Lead is a heavy and soft metal that is traditionally used as the main ingrediend in projectiles. Since it is a maleable substance, a lead bullet will mushroom on impact, which causes more damage to the internal organs and helps provide a quick harvest. Lead shot pellets deform due to air resistance on their way to the target. They will generally have a much larger pattern area than other materials, such as steel.
The problem is however, that lead is a toxic substance. When lead pellets are used by waterfowl hunters, most of the pellets will fall to the ground and the toxic metal will slowly leech into the soil and water table. The lead pellets are also occasionally eaten by migratory birds. The poisonous lead accumulates in the animal's body as the animal eats more and more, and quickly becomes a case of lead poisoning. This can be very dangerous to the animals and the ecosystem.
For this reason, federal regulations now state that it is illegal to hunt waterfowl with lead shot. Hunters must use non-toxic shot, such as steel, tungsten-iron, or tungsten-matrix for waterfowl hunting.