The UK finally ended sales of leaded petrol in 2000. From 2018 we should start to see the benefit of this in terms of a substantial reduction in the need for prison places. London's sparrows, sadly, are unlikely to recover.
I suspect readers are divided amongst those who know exactly what the para above is above and those frowning in puzzlement. Well, Lead in petrol and crime rates are very closely correlated, and as nations banned tetraethyl Lead as a petrol additive from the 1970s onwards they began to notice, with a lag of about 19 years, substantial reductions in many crimes as blood lead levels dropped in young people at peak offending age. The science behind this is very robust – each nation's drop in crime follows very closely from the drop in leaded petrol. And not only crime but teenage pregnancies, ADD, and other problems.
There's a good quality paper HERE from Mother Jones with good links to many of the research papers.
Sparrows? Well, they have disappeared from large cities as air pollution from leaded petrol has decreased. No one knows why. Populations in smaller cities and market towns are fine – places where traffic concentrations and low speeds were not a problem.
Prisons are having a crisis at the moment due primarily to overcrowding and understaffing. The government, rightly, are holding back from building yet more prisons. In a few years, unless we discover even more widespread Muslim gang child sexual abuse, demand for prison places will fall. The crisis will solve itself.
I once had an officious safety official demand that I take steps to remove Lead-contaminated dust from a building conversion in London down to a level of 25ppm (25mg/kg). I was happy to refuse and tell him to get lost. The background Lead level in the soil outside, and for miles around, was in excess of 350ppm. Two thousand years of Lead pipes, roofs, windows, industry, crafts, glass, potteries and half a century of leaded petrol have gifted our big cities with Lead levels in the soil that are off-the-scale – but not a major health issue. You really need either to eat Lead or to breathe it in from vehicle exhausts or burning processes to be affected. It's very insoluble and does not penetrate the skin at all easily.
Funny though that all our proactive scientists who are so good at telling us what's bad for us actually managed to miss Lead completely.