Britain's National Physical Laboratory monitored air quality at 17 sites and determined the presence of harmful heavy metals such as lead, iron and copper had diminished.

The results show a 70 percent reduction in all heavy metals tested. Lead showed a particularly sharp decline, falling from 556 nanograms per cubic meter in 1980 to 19.95ng/m3 -- a reduction of 96.5 percent. A nanogram is one-billionth of a gram.

The decrease in air pollution reflects advances in environmentally focused technology such as unleaded fuel, said Richard Brown, NPL's principal research scientist.

"Taking lead as an example, the steady decline of emissions from coal and oil combustion along with the change in fuel usage, and reductions in industrial output, has resulted in a significant reduction of lead in the atmosphere," said Brown. "We expect to see this decline continuing across the board until levels finally bottom out and become close to those occurring naturally."

Heavy metals monitored since 1980 were cadmium, chromium, copper, iron, manganese, nickel, vanadium, zinc and lead. Elements monitored since 2003 include arsenic, platinum and mercury.