Because an insulated barn maintains a slightly greater temperature difference between the inside and outside of the structure, air movement tends to be slightly intensified. Insulation also reduces the risk of condensation buildup. It's not always easy to install insulation in an existing structure, but it's an option worthy of exploration. According to the Equine Research Centre's studies, it's hay. Susan Raymond, research associate with the Respiratory Health and Air Quality project at the ERC, explains that in one study, dry hay demonstrated dust levels dozens of times higher than hay cubes or pellets.
Bedding is another major source of inhalant particles in the equine environment.
The worst offender, according to the ERC studies, is straw. Even the cleanest of wheat straw, it has been found, contains significantly more small, respirable fungal spores than most other beddings, such as shavings, peat moss, or shredded newspaper. Deep litter bedding systems, whether done with straw or shavings, are a major mold-producer. They allow molded bedding and fecal material to accumulate, and have the added disadvantage of allowing the buildup of noxious gases such as ammonia, not to mention infectious bacteria and the larvae of internal parasites. Such a setup might mean your horse is inhaling irritant particles for hours every day.