"shaving" old key leads

[frank]

How can you be sure that "shaving" old key leads isn't harmfull? Maybe 
some of you like the sweet taste you get in your mouth after breathing 
in the dust, but I think you better be carefull with the oxidised lead. 
Use a good vacuum cleaner when doing the job.
Any thoughts how it became oxidised in the first place? To my knowledge 
good pure lead lasts thousands of years (the Romans used lead piping in 
their bathhouse in Bath (GB) wich are still intact), so why does it 
oxidise in some piano's? You can even have very good leads and very 
oxidised in one piano, even in one key.
Impure lead? Something in the wood of the keys?