A large study has linked a diet of 3 burgers weekly to a higher incidence of asthma, and a diet rich in fruits, vegetables, and/or fish to a reduced risk. While they do note that 3 burgers/week probably indicates a person has other unhealthy habits (and are there fries going along with those burgers?), this immediately reminds me of the anti-inflammatory diet and the link between inflammation and many diseases - asthma, heart disease, cancer, arthritis...
We consume a large amount of food each day and it makes sense to begin there to reduce health issues. Following are some links to information about an anti-inflammatory diet.
Reducing inflammation - the natural approach
Anti-inflammatory diet tips
Anti-inflammatory diet summary including a brief discussion of omega-6 and omega-3 fats and how the balance between them has shifted in the American diet over the past century