Does salt work...
... as an air dehumidifier?
How fast can salt absorb or release humidity from or into the air, at which air temperatures and to which extent?
Sodium chloride - NaCl - can absorb humidity from the air as fast as the dissolution process in water takes. This process highly depends on the relative humidity and in particular on the salt surface. A single big rock salt crystal offers a rather small surface to humidity, while salt in the granulation of a standard table salt offers a comparatively large surface to humidity. This means that fine salt will absorb humidity in principle faster and, thus, will also agglutinate faster because the dissolution process started.
One gram of salt will dissolve in 2.8 grams of water. This means that one gram of salt can absorb up to 2.8 grams of humidity until complete dissolution. The release of water, on the other hand, will happen only through evaporation, which takes place more quickly with temperatures increasing.
Furthermore, sodium chloride itself is not highly hygroscopic. The absorption of humidity results from traces of other minerals contained in the salt, such as magnesium chloