Gary McDonald, age 13, of Newport News, Va., for his question:

IS LEAD THE ONLY RADIOACTIVE SHIELD?

Our remote ancestors coped with plagues and endless other natural disasters. They also found time to invent destructive devices of their own. Modern man has walked on the moon and invented nuclear bombs  the most ghastly of all destructive weapons.

An atomic explosion spreads clouds of radioactivity and some of it remains dangerous for generations. Naturally nobody likes to think about this radioactive fallout. But no problem improves by refusing to face it. In this case, what we know about radioactive shielding may save our lives.

The problem is complex because a nuclear explosion releases a variety of radioactive energies, some more dangerous or longer lasting than others. Distance also may make a difference.

In underground shelters, the walls and floors are shielded by earth. However, a shield of earth at least three feet thick must be piled on their roofs.

So lead is not the only shield against radioactivity. In most cases, even the most dangerous emissions are reduced as they pass through thick and thicker layers of dense material, such as hard rocks.