One thing we can point to is the use of Lasix. It's only allowed in America, and we also seem to be the world leader in EIPH. Of course we also race year round and often times do so on dirt. Because of the nature of the game, horses rarely get any exercise in the aerobic heart rate zone at most US training centers.
Other countries forbid Lasix, have shorter racing seasons, and spend much time training on turf. In addition, horses spend much more time exercising, a lot of that slow gallop work taking place at the appropriate intensity level for aerobic development.
Very preliminary findings on my part show that in terms of the stress put on a breezing thoroughbred, 6 furlongs on polytrack is equal to 4 furlongs on dirt. Big difference. If indeed, the respiratory system fails to respond to training, as many experts believe, then training/racing over dirt year round is bound to be a leading factor.
So, how can you attempt to prevent bleeding?
First of all, just because your horse isn't gushing blood from his nostrils after a work doesn't mean he's not bleeding. Ruptures of sacs deep within the lungs probably take place quite often when travelling over a hard surface, and this accumulates over time. Aerobic work in the 70-80% intensity zone can help the horse form more capillaries that will aid in decreasing blood pressure and mitigate some of this damage.
Secondly, try to keep the stable environment as dust free as possible, which probably goes without saying. Thirdly, if you have access to one, expose the horse post breeze to some hyperbaric chamber treatments where the increased oxygen delivery can help speed the repair process deep within the lungs.
Most importantly, follow a structured progressive plan like the one in another post on bucked shins. Taking 15 small steps from legging up to racing is better than taking 5 huge ones. The more stress/recovery cycles you can hit perfectly, the better off you are.
In a perfect world, every horse would have access to pools, treadmills, turf, poly, dirt and be able to cross-train much like humans. The mechanics of running on each surface are different, like hitting a curve ball in baseball - some can do it from birth, others need to practice. Dirt will build stronger bones, but you will always be on the knife-edge for injury. Synthetics will build stronger soft tissues like suspensories, but bone density may very well suffer a bit.
Don't let Lasix do all of this work for you, fine tune the training process in order to be able to use Lasix as an edge on raceday (if your country allows it), not just as a survival tool-