Poor air quality near airports is the results of planes that use leaded-fuel piston-engine, despite leaded gasoline's phase out in the 1996 Clean Air Act. It is used for a fleet of 170,000 piston-engine airplanes and helicopters, and now makes up "the largest remaining aggregate source of lead emissions to air in the U.S.," according to the Environmental Protection Agency.

But if it was phased out 25 years ago, how is it still being used in thousands of aircraft today?