I guess it's possible, but not probable.... That window is a long way from your tail pipe and if you have a window or two or a roof vent open in front of that rear window, then the horse are would be pressurized with fresh air. The air will be flowing through the trailer from front to rear as your moving along the road. If you want to see pretty much where your exhaust is going, watch the road spray coming off your truck and trailer tires while traveling on a rainy day. Some vapors may enter the windows but it will be very diluted. If you got any doubt, place a carbon monoxide detector in the trailer and hit the road. Place a call from the cell phone in the trailer (leave this phone in the trailer) to the phone you got on you so you'll know when the alarm go's off, If it's going to detect, it should do it within a mile....

 Now with a pickup truck and a camper shell.... Well that brings on a whole nutha' twist...! The exhaust is at the rear of the truck, if the camper shell over head door is open if will create a vaccum as the truck travels down the road allowing carbon monoxide vapors to enter the truck bed..... Trucks with exhaust pointed straight out the rear are more susceptible to have exhaust enter the bed area than trucks with the exhaust exiting the sides.

Is that why exhaust goes to the side now?  And why we can no longer ride in the camper above the cab :)

Very interesting

 Not sure about that, just going by my observation of 52 years, me being raise on an eastern North Carolina, tobacco, hog, cattle and Tennessee Walking Horse farm...

It's really just another S.W.A.G with some real life experience, from Retento...

Thanks for the info.  I was told that a long time ago and never open the rear windows in my trailers, but with the excessive heat and humidity we've been having in Oklahoma, I want as much air moving through the trailer as possible.

I had to take a horse from OK to TX yesterday and I opened all the windows and vents in the trailer, including the rear window.  The horse jumped off the trailer when I arrived at my destination in great shape.  And not stressed either.      

The rear window of a trailer is similar to the rear window of a pick up truck; the aerodynamics are such that a low pressure area is created, and air is drawn into the rearmost vertical plane. The trailer's side windows are similar to a car's in that the air is usually drawn out at the front, and some enters at the back. The top vents if opened forward, will tend to pressurise the inside with ram air.

Because the towing vehicles' exhaust mixes with and is diluted by the moving air, little contaminated air will enter the back of the trailer, many feet behind the exhaust's outlet. It would probably be less contaminated, than a person breathing the air while walking around a parked, running vehicle.

We had a stock type trailer, where the top part of the rear door was open to the roof. We had a two horse ST load that had the same configuration. Many trailers are similarly equipped. I don't know of any owners complaining about ill horses, caused by trailering under these conditions.