<P> Lead is a toxic metal that accumulates in the body and is associated with subtle neurotoxic effects especially at low exposure levels, such as low IQ and antisocial behavior . It has particularly harmful effects on children . These concerns eventually led to the ban on TEL in automobile gasoline in many countries . Some neurologists have speculated that the lead phaseout may have caused average IQ levels to rise by several points in the US (by reducing cumulative brain damage throughout the population, especially in the young). For the entire US population, during and after the TEL phaseout, the mean blood lead level dropped from 16 μg / dL in 1976 to only 3 μg / dL in 1991 . The US Centers for Disease Control considered blood lead levels "elevated" when they were above 10 μg / dL . </P> <P> Lead exposure affects the intelligence quotient (IQ) such that a blood lead level of 30 μg / dL is associated with a 6.9 - point reduction of IQ, with most reduction (3.9 points) occurring below 10 μg / dL . </P> <P> Reduction in the average blood lead level is believed to have been a major cause for falling violent crime rates in the United States and South Africa . Researchers including Amherst College economist Jessica Wolpaw Reyes, Department of Housing and Urban Development consultant Rick Nevin, and Howard Mielke of Tulane University, say that declining exposure to lead is responsible for up to a 56% decline in crime from 1992 to 2002 . Including other factors that are believed to have increased crime rates over that period Reyes found that this led to an actual decline of 34% over that period . </P> <P> A statistically significant correlation has been found between the usage rate of leaded gasoline and violent crime: taking into account a 22 - year time lag, the violent crime curve virtually tracks the lead exposure curve . After the ban on TEL, blood lead levels in US children dramatically decreased . </P>

When did they take lead out of gasoline