Diesel Fumes and Hauling

Hello Laura,
Hmmm, I have no knowledge of horses getting ill from the exhaust fumes while being towed. That is not to say there are no concerns; carbon monoxide (CO) poisoning is a real thing, and its effects can be cumulative. CO is odorless and colorless so you cannot “sense” it. It is also much heavier than air. Conditions can vary so much there is no single answer. While moving, I would think dilution would rule making it of little concern. While stopped, I still think dilution between the tailpipe and the trailer would probably ameliorate CO concentration down to an acceptable level in all but the most unusual circumstances. The wind would blow it away, if the air is still it falls to the ground. Try standing where your horse is while the motor is running on a still day and see if the fumes are noxious.
DrO