O2 Safe Shock is a non-chlorine shock (but not a persulfate compound). It yields more than 12% active oxygen (3 times more oxygen than persulfate compounds). O2 Safe Shock can be used as an acid wash pretreatment, making tough copper stains much easier to remove. Please Note: Not recommended for maintenance in chlorinated pools, but ideal for biguanide, ozone, and ionization systems.