A couple of ailments lead to asthma in horses. These include heaves, which your veterinarian may call chronic obstructive pulmonary disease (COPD) or recurrent airway obstruction (RAO), and summer pasture-associated obstructive pulmonary disease (SPAOPD). The symptoms of horse asthma are difficulty breathing, especially a wheezing sound while breathing; mucus discharge from the nose; and coughing. The best treatment for your asthmatic horse is to prevent the symptoms from appearing.
The best scenario for most horses with heaves is to move them out to live predominantly on grass pasture. If that's not possible, or if your horse displays more reactions when outside, you can reduce the allergens your horse breathes in by opening the windows and doors in your stable to allow plenty of airflow. You can also use a fan to help move air around, but be sure to install the fan off the floor so that it doesn't blow around.
Common barn chores such as stall cleaning, moving bedding or hay, and sweeping or raking the aisles can move around a lot of dust. You can reduce the number of allergens your horse inhales when these chores are going on by moving your horse outside, or by spraying down the aisles with water, in addition to the ideas listed previously. Don't spray enough water to cause puddles in your barn; use just enough water to dampen the surface.
If you have access to a complete feed that includes your horse's daily roughage requirements as well as energy requirements, this is your best option to prevent the inhalation of dust while your horse is eating. If not, and feeding hay is your only option, be sure to soak the hay with water before feeding it to your horse. Also, keep the feed on the ground, or in a ground-level feeder, to prevent your horse from getting dust and other allergens up his nose while eating.
Riding in an arena can kick up a lot of dust that both you and your horse will be breathing for the entire length of your ride. Riding outside of the arena would be best, but if you must ride in an arena, be sure to water it down. Again, don't use so much water that you create puddles. You will have to water the arena every few days to keep the surface damp enough to avoid a lot of dust. If you are looking for a more permanent option, you can have a commercial dust suppressant applied to the arena. This will last a few months or longer.
Your veterinarian can come out and give your horse a full examination. Afterward, your veterinarian should offer recommendations on management practices you can use to care for your horse, as well as prescribe a bronchodilating medication. Your veterinarian may also decide medication is unnecessary in your horse's case.