Heaves, now called recurrent airway obstruction (RAO) in horses is essentially similar to asthma in human beings. As such, it is a respiratory problem. Therefore, anything in the horse's environment that he breathes into his lungs and is allergic to, could potentially cause RAO. Typically these allergens are dust and mold spores that are found in high concentrations inside a barn when dusty or moldy feeds are present. Usually, horses kept outside do not suffer from RAO for this reason. Although it is rare, it is possible that your horse could have RAO even though he is kept outside. Just as some human families have a greater incidence of asthma, some horse families may have a greater of incidence of RAO, so there may be a genetic component. He may also have summer pasture-associated obstructive pulmonary disease (SPAOD), which is very similar to RAO, except it happens in horses that are kept outoors in a hot a humid environment, and thus the treatment would involve keeping him in a dry and cool stall.