A Musty odor detected in your basement is a result of airborne mold spores. Mold can grow on any organic surface as long as there is at least 14% moisture content on the host surface. Everything porous in your basement, cardboard boxes, furniture, clothing, carpeting, and even masonry itself, “absorbs” moisture and can serve as a host for mold. When the moisture content goes over 21% the mold starts to shoot spores into the air in an attempt to grow faster. The EPA says the best way to detect active mold is to use your nose. So when you smell a musty odor it is a sign that mold spores are spreading actively through the air.
Since moisture can make its way into your basement through the walls, floor, and/or air it is important to control all the sources of moisture in your basement. A dehumidifier might provide temporary relief by lowering the moisture in the air, but if the cause of the moisture is not identified and fixed correctly the musty smell will most likely come back.