Under hot, humid conditions, stored away stuff (clothes, shoes, musical instruments,..) can get moldy after a while and/or develop that moldy smell.
For bigger rooms, running the air con or electric dehumidifier works wonders, but this does not work really well for stuff stored away in enclosed spaces like cupboards or storage cabinets. This is especially problematic if you are traveling and no one is home to run the air con. The best solution to keep the air in small enclosed spaces dry tends to be using those desiccant based dehumidifiers. Here in Singapore, people refer to them as "thirsty hippos" although a range of cheaper brands are available as well.
These dehumidifiers all contain calcium chloride salt (CaCl2). This salt is extremely hygroscopic: it will suck the moist out of the air and dissolve in the process. When all the solid salts have dissolved, the humidifier won't absorb any more moist and should be disposed of.
The used dehumidifiers all end up in the domestic waste. Here, domestic waste is incinerated, but since CaCl2 won't burn, it will end up being dumped with the ashes.
This is a waste, as the chemicals don't get consumed in the process; all they do is absorbing water. It is actually really easy to recycle the CaCl2: all it takes to restore it to its solid state is simply boiling off the absorbed water again. This can be done by boiling the solution on a simple domestic cook top.
I will demonstrate how to do this, and how to construct a safe new container to put the calcium salt into to use as a humidifier.