Most homeowners know the word mold.

They have seen it. Maybe in a bathroom corner. Maybe on a basement wall. Maybe somewhere they were not expecting it — behind a piece of furniture, on a ceiling tile, or along a window frame.

But understanding what mold actually is, and why it matters as an indoor air quality concern rather than just a surface problem, is where most people have significant gaps.

This post covers both of those things in plain language.

What Is Mold?

Mold is a type of fungus. It exists naturally in the environment both indoors and outdoors and plays an important role in the natural decomposition of organic matter.

Mold reproduces by releasing microscopic spores into the air. Those spores are present in most indoor environments at some level. The presence of mold spores in a building is not by itself unusual — it is a normal part of the indoor environment.

Mold becomes a building concern when conditions inside the structure support active mold growth. At that point, mold...