In January, Kevin Drum from Mother Jones published a shocking article linking the rise and fall of crime rates from the 1960s through the 1990s to lead gasoline emissions. During the 1990s, the huge crime rates in New York and in many cities severely dropped. In 2010, violent crimes rates in New York City had decreased 75% from their peak in the early 1990s.
Many theories came to explain this trend, but none of them were perfect. Rick Nevin, a consultant working for the US Department of Housing and Urban Development on lead paint, worked on the link between leaded gasoline emission and violence. From 1940s to 1970s, consumption of leaded gasoline quadrupled. Then, emissions fell very quickly because unleaded gasoline replaced it. Nevin found that the curve fitted the evolution of crime rates with a 23 years lag. Even if it was still just a correlation, it was already an exciting theory.
We know that lead exposure, mainly during childhood, causes lower IQ, hyperactivity, behavioral problems and learning disabilities. Neurological studies explain that lead degrades brain connections and executive functions—which means, among others, emotional regulation and impulse control. The safe level of lead exposure decreased over the year and is now zero.
Then, Drum explains how correlation became causation. Jessica Wolpa Reyes, a graduate student, found that the reduction of lead emissions was not uniform in all states and cities. During the 1970s and 1980s, the catalytic converter was introduced and the EPA rules became stricter and stricter, but progressively. Reyes found that, in cities or states where leaded gasoline emissions declined quickly, so did crime rates. In 2007, Nevin published a paper studying the different crime trends around the world and found the same correlation in other countries. Researcher Howard Mielke and demographer Sammy Zahran released a study focused on 6 American cities that had good lead and crime rate data, and it confirmed the theory as well. In New Orleans, Mielke even studied this relationship at a neighborhood level, and lead concentration maps matched crime maps.
These studies at international, national, state, city and neighborhood levels are supported by many others following groups of children that found a link between blood lead levels and adult arrest rates. They also partially explain the difference of crime rates between small and big cities. Some researchers found similar trends with lead paint at the beginning of 20th century, but with less data.
Why did it take so much time for this theory and studies to come out? First, it is because the phenomenon was studied by criminologists—who are mostly sociologists. They look for social reasons, not medical ones, so might have missed the connections between bodily lead levels and behavior. Second, conservatives defended the idea that the rise in crime was due to the social upheaval in the 1960s, and thus put pressure on other theories.
There is still lead in a lot of places, especially in the soil. Mielke found that soil levels directly correlates with blood levels. For instance, children who live in neighborhoods with a soil level of 100 ppm have average blood lead concentrations of 3.8 μg/dL while those living in areas at 500 ppm go up to 5.9 μg/dL. According to Mark Laidlaw on the website Urban Lead Poisoning, people and pets bring lead dust from soils in the house and children ingest it via hand-to-mouth contact. Also, every summer, all the lead is back in the atmosphere. Many houses still have lead paint on walls. Drum wondered why it takes so long to renovate and clean up this lead. The first answer he received was that the most concerned areas are low-income neighborhoods. He gives the example of a reporter from the Baltimore City Paper who asked the same question to Herbert Needleman, a pioneer researcher on the effect of lead on behavior. He answered: “Number one, it’s a black problem.”
Political leaders must act quickly on this issue since the dangerous and lasting effects of leads are proven. Mielke and Zahran estimates that if the US adopted the standards of a country like Norway on soil levels (100 ppm or less), it could bring roughly $30 billion in annual returns from the cognitive benefits only. Of course, it is difficult to evaluate the exact returns of such policy, but the cost of inaction is too high to stay where we are today. The legacy of lead has dreadful consequences, so dreadful that the Baltimore-based lead-litigation attorney Saul Kerpelman qualify lead as a “civil-rights issue”. We need to consider and address such environmental impacts on health as a top priority if we want to build a sustainable society.