I only get that smell in summer, although I have a dry basement. A dehumidifier, emptied once a day does the trick. That smell is mildew and/or mold spores in the air.
One thing that worked for me was sprinkling borax all over the floor, letting it sit for a few minutes, then sweeping it up. It absorbs the moisture and also helps sweeten it. It makes quite a difference. Also, get as much air moving through the space as possible with fans or opening windows if there are any down there.
Susan from ThriftyFun
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