Exposure to allergens is a major factor, with the house dust mite a prime culprit. Secondhand tobacco smoke has also been blamed. Some studies show that lowered intake of fish may weaken asthma resistance. Some evidence suggests that intake of omega-3, a fatty acid found in fish oil, may be beneficial in keeping the disease away. Omega-3 has also been linked to healthy cardiovascular and immune systems and the development and functioning of the brain and eyes.
Dust mites are the most important to avoid, followed closely by cats and certain types of mold.
Dogs and maybe cockroaches figure less importantly, then plant pollens. Zero environmental tobacco smoke is a must. Breast feeding is certainly protective as is a diet of fresh fruit and vegetables, oily fish and low salt.