Home mold test kits vs. professional testing

Home mold test kits are popular, and they're cheap. The most common type is a settle plate: you open a dish of growth medium, leave it out for an hour, close it, and see if anything grows a few days later.

Why kits almost always come back positive

Mold spores are a normal part of outdoor air, and some always come inside on air, clothes and pets. A dish of growth medium left open in nearly any home will grow something. So a positive kit doesn't tell you that you have a mold problem. It tells you that you have air.

Settle plates also don't measure a known volume of air, they favor spores that are heavy enough to fall and species that grow easily in a dish, and they give you nothing to compare against. You can't tell whether what you caught is normal or elevated.

What a professional test measures

• A measured air volume. A calibrated pump pulls a known amount of air through a spore trap, so results are reported as spores per cubic meter.
• An outdoor control. The same sample is taken outside at the same time. Indoor results are judged against it, since there are no federal limits for indoor mold.
• Lab identification. An accredited lab counts and identifies the spores, so you know which types are present, not just that something grew.
• The moisture source. A good inspection also looks for why mold is there, which no kit can do.

When a kit is fine

If you can already see a small patch of mold in a bathroom and you know why it's there, you don't need a kit or a professional test. Fix the moisture, clean the surface, and watch for it to return.

When the source is hidden, someone is having symptoms, you're buying a home, or you need documentation, professional air quality testing is the way to get a real answer.