None of these have yet been proved conclusively. Here's a link to the PPP fact sheet on asthma which contains a more detailed description of asthma and its management. The combined effect of these is to make breathing difficult. If you are prone to asthma, then you will know that attacks can be triggered by a variety of factors. Intrinsic factors include stress, exercise, fatigue, and emotional disturbance.
Some people find that attacks can be triggered by industrial fumes, or certain drugs or foods.
If you know what triggers your asthma, then the best thing to do is to avoid the trigger factor. For some people, this means keeping cats or dogs out of the house, or removing all carpets and soft furnishings. For others, it may mean a change of job, or moving to a house near the coast. You could also try the Buteyko Breathing Technique as a means of helping - apparently it is especially suitable for children. They may be prescribed singly or in combination, and are most commonly administered using an inhalor or nebuliser.