What kills black mold and how to use it

Bleach, vinegar, hydrogen peroxide, and borax all kill black mold on hard surfaces. Bleach works fastest — mix one cup per gallon of water, spray it on, wait 10 minutes, and scrub. Vinegar takes longer but is safer indoors; spray undiluted white vinegar, let it sit for an hour, then scrub. Hydrogen peroxide (3 percent from the drugstore) works similarly to vinegar. Borax powder mixed with water creates a solution that kills mold and prevents regrowth for weeks.

The choice depends on where the mold is and what surface it covers. Bleach is strongest but produces fumes and can damage or discolor porous materials like wood and drywall. Vinegar and hydrogen peroxide are gentler on surfaces and safer to breathe. Borax is best for prevention after you've cleaned, because it leaves a residue that stops mold from coming back as fast.

For any method, wear gloves and eye protection. Open windows or run a fan to move air out of the room. If the mold covers more than 10 square feet, or if it's inside walls or HVAC ducts, stop and call a mold remediation company — home methods won't reach it all.

Key Takeaways

  • Bleach solution (1 cup per gallon of water) kills black mold in 10 minutes but produces strong fumes and can damage porous surfaces.
  • White vinegar or 3 percent hydrogen peroxide work more slowly but are safer to use indoors and won't bleach or weaken materials.
  • Borax powder mixed with water kills mold and leaves a protective residue that slows regrowth for several weeks.
  • Mold larger than 10 square feet, or mold inside walls and ducts, requires professional removal because surface cleaning won't reach all of it.
  • Always wear gloves and eye protection, and ventilate the room during and after cleaning to avoid breathing mold spores and fumes.

Bleach: fastest but with tradeoffs

Bleach kills black mold on tile, glass, and sealed stone in about 10 minutes. Mix one cup of unscented household bleach (5 to 6 percent sodium hypochlorite) into one gallon of water. Pour it into a spray bottle, spray the moldy area until it is wet, set a timer for 10 minutes, then scrub with a stiff brush and rinse with clean water.

Bleach has two major drawbacks. First, it produces chlorine gas fumes that irritate your lungs and eyes — you must open windows and run a fan the whole time you work. Second, bleach can discolor or weaken porous materials like wood, drywall, grout, and fabric. If the mold is on painted drywall, use vinegar or hydrogen peroxide instead. If it's on wood trim or subflooring, vinegar is safer.

Bleach also does not penetrate porous surfaces deeply, so mold roots that have grown into wood or drywall will regrow. For those materials, you need to remove and replace the damaged section, not just clean the surface.

Vinegar and hydrogen peroxide: safer for most indoor spaces

White vinegar (5 percent acetic acid) and 3 percent hydrogen peroxide both kill black mold without fumes or surface damage. Spray undiluted white vinegar directly onto the mold, let it sit for one hour, then scrub and rinse. For hydrogen peroxide, spray it on, wait 10 minutes, scrub, and rinse. Both work on tile, grout, wood, drywall, and fabric.

Vinegar works more slowly than bleach but is safer to breathe and won't bleach or weaken materials. The smell is sharp but fades as it dries. Hydrogen peroxide works faster than vinegar — about as fast as bleach — and leaves no smell. Both are cheap and available at any drugstore or grocery store.

Neither vinegar nor hydrogen peroxide prevents regrowth as well as bleach or borax do. After cleaning, the mold can return in a few weeks if the moisture source is not fixed. If you use vinegar or hydrogen peroxide, plan to clean again in three to four weeks, or switch to borax for longer protection.

Borax: best for prevention after cleaning

Borax powder is a mineral salt that kills mold and leaves a residue that slows regrowth for weeks or months. Mix one cup of borax powder into one gallon of water. Spray or brush it onto the cleaned surface and do not rinse it off — the powder that stays behind is what stops mold from coming back.

Borax is slower than bleach or hydrogen peroxide at killing active mold, so use it as a second step: clean with vinegar or hydrogen peroxide first, let the surface dry completely, then explore borax solution. The combination kills the mold and protects against regrowth.

