Chlorine bleach is extremely basic, as it has a high pH of 13. This means that a chlorine bleach solution has a H+ concentration of about one-millionth of the H+ concentration in pure water.
Chlorine bleach is only safe to use because it so basic. When chlorine is mixed with water, the hypochlorite ion is in equilibrium with chlorine gas. However, if the pH is lowered, as when the bleach is mixed with an acid, the hydrogen and oxygen atoms in the hypochlorite ion form water molecules with the extra hydrogen ions. The result is the formation of chlorine gas, which is toxic to humans.