You found mold. What to do first.

Find the water first. Mold is not the problem, it is the evidence of a moisture source, and every hour spent scrubbing a surface without finding the leak is an hour you will spend again in a month. Then work out how much of it there is, because the size decides whether this is a Saturday job or a contained one.

Do not start by putting bleach on it, and do not start by getting it tested.

Why the water comes first

Mold spores are in every building already. They are not the variable. Moisture is. A patch of mold means that somewhere near it, something has been wet for long enough, and unless that stops the mold comes back on whatever you clean or replace.

So the first hour is a search, not a cleanup:

• Is there a leak above it? A bathroom, a pipe in the wall, a roof, a window.
• Is it on an exterior wall in a cold corner? That is usually condensation rather than a leak, which is a ventilation and humidity problem.
• Is it near a plumbing fixture? Under a sink, behind a toilet, at a washing machine.
• Did anything flood here? Even months ago.
• Is the room simply humid? A bathroom without an exhaust fan, a basement without a dehumidifier, drying laundry indoors.

If nothing is obvious, the way to find a hidden water source is by when it appears, not where.

Then measure it, roughly

The size of the patch is what decides the job, and there is a widely used threshold: the EPA’s guidance is that an area of mold smaller than about ten square feet, roughly a three by three foot patch, can usually be handled by a householder. Larger than that, or inside the building fabric, and it needs professional containment.

That number is guidance rather than law, and it assumes the water source is fixed, the material is hard or replaceable, and nobody in the house is vulnerable. Where the boundary really sits, and what changes it.

What not to do today

Do not put bleach on porous material. It is the standard instinct and it is mostly wrong. On drywall or wood, bleach takes the color out of the surface while the growth continues in the material underneath, so the patch looks cleaner and is not. Detergent and physical scrubbing does more, and porous material that is genuinely colonized gets removed rather than treated.

Never mix bleach with anything containing ammonia, including many general purpose cleaners. It produces a toxic gas.

Do not paint over it. Mold resistant paint prevents growth on a clean surface. Over an existing colony it is a lid.

Do not put a fan on it. Air movement across a disturbed colony is how it gets distributed through the rest of the house.

Do not get it tested yet. In almost every household case the test does not change what you do, because the answer is the same regardless of species: remove it and fix the water. When a test is actually worth paying for.

What to do today

1. Fix or stop the water. If that means turning a valve off or putting a bucket under something until a plumber comes, do that.
2. Shut the door on the room if the patch is large or if it is being disturbed.
3. Photograph it, with something in frame for scale, before anything is cleaned. If a claim happens later this is the only record of what it looked like.
4. Get the humidity down: open windows if the outside air is drier, run the exhaust fan, run a dehumidifier if you have one.
5. Move anything porous and valuable out of the room.
6. Decide, from the size and the material, whether this is yours or not.

Is it even mold?

Worth ruling out before you plan a remediation. Soot from a candle, mildew on a grout line, dirt in a condensation streak, and efflorescence, the white mineral bloom on masonry, all get mistaken for it. How to tell them apart in about a minute.

And the question everyone asks first

Whether it is the dangerous kind. The short answer is that the color does not tell you much, the specific claims made about black mold are not well supported by the evidence, and all indoor mold gets removed regardless of species. The longer answer, and what is actually established.

The short version

Find the water and stop it, before anything else. Measure the patch: about ten square feet is the usual threshold between a householder job and a contained one. No bleach on porous material, no paint over it, no fan on it, no test yet. Photograph it before you touch it.