I don't particularly believe lead in avgas is a risk to the environment, but it's toxic enough that I would prefer not to have to deal with it myself.
By the time I've dipsticked the tanks, refuelled, and tested the tanks for water, I'm as likely as not to have gotten at least a splash on me. By my reckoning 5 mls of 100LL would have about 2.5 milligrams of the stuff, or about a milligram of lead, which is about 170 times the targeted maximum daily intake for a Californian (6 micrograms).
Assuming it doesn't depend on something even nastier, bring on unleaded.