While bleach kills the mold it comes into contact with, it is not efficient for killing mold growing on porous substrates because it does not penetrate these substrates well. Bleach is suitable for removing mold growing on hard, non-porous surfaces only.
Mold growing on a porous substrate may spread deep under the surface. If chlorine bleach is applied to a moldy spot, it kills the mold on the surface and bleaches the dark mold stain. However, the mold may remain alive under the surface.
Bleach causes damage to some materials, such as wood. It is also toxic and, if mixed with ammonia, it produces dangerous mustard gas. Therefore, it is necessary to use chlorine bleach with caution.