As long as automobiles have existed, they have spewed dangerous toxins into the air. The content and volume of these emissions have changed over the years. When leaded gasoline was the norm, the blood lead levels of the American population were significantly higher than they are today. Before catalytic converters, smog was an even worse problem, especially in car-heavy cities like Los Angeles. Currently, America's cars contribute to a staggering one-fifth of our nation’s carbon emissions and almost half of global automotive carbon emissions.