The terms mold and mildew are often used interchangeably — but they refer to different organisms with different characteristics, different treatment requirements, and different implications for your home and health.

What Mildew Is

Mildew is a surface fungus that grows on damp warm surfaces — typically appearing as a flat powdery growth that is white, gray, or yellowish in color. Mildew is common in bathrooms, on window sills, and on surfaces exposed to regular moisture. It grows on the surface of materials rather than penetrating into them. Mildew smell is typically described as musty, earthy, or similar to wet paper or cardboard.

What Mold Is

Mold is a broader category of fungi that can penetrate into porous materials — drywall, wood, carpet, insulation. It comes in many colors including black, green, white, and orange, and can grow extensively behind walls and under flooring where it is not visible. Mold smell is typically more intense than mildew — often described as musty but also earthy, sometimes with a sour or ammonia note.

Why the Distinction Matters

• Health implications: Mildew is generally considered less harmful than mold, particularly black mold which is associated with respiratory symptoms. If you suspect significant mold growth, a professional mold assessment is appropriate before remediation.
• Treatment approach: Mildew on surfaces can often be cleaned with appropriate products. Mold in walls or structural materials requires professional remediation — cleaning the surface without removing contaminated material is not sufficient.
• Recurrence: Both mildew and mold will return if the moisture source is not addressed. Odor treatment without moisture remediation produces temporary results.