Hi, I'm Dr. Jill. If the roof truly contains asbestos instead of a newer safer insulating material, then no one, horse human or otherwise, should be inside the barn. Numerous reports have linked chronic inhalation exposure with several lungs diseases in humans (including a specific form of lung cancer). There is no published peer reviewed research establishing a link between chronic asbestos inhalation exposure with disease in horses, but there is a suspected link between chronic exposure to another silica compound called crystobalite (asbestos is also a silicate compound, as are many other commonly found minerals such as quartz) in the soil of certain regions and silicosis lung disease in horses, similar to pneumoconiosis in humans. There may also be concern about asbestos particles falling and becoming lodged in your horses hair that would present a risk to others outside the barn with brushing or other activity that would cause them to become free floating in air again.

I hope this is helpful. If you have any additional questions, please let me know.