I think that if you consider emissions by engine displacement the model engine is far more pollutant than anything out there. Consider the fact that the engines are always run rich of stoich. Glow engines use a rich mixture to hold back detonation. In such a small cylinder combustion efficiency is pretty poor. I don't know what nitromethane does to emissions, but would be interesting to find out. A 2 stroke glow engine is about 11% efficient at best. OS's move with PVC and other things is in my opinion a preemptive effort on their part to avoid regulation. One would just need to figure out how many gallons of fuel are sold to get an idea of what kind of pollution occurs. but in the grand scheme of things vehicles will always be number one pollution contributors by volume.

At least castor oil and methanol are bio products. So we are pretty much emissions neutral in that regard.

It will be quite a while before you have to buy carbon credits to fly model aircraft.