I read a report which claimed a common asthma trigger was sulphur dioxide, given off by a group of additivies, commonly found in foods. Check food labels for additives E220-E227. Where possible go for organic foods if you're asthmatic. particular foods that may contain these additives include juices, fruit pie fillings, jams and sausages. I have also read that dairy products can cause a build up of mucous in your system which exacerbates breathing difficulties and asthma
All the common allergy triggers - sulphur dioxide, milk products, chocolate, nuts - have been implicated, if you want to try to find your trigger, cut everything out of your diet for a week then add in one thing at a time on a four-day basis until you notice a difference. Ive found that drinking two mugs of gunpowder green tea a day cuts down on my drug use for asthma control. But then I like the stuff.