So we had 41 loads total to spread and we were sure glad we had invested in a loader for our tractor - that made things go fast and smooth. In the past, we hand-loaded the manure wagon and that was hard labor. A healthy horse barn is a well ventilated barn which means that windows and doors are often open. This can result in the breezes carrying in fine dirt which settles on almost everything in the barn. And if you are forced to store hay in your horse barn, you'll find that no matter how clean the hay is, it does bring in some field dirt and dust which becomes airborne as you feed.
Bedding also varies in how much particulate matter it puts in the air.
The end result of all this means that you really should perform a thorough annual barn cleaning to remove dust and dirt in order to protect your horses' respiratory systems. I used an industrial vac that has a cloth bag which you empty and shake out periodically. Every hour or so, I'd remove the bag and find it was quite heavy. So when my horses are forced to spend time inside during severe winter storms, they will have healthy air to breathe. A main contributing factor to heaves is unhealthy air in a closed building.