There's a lot of dirt & dust in there no matter what; on ours anyways. Before I cut the holes, it wasn't just heat coming through, it was a pile of dust, too, once the hole was cut in the back, about 90% of the dust went out through that hole.
I think that the original design sucked, the snorkel just sucked in dust that was there from the tires; I thin that there's pressurized air from the clutch combined with heat, that makes it all want to naturally rise, if there's a legitimate hole somewhere higher, it has a major tendency to follow the easiest route; out the back. If you're going over 10 MPH or so, there's a slight vacuum at the rear of th machine, I think this helps greatly to help pull that hot air/dust out the back end.
I could be wrong....