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Source: BTS.
Emissions of Major Air Pollutants in the United States, 1970-1991 (in million metric tons)
There are major changes in the contribution of transportation to the emission of major pollutants in the United States between 1970 and 1991. In absolute numbers, total emissions have stabilized or decreased significantly even if the consumption of fuel by road transportation modes has increased from 92.3 billion of gallons in 1970 to 128.6 in 1991. It can be mostly attributed to emissions regulations for the internal combustion engine, like the diffusion of the catalytic converter and better fuel consumption norms. The most significant change is from lead emissions, the result of a ban of lead tetraethyl as an anti-knock agent in most developed countries during the 80s. All this suggests that externalities are shifting from direct to indirect consequences because the number of cars is increasing while their total emissions are lessening.