Yamataians and Neko both are Androids, so they regenerate. Plus a visit to the Hemosynth tank can help accelerate the repair process.
As for a tolerance, everyone has a different one. Body mass etc, are factors.
But yeah, no earth, and no Irish, but he can claim norse blood because its estalished that the settlers of this sector are of japanese and Nordic descent.
Technically that means earth existed, but its been long forgotten, more like a myth at most these days