lead is only an issue if you inhale the dust or lead fumes, or manage to ingest it, which again is usually down to dust particles
i spent 2 years running a press in a battery factory using lead oxide plates and had to have blood tests every month. If lead in blood rose to above something lime 32micrograms in 10ml of blood wr were moved to a 'clean area' and told to drink lots of milk and fizzy water to flush the lead out
The lead content is very small it's only .25%. Its also contains sulpher to help the machinability. In short you should not breath anything that the lungs are not meant to breath, which of course as we all know is impossible in the society we live in. Talk to any respiratory team dealing with lung conditions on a daily basis and they will all tell you the same thing. Would I stand there breathing in lung fulls of welding fumes from any type of welding or material? Erm no, not by choice. Will it kill you welding it?........ probably not, but when you have a lung condition appear when you are 60 you may wonder if that welding you did all those years ago has attributed to the problem.. If in doubt use a fume mask, cheap throw away one are available at a 3 quid each or so from screw fix etc. It's always better to be safe than sorry.
Be aware that En1a and En1a leaded are not recommended for welding due to strength issues. That said if its something non structural and no strength requirement is needed then yes it will do a turn. It can undercut a bit and also a bit of porosity can occur, technique can help resolve this but as always takes bit of figuring out.
It's ok to glue things together and will do job of sorts, but it is what it is, a grade not recommended for welding and purpose made for machining. The odd thing for decoration it will pass but that's as far as it goes.
Maybe but if I find red s### in a tissue when I blow my nose after cutting some bricks I class the mask as useless, the filter media may be ok but the face seal and head straps aren't up to much.