If you have a basement, you probably have a moisture issue of some kind. Either through the block walls (or cement floor) or as a result of humid, stagnant air.

A dehumidifier can really help.

Over the past two years, I have been gradually finishing the basement in our house. When we bough the place, the basement was musty and had mold issues as a result of water leakage from our water filter system made worse by the lack of decent airflow. I got a handle on this problem by (first) fixing the leak, then installing new windows (which actually opened - so we could let outside air in and vent the place) and finally, by buying a 70 pint dehumidifier, which kicks on whenever the relative humidity climbs to more than 60 percent.

During the summer, when it is humid, I usually have to empty the bucket about once every 2- days. In fall/winter, it may not need to be emptied more than once a month.

The air quality in the basement is now as good or eve better (on hot days, when the basement is cooler) than it is upstairs!