Does bleach kill mold?

Porous and non-porous is the only distinction that matters

Bleach is a strong oxidizer and it genuinely kills mold on contact. The problem is not the chemistry. The problem is that it cannot reach what it needs to reach.

On a hard non-porous surface such as glazed tile, glass, a bath, a sealed countertop or a metal vent cover, the growth sits on top. Bleach contacts all of it, kills it, and you wipe it away. That is a real result and there is nothing wrong with doing it.

On a porous material the growth is not sitting on top. Mold puts hyphae into the material the way roots go into soil, and on drywall, plaster, bare wood, insulation, grout and particle board the visible patch on the surface is the smallest part of the organism. Household bleach is roughly 94 percent water. The chlorine is a large molecule that stays near the surface; the water soaks straight past it into the material.

You bleach the stain and water the roots. The surface looks clean for a season, and the colony has been given a drink.

This is why people are convinced bleach works and equally convinced it comes back. Both observations are correct and they are the same observation.

Vinegar, hydrogen peroxide and the rest

The same test applies to every product, which is more useful than a ranking.

• Vinegar penetrates porous material better than bleach and is less aggressive on the material itself. It is slower and it does not remove staining, so people assume it failed when it often did more than the bleach did.
• Hydrogen peroxide works on contact, penetrates reasonably, and breaks down into water and oxygen. It also bleaches fabrics and finishes, which surprises people.
• Ammonia has the same surface-only limitation as bleach, and must never be mixed with it. Bleach and ammonia together produce chloramine gas. People do this by accident, cleaning one surface after another in a small bathroom with the door shut.
• Concentrated cleaners marketed as instant mold killers are generally one of the above with a fragrance. The question to ask is not what is in it but whether it can get where the growth is.

The part everybody skips

Every product on that list treats the growth. None of them touches the reason it grew.

Mold needs sustained moisture. If you clean a bathroom ceiling and the fan still vents into the attic, you have not changed anything about the conditions; you have only reset the clock. The same is true of a wall fed by a sprinkler head, a closet on a cold exterior wall in a full house, or a crawl space with bare soil under it.

The single most useful thing a homeowner can do about a mold patch is not to attack the patch. It is to work out what is wetting it, which is usually findable and often cheap to correct.

When cleaning it yourself is the right call

Reasonable, and worth saying plainly rather than pretending every spot needs a company:

• The surface is hard and non-porous
• The area is small, roughly a square meter or less
• You know what wet it, and that has been fixed
• Nobody in the household reacts to mold, and you are not immunocompromised
• It is not recurring in the same place

Ventilate, wear gloves and an N95, and do not dry-brush or sand it, which puts spores into the air you are standing in.

When to stop

Stop and have somebody look if the material is porous and the patch is larger than that, if it keeps coming back in the same place, if there is a smell with nothing visible, if it followed a leak or a storm, or if anyone in the house has symptoms that clear up when they leave. Those are not upsell triggers; they are the cases where surface cleaning is measurably the wrong tool.

What a treatment does differently is not a stronger chemical. It is reaching the cavity the growth came out of, removing the dead material rather than leaving it in place, correcting the source, and then having somebody independent read the result instead of judging it by eye.

If you are looking at a patch right now and are not sure which category it falls into, that is exactly what the free inspection answers, including when the answer is that you can handle it yourself.