I fight a constant battle with mildew in the Pacific Northwest. I can buy solid …

I fight a constant battle with mildew in the Pacific Northwest. I can buy solid chemicals to put in my closets, which take water out of the air, eventually creating a bucket full of water. Do these devices actually lower the moisture content of the air or do they just make me feel like I’m doing something? — MD

How much effect these drying agents have depends on how much air they’re exposed to. Water molecules are continuously going back and forth between the air and everything exposed to that air—your clothing, your hair, the walls of your home, the contents of a saltshaker, and the drawers in a wooden bureau. The water molecules land on and take off from every surface, like busy miniature airports. The rate at which water molecules land on an object depends on how humid the air is. The rate at which water molecules leave that object depends on how hot the object is and on how tightly water molecules cling to it.

The landing and leaving processes are in perpetual competition and the fastest one wins. If the air is humid and the object is cold or attractive to water molecules, the landing process dominates and water condenses out of the air and onto the object. If the air is dry and the object is hot or doesn’t bind water molecules well, taking off dominates and water evaporates from the object into the air.

Your problem is that the air in your closets is very humid and landing is winning—too much water is condensing on your walls. To stop this condensation, you either have to heat the walls, so that water molecules leave them faster, or reduce the humidity of the air, so that water molecules land less often. Putting a material that binds water molecules into your closets changes the balance of landing and taking off—water molecules that land on this material don’t return to the air often so the humidity of the air diminishes. With less humidity in the air, the rate at which water molecules land on the walls also diminishes.

But this drying effect only works if the air in the closet is trapped there. If your closet exchanges air quickly with outdoor air, the water molecules removed by the drying agent will be quickly replaced with new water molecules from outside. In effect, you will be trying to dry the great outdoors, a hopeless task. To make the most of this drying agent, you should let it work on as little air as possible by sealing the closet and slowing the exchange of air with outside. Better yet, replace the drying agent with a dehumidifier. A dehumidifier accumulates water molecules from the air by presenting the air with a chilled surface. Water molecules land on the cold surface and then don’t have enough energy to return to the air. They are trapped by the cold rather than by chemical binding.

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