Article: Do this as needed to keep plenty of air moving through the trailer.  Horses release a lot of heat and moisture as they breathe. Exhaust fumes and ammonia can build up in a closed trailer, causing respiratory distress for the horse. Places to check include doors, wheels, and the hitch. Make sure nothing is amiss and no tools, lead ropes, etc. have been left on the ground or trailer fenders, or are leaning on the trailer. Take corners, accelerate, and stop slowly and smoothly. Remember that the horse is standing up, trying to keep its balance.  Practice driving the trailer before you try with a horse loaded. Get used to taking turns slowly and changing lanes gradually. Your horse needs time to rest and unlock its legs.  Stop at least every 3-4 hours. Do not remove the horse from the trailer. Park in the shade. Give the horse some water. Check for injuries and make sure the trailer is still in good condition.

What is a summary?
Open air vents on windows and doors. Do one last check. Drive gently! Take frequent breaks on long trips.