Is it mold, mildew, or just dirt?

Clean a small test patch with detergent and warm water, dry it, and look at it again a week later. Dirt and soot come off and stay off. Surface growth comes off and comes back. Growth that has gone into the material barely lightens at all, and that is the one that decides the job.

You will learn more from that week than from anything you can buy.

The quick tells, before the test

Smell. Active growth has a distinct musty, earthy smell. Dirt and soot do not smell of anything much. If a cabinet smells and looks clean, believe the smell.

Where it is. Growth appears where moisture collects: cold corners of exterior walls, behind furniture pushed against an outside wall, around window frames, on bathroom ceilings, along the bottom of a wall, under a sink, on the back of a closet. Dirt appears where air moves and where hands go: around vents, above radiators, at light switches, along baseboard where a vacuum cleaner has been.

The shape. Growth spreads in patches with irregular, fuzzy edges, sometimes in a rough circle from a starting point. Dirt from air movement forms streaks and even shading, often sharply following a stud, a joist, or the line of a draft. Soot from candles is a fine even gray film, worst near the ceiling and on cold surfaces.

Texture. Growth is often slightly raised, fuzzy, or speckled when you look closely. Dirt is flat.

Things that get mistaken for it

Mildew is really just the early, surface stage of growth on a hard, non-porous surface. Grout in a shower, a window frame, a windowsill. It cleans off properly and it comes straight back if the humidity does not change. It is not a different problem, it is the same problem at a stage where cleaning still works.

Soot from candles, an open fire, or a poorly burning appliance. Fine, gray, evenly deposited, and worst on cold surfaces and near the ceiling. If you burn candles regularly and the marks are above them, that is what it is.

Efflorescence is the white or gray crystalline bloom on brick, concrete, or a basement wall. It is salt left behind as water moves through masonry and evaporates. It brushes off as powder, it is not alive, and it is a moisture signal in its own right: something is bringing water through that wall.

Dust and lint collecting on a cold surface, particularly in a pattern that follows the framing behind the plaster. That pattern, called ghosting, means the wall is colder along the studs and moisture is condensing there. It is a sign the insulation is poor and the room is humid, and it will become growth eventually.

Water staining without growth: brown or yellow tide marks, dry and flat. A past leak. Worth finding the source anyway.

The test in detail

1. Pick a patch about the size of a hand, somewhere it will not matter if the finish changes.
2. Wear gloves and, if you have one, an N95. Do not do this at all if you have asthma or a lung condition, or if you are immune compromised or pregnant.
3. Clean it with detergent and warm water and a cloth, physically scrubbing. No bleach, and nothing that could contain ammonia.
4. Dry it. Photograph it clean, next to an uncleaned area.
5. Improve the ventilation in that room and do nothing else to it.
6. Look again after a week.

Came off, stayed off: it was dirt or soot. Clean the rest.

Came off, came back: surface growth with a moisture source that is still running. Clean it again if you like, but the job is the humidity or the leak.

Barely came off: it has gone into the material. On drywall, wood, or anything porous, that patch is not cleanable and the material comes out.

What it means for the plan

If it is growth of either kind, the next question is not what species it is but how much there is and what it is on. Find the water first, and measure the area, then work out whether the size makes it yours or a contractor’s.

If the test says dirt but the room still smells musty, that smell is coming from somewhere else, quite possibly inside a wall or under a floor. The signs of growth you cannot see.

When to skip the test

Do not spend a week on this if:

• The patch is large, roughly bigger than three feet by three feet.
• It is on something that got wet in a flood, a leak, or a backup.
• Anyone in the house is having symptoms that improve when they leave.
• It is spreading visibly week to week.

In all of those the answer is the same whichever it is, so go straight to dealing with it.

The short version

Clean a test patch with detergent and look again a week later. Off and stays off is dirt. Off and back is surface growth with a live moisture source. Barely comes off means it is in the material and the material goes. Smell beats appearance, and efflorescence is salt, not mold, but it still means water.