I have sand in all my coops and pens too, even in some of the nest boxes lol.
I'm in northest Florida, it was 27 outside earlier. I cover the vents on the coops with old towels to prevent drafts on the windward side of coops. The vents on the leeward side are uncovered.
It's possible it might be ammonia fumes from soiled bedding inside the coop and/or improper ventilation. A coop clean out might be in order.
Also, inhalation of feed dust or dust from dust bathing could be a problem. Pollen, insecticides, lawn fertilizer are issues, mold or mildew inside the...