Less than 30 years ago, England had been heavily polluted by industrial chemicals. But the air quality has improved dramatically in the last 25 years (as it did in many other cities in the industrialized world), largely because of well-enforced government regulations, reduced industrial output, closure of coal-fired power stations, and the switch to central heating from coal stoves.

A study last year by Britain's National Physical Laboratory showed a 70 percent reduction at 17 sites around the country in the average levels of all the heavy metals that were tested, including lead, iron, and copper.