Mold

Mold and mildew causes this damp, musty smell. These are musty smelling fungi that thrive in moist conditions. Keeping humidity down reduces the chance for mold to grow. In general, mold will not grow at less than 50% relative humidity! Dehumidification is the key. Of course, any groundwater, plumbing or other leaks should be fixed, as part of a strategy to eliminate mold .

Allergens in the Home
Mold spores will certainly irritate someone with allergies. The number one allergen however, is the fecal matter of dust mites. Dust mites don't drink water, but absorb it out of the air through their skin. In order to live, dust mites need relative humidity over 50%. Reducing the relative humidity below 50% will cause dust mites to die off.