It is the middle of winter and probably cold and wet outside where you live. If you are residing in a house that is older and not airtight, you might have a problem with closet dampness. Anyone who can't afford to keep the heat on all the time might also be living in a house where some of the rooms are colder and you might have a mildew problem
Basements usually are the main source of frustration with dampness but closets can be a problem as well. If you have closets that are damp, they might also smell bad and that is a very unpleasant situation. Smelly closets that are wet are not a good place to keep clothes and you need to do something that will eliminate the odor and wetness.
Chalk works well for this and you should put chalk in a paper bag and put it in a corner of the closet. The damper your closet is, the more chalk you should use and it is best if it is kept there year round. Just plain old white chalkboard chalk is all that is needed and it should be changed every six months or so. It is kind of like putting baking soda in your refrigerator to eliminate the smells.
If your closet really has a problem and is very wet and full of mildew, this is one of those closet ideas that might not work. You need to attack the problem before it gets too bad. Likewise, if you have a big walk in closet, you might need to put chalk in every corner. This closet tip for dampness definitely works best with small closets.