Do home mold test kits work?

What does a home mold test kit actually measure?

Most hardware-store kits measure whether mold spores land on a dish of growth medium. The common type is a settle plate: you open a petri dish, leave it in a room for a set time, close it and watch for colonies over the next few days. Some kits also include a swab or tape for lifting material off a surface, and many offer lab identification of whatever grows for an extra fee.

That is all the kit measures. It doesn’t measure moisture, the size of any growth, or what is happening inside a wall.

Why does almost every kit come back positive?

Because there is mold in the air of nearly every home. The EPA says it is impossible to get rid of all mold and mold spores indoors, and that some spores will be found floating through the air and in house dust.¹ Leave an open dish of food for mold in any room and something will usually land on it and grow.

A positive plate therefore can’t separate normal background spores from an actual problem. A clean house in Silver Lake and a house with a slow leak behind the dishwasher can both grow colonies.

What can’t a kit tell you?

The questions that matter are where the water is, how far the growth goes and what is causing it. A kit answers none of them.

It also gives you nothing to compare against. The EPA notes that no federal limits have been set for mold or mold spores.² The California Department of Public Health goes further: it recommends against measuring indoor microorganisms to judge the level of health hazard or the need for urgent remediation, and recommends finding and fixing the water source, drying damp materials and cleaning up mold instead.⁴

Is lab analysis of a home kit any better?

Lab identification tells you what grew on the dish, but the species rarely changes what you should do. The CDC says it is not necessary to determine what type of mold is growing in your home, and that all molds should be treated the same with respect to potential health risks and removal.³

When sampling is useful, the EPA says it should be done by professionals with experience designing sampling protocols and interpreting results.² A sample is only as good as the question it was taken to answer.

What works better than a test kit?

The checks that find moisture, because moisture is what lets mold grow:

1. Use your nose. A musty smell that is strongest in one spot points to a local source. Our guide to a musty smell in the house covers where to look.
2. Look where water goes. Under sinks, behind toilets, around windows, near the water heater and AC air handler.
3. Buy a hygrometer. The EPA recommends keeping indoor humidity below 60%, ideally between 30 and 50%, and notes that humidity meters are inexpensive.¹
4. Watch for condensation. Water on windows or pipes is a sign of high humidity.¹

If you’ve found something and aren’t sure what it is, send us a photo or take the mold check.

When should you call an inspector instead of using a kit?

You often don’t need either. A small patch of mold under about 10 square feet on a hard surface with an obvious cause, like shower grout, can usually be cleaned with detergent and water.¹

An inspection makes sense when:

• A musty smell persists and you can’t find the source
• There was a leak and you don’t know whether the area dried
• Growth keeps coming back after cleaning, or covers more than about 10 sq ft
• You’re buying a home and want the moisture picture documented

A mold inspection is $495 for one dwelling up to 2,000 sq ft and includes moisture readings, photos and a written report. Surface samples are optional at $125 each, and we only take them if a lab result would change your next step. If a kit already came back positive, that alone isn’t a reason to book. Tell us what you’re seeing and we’ll say whether a visit is worth it.