Not cleaning out stalls daily and removing soiled bedding and manure contribute to stale air in barns. The horse, as an athletic animal, must inhale large volumes of air. Things that reduce this ability, including poor air quality, can affect the horse's health and ability to perform. A horse in a dusty environment is more prone to respiratory infections than a horse in a cleaner environment. Bacteria, viruses, airborne dust and noxious gases are major causes of small airway diseases in horses.
Mold spores are the most common cause for concern in the air of stables.
Large numbers of spores cause inflammation and irritation of the horse's small airways. A horse with a respiratory tract infection in a dusty stall takes much longer to recover than if breathing fresh air. The two major sources of dust in a stable are hays and bedding. The major source of mold spores is hay. In fact, all hays have some mold spores.