06/22/2026
For three decades, every cleaning aisle pushed the wrong answer.
Chlorine bleach is about 90 percent water. On a nonporous surface like glass or tile, the chlorine has time to oxidize the mold cells before it evaporates. On porous material like wood studs, drywall paper, or grout, the chlorine stays on the surface while the water carries deeper into the material, hydrating the mycelium that lives below the visible patch. The hyphae, the root-like threads of the colony, often regrow within days, sometimes hours, because the cleaner itself delivered the water they needed.
The EPA's official Mold Remediation in Schools and Commercial Buildings guide, updated in 2008, explicitly stops recommending bleach for porous surfaces. OSHA followed in its own 2013 mold guidance, and the New York City Department of Health did the same. Most homeowners never got the memo because the labels on bleach bottles never changed, and the same product still appears in every "mold killer" listicle on the internet.
What works on porous material is physical removal. Cut out the affected drywall about 12 inches past the visible growth, sand the wood, and dry the cavity to below 18 percent moisture content using a fan and dehumidifier. A moisture meter from a hardware store costs about $25 and confirms the cavity is actually dry rather than just dry to the touch. Treat the exposed studs with a borate-based wood preservative before you close the wall back up, since borates pe*****te the wood fibers in a way chlorine never could.
For nonporous surfaces, plain dish soap and water is enough to lift the colony so it can be wiped away. The mechanical action of scrubbing does more than the chemical does. Rinse the cloth often so you are not just spreading spores around the room, and finish with a 70 percent isopropyl alcohol wipe which evaporates fast enough not to feed the colony.
If you must use a sprayed product on porous material, a 3 percent hydrogen peroxide solution pe*****tes better than bleach and leaves no chlorine residue. Even then, treat it as a stopgap until the porous material can be replaced. [0OHVT]