritz offers informative insights on being an asthmatic Siamese
Veterinary Information Network