MX tracks usually have water lines all around the track to keep the dirt moist, so dust wont float up in the air. Regular parks with single trails and such dont. I am involved in the sport, I have an 03 Honda CRF450 dirtbike.
I try to do my part in keeping the trails clean, while most of us offroaders are good people, their are also alot of assholes too who cut up and ruin it for the rest of us. And at the same time their environmental groups who are after us for alot BS reasons.
As for 2 cycle exhaust, yes it can get obnoxious, though their are still alot 2 strokes dirtbikes, bike makers like Honda, Yamaha, Kawasaki, and Suzuki have completely switched over to 4 strokes except on the smaller MX bikes in the 85cc class. Honda so far has been the first the produce a 4 stroke alternative to their CR85, which is the CRf150R. Problem is, I dont think their would alot of buyer for it because it costs 1 thousand dollars more than the CR85.
Fuel usage, 2 strokes do use alot of fuel! They can burn through 2 gallons of gas in about 2 hours or so. 4 strokes can burn 2 to 3x less. Going by the trip meter on my bike, riding on trails I usually get up to around 65 miles on a 2.5 gallon tank, thats 26mpg.
You want to do something about people getting hurt? Open more offroad parks. Alot of parks have been getting crowded becuase of the closers of other parks, thus the open parks getting more crowded and then you have more people getting hurt.