How concentrated? Concentrated hydrogen peroxide (> 90%) is a rocket fuel. Above 8% is considered somewhat hazardous. Above 28%, hazmat precautions (corrosive, burns skin). Above 35%, unstable. Above 50%, even large industrial users usually don't go.[1]
I'm not sure, I believe it's 35% or more. She once mentioned that "it's not the legit stuff unless you have to sign a hazardous materials sheet" or something to that affect.
[1] http://www.h2o2.com/technical-library/default.aspx?pid=76&