The prevalence of lead may explain crime rates for the past century, according to studies and recent reporting.

"Neurological research is demonstrating that lead's effects are even more appalling, more permanent, and appear at far lower levels than we ever thought," wrote Kevin Drum in Mother Jones magazine. According to research at the University of Cincinnati, lead promotes cell death in the brain and can damage brain tissue — including impacting neuron connectivity and reducing gray matter in the prefrontal cortex.

"Even moderately high levels of lead exposure are associated with aggressivity, impulsivity, ADHD, and lower IQ. And right there, you've practically defined the profile of a violent young offender," wrote Drum.

Lead may also explain why cities have more crime per capita than other areas — lead (and therefore lead exposure) is more concentrated there. A study published in Environment International last year found a link between childhood lead exposure and crime rates 22 years later in four major U.S. cities.

"Children are extremely sensitive to lead dust, and lead exposure has latent neuroanatomical effects that severely impact future societal behavior and welfare," said toxicologist Howard W. Mielke. "Up to 90 percent of the variation in aggravated assault across the cities is explained by the amount of lead dust released 22 years earlier."

Researchers compared lead and crime rates in six cities over 35 years. Mielke said that vehicles using leaded gasoline contaminated cities' air, which in turn led to increased aggravated assault in urban areas.

In an international examination of crime trends, researcher Richard Nevin found a similar connection. Studying nine countries, Nevin found a "very strong association" between preschool blood lead and subsequent crime rate trends.

Even though U.S. gasoline is no longer leaded, lead molecules that have settled in the soil and lead in old buildings can have the same harmful effects — especially during renovations or construction, as lead molecules are redistributed into the air.

Drum concludes that lead reduction would have significant economic benefits — upwards of $200 billion a year due to higher lifetime earnings and crime reduction.

"Not only would solving our lead problem do more than any prison to reduce our crime problem, it would produce smarter, better-adjusted kids in the bargain," wrote Drum. "Cleaning up the rest of the lead that remains in our environment could turn out to be the cheapest, most effective crime prevention tool we have."

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