Water. If your child is suffering from asthma, make sure they are drinking enough water. When water is low, the brain sends a signal to the lungs telling the bronchioles to close in order to preserve water. Make sure your child is drinking at least 4 glasses of water per day or drinks two glasses of water at the onset of asthma symptoms. They should always have water with them wherever they go. Be careful not to over-water as this can deplete the body of precious minerals including sodium, in which case they should also increase their sea salt intake. Read the book about the “body's many cries for water”.
My son had a child in his class that was always asking to be excused to get a drink of water, annoying the teacher who would deny her, and unfortunately she passed away from an asthma attack later that same year. I personally believe asthma may be caused by long term dehydration.