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Leaded gas was introduced in the 1920s; leaded gas combustion produces Pb vapor, the human body treats Pb as though it's Ca, mothers pass Pb contamination on to their infants in utero & via nursing, Pb causes neurological developmental deficiency, brain damage resulting in lowered IQ... ...so if you're wondering why a disproportionate number of U.S. voters are so freakin' dumb...