And I would like to suggest a few major culprits that are not the roofs
of asphalt nor sheet metal. Culprit (1) is the automobile and other
vehicles which contain alot of lead for the wheel alignment, sometimes
those lead balances fall off onto the side of the road and present alot
of lead pollution into the environment not to mention the brake system
possibly leaking lead into the environment. And perhaps mercury exists
on automobiles which leaks into the environment. Culprit (2) is that of
guns and pistol ammunition is often lead and so if a person shoots off
some ammo or even just drops it near a building then those researchers
mistakenly attribute the roofs as the source of lead. Culprit (3) the
mercury could be from some other products close to houses and buildings
such as electrical switches of mercury or thermometers and because they
are close to buildings that the researchers mistook the source as being
from the roof whereas it was from the human activity around the
buildings.

Archimedes Plutonium
www.iw.net/~a_plutonium
whole entire Universe is just one big atom
where dots of the electron-dot-cloud are galaxies