Borax is safe to touch and breathe, but do not ingest it and keep it away from pets and small children. Wear gloves when mixing and explore it. Borax costs less than a dollar per pound and is sold in the laundry aisle of most grocery stores.

Where you can clean yourself and where you need a professional

You can safely clean black mold yourself if it covers less than 10 square feet, is on a hard surface you can see and reach, and is not inside walls, attics, crawl spaces, or HVAC ducts. Bathroom tile, kitchen counters, basement concrete, and window sills are all safe to handle yourself.

Stop and call a mold remediation company if the mold covers more than 10 square feet, if it is inside drywall or wood framing, if it is in your HVAC system, or if you cannot see where it ends. Mold inside walls spreads through the wood and insulation and cannot be killed by surface spraying. Professionals use containment, HEPA vacuums, and sometimes remove and replace the affected materials.

If you have had water damage — a burst pipe, roof leak, or flood — and the materials dried more than 48 hours after the water stopped, mold has likely grown inside the walls. Do not assume surface cleaning is enough. A mold inspector or remediation company can tell you whether the damage is visible or hidden.

Fixing the moisture source so mold does not come back

Mold returns if the surface stays wet or very humid. After you kill the mold, find and fix what made it grow. Common sources are poor ventilation in bathrooms, leaking pipes under sinks, cracks in basement walls that let in groundwater, roof leaks, and air conditioning condensation.

In bathrooms, run the exhaust fan during and for 20 minutes after showers. Wipe down wet surfaces and leave the door open to dry the room. Under sinks, check for drips and tighten connections. In basements, seal cracks with hydraulic cement and make sure gutters and downspouts drain water at least 4 feet away from the foundation. If you have a persistent leak, fix it before you clean the mold, or the mold will grow back within weeks.

If you cannot find the source, or if mold keeps returning after you clean it, call a plumber or water damage specialist. They can use moisture meters and thermal imaging to find hidden leaks and condensation problems you cannot see.

Safety steps while cleaning

Wear nitrile gloves, eye protection, and an N95 mask if you are sensitive to mold or if you are using bleach. Open all windows in the room and run a box fan or window fan to push air outside. Do not mix bleach with ammonia, vinegar, or other cleaners — the combination produces toxic gas.

If you feel dizzy, short of breath, or your eyes burn, leave the room when ready and get fresh air. Mold spores can trigger asthma and allergies, so if you have either, wear a mask and consider having someone else do the cleaning.

After you finish, wash your clothes and take a shower to remove mold spores from your skin and hair. Dispose of heavily molded materials like drywall or insulation in a sealed bag in the trash — do not leave them in the garage or basement where spores can spread.

Frequently Asked Questions

Can I use bleach on drywall or wood?

No. Bleach can discolor and weaken drywall and wood. Use white vinegar or hydrogen peroxide instead. If the mold has grown deep into drywall or wood, the material may need to be removed and replaced, because surface cleaning will not kill the roots.

How long does it take for vinegar to kill mold?

Vinegar needs about one hour of contact time to kill black mold. Spray it on, set a timer, then scrub. Hydrogen peroxide works faster — about 10 minutes — but vinegar is safer on delicate surfaces and does not bleach.

Will the mold come back after I clean it?

Yes, if the moisture source is not fixed. Mold returns within a few weeks if the surface stays wet or humid. Find the leak, poor ventilation, or condensation problem and fix it first, or clean again every three to four weeks using borax to slow regrowth.

Is black mold more dangerous than other mold?

Black mold (Stachybotrys) can produce toxins, but so can other molds. Any mold in your home can trigger allergies and asthma. The safest approach is to remove all mold and fix the moisture source, regardless of the type.

What should I do if mold is in my air vents?

Do not try to clean mold inside HVAC ducts yourself. Call a professional mold remediation company or HVAC contractor. Mold in ducts spreads spores throughout your home every time the system runs, and surface cleaning will not reach it all.