Here's why:

* Air has mass: Air is made up of molecules (nitrogen, oxygen, etc.), and molecules have mass. This means air has weight.

* Air exerts pressure: The weight of air pressing down on the Earth's surface creates atmospheric pressure.

* Air composes matter: Air is a mixture of different gases, which are forms of matter.

Mold is a type of fungus, and while it might be present in some environments, it's not a fundamental component of air itself.