I discovered when trying to get rid of a cold that zinc + vitamin C and pelargonium extract not only helped the cold symptoms, but also improved my asthma. I've carried on taking these supplements together with my preventer inhaler and am delighted to find that I can now go for days without using my reliever inhaler. This is a massive first for me in the 18 years I have had asthma, during which I've been getting through 2-3 reliever inhalers per month.
A study in Egyptian children found supplementing their diets with zinc + vitamin C + omega-3 reduced lung inflammation, improved lung function and decreased the severity of asthma symptoms and attacks.
I've written a hub on the effects of zinc on asthma