Lead
Hence the tip of not eating anything using your fingers during or after tying unless you wash them first. There are a number of other chemicals on tying materials (recently covered in another thread), so keep your hands away from your mouth and tying materials out of your mouth.
There are very few bodies of water that do not allow lead, but there are enough out there that you should check the fishing regulations on any waters you are not familiar with.
Larry ---sagefisher---
Organizations and clubs I belong to:
Fly Fishers International Life Member
FFI 1000 Stewards member
FFI Presidents Club
FFI Fly Tying Group Life Member
Washington State Council FFI
V.P. Membership
Alpine Fly Fishers Club
President & Newsletter Editor--The Dead Drift
North Idaho Fly Casters club