Lead is a poisonous substance to animals. It damages the nervous system and causes brain disorders. Excessive lead also causes blood disorders in mammals. Like the element mercury, another heavy metal, lead is a potent neurotoxin that accumulates both in soft tissues and the bones. Lead poisoning has been documented from ancient Rome, ancient Greece, and ancient China.
...Very chilling. (not really) A little known fact... lead by itself (or alloyed with antimony and/or tin) combined with common propellants can be used as an efficient and economical 'substance' to incapacitate animals from a distance. This combination (known to some as 'hot lead', 'ammo' and 'shot') can also be used for recreation and self defense. Other non-lead containing alloys can also be used for the same effect, but those alloys have been limited for use by laws and regulations.
It seems that the EPA is very concerned about lead and how it acts as a poisonous substance... so much so that they want to ban its use as ammunition. The alternatives to lead based ammo are very expensive, and in many cases illegal to use.
Take a look and Vin Suprynowicz's article for more information: Birdshot petition gets taken for a ride
(Thanks goes to C.Anon for forwarding this article to my attention)