It's not 8% of some hypothetical dose dose that would be known to be harmless to humans. It's 8% of a dose that seemed harmless to mice, when administered for no longer than a mouse's lifespan, as far as the EPA looked.
While true, I believe the point is that mice don't live very long, so long term effects may not be visible.
(I don't know how true that actually is, perhaps mice show effects faster, but this is one of the reasons I'm dubious about the value of animal testing even if we don't have much better methods available yet).
