Seems rather perplexing that there would be a correlation between these two things, but scientists in the US have found that cities with higher levels of lead in the oxygen experience higher rates of crime in general. However, the effects of the lead are delayed (20 years are needed for crime activity to increase) so it wasn’t until recently that this connection was even spotted. The current theory behind this explains that exposure to lead at a young age can cause neurological damage during the child’s development.
It’s a whole chain off effects. Lead reduces IQ, which then reduces concentration. Diminished concentration accounts for learning difficulties and an impulse behaviour, which over the years can serve as the foundation of future criminals. Statistics show that cities, which were to adopt lead-free petroleum and paint, experienced a gradual decline in criminal activity over the next two decades.