Be sure the horse cannot reach the fan or the electrical cords supplying the fans. Electrocution deaths as a result of a horse chewing on an electrical cord are fairly common. While there are usually breakers or fuses that will shut the power off, fires can result from malfunctioning fans in barns. With stabled horses, keep stalls clean. Urine-soaked bedding and manure ferment more rapidly in hot humid weather.
Fermentation produces both heat and ammonia, which increase the risk for lung disease.
Picking stalls out once or even twice a day will reduce both problems. Try to work horses early in the morning. This takes advantage of the cooler part of the day, plus gives the horse time to normalize its body temperature. Call a veterinarian, get the horse in shade, use fans and bathe with cold water. Bathe the head, underside of neck, inside of legs, and the entire body.