I found this article, I knew there had been a lot of research into cooling a hot horse prior to the Atlanta Olympics.
Generally it is not a problem here in the UK!
The proper way to cool a hot horse
Many owners and trainers have never been taught how to properly cool a hot exercising horse.
How hard can it be -- just hose your horse down with water, right?
Actually it is very important to understand how to safely and quickly cool down a hot, sweaty horse.
The 1994 FEI Samsung Equine Sports Medicine Conference in Atlanta addressed problems that were anticipated in the intense heat and humidity that was likely to occur during the 1996 Olympic Games. Research that came from that initiative showed that simply hosing off a horse did little good -- it did not significantly drop the horse's temperature and may in fact have been detrimental.
Researchers showed that cool-to-cold water being poured or sprayed over a horse's skin wets the horse and the very thin layer of water actually contacting the horse's skin is super-heated quickly to the horse's body temperature.
The rest of the water being sponged or dumped or sprayed on the horse merely sheets over that initial one- to two-cell water layer on the horse. The initial water actually acts like a raincoat and does not allow other water molecules to contact and cool the skin, so the horse may in fact become hotter in the process.
The best method for cooling a horse is to spray or sponge on water with one hand and to almost immediately scrape off the now super-heated water with a sweat scraper in your other hand. "Spray, scrape, repeat" is the cooling method of choice. This means of cooling can significantly drop a horse's core temperature and requires less actual water since large volumes are not being wasted as they are usually dumped on a horse with little chance of having any effect.
The water can be fairly cold with this method of cooling because it does not stay on the horse's body long enough to cause muscle cramping which had been a previous concern when cooling horses with the application of cold water.
Increasing airflow can also help with cooling, so fans and a location with a breeze may help cool an exercising horse but researchers have shown that water is still 20 times more efficient at cooling horses.