Went to a seminar many moons ago and if I remember correctly Shell said that the amount of lead emitted into the atmosphere world wide per year from Aviation was about 4 tonnes, that is a lump about the size of a coffee table.. Not exactly a lot is it.
One problem they had was if they couldn't get the specific rating out of a batch of fuel, previously they would of blended it to produce car fuel, with the leaded version of that gone, they now had to pay a company to take it away, burn it and filter the emissions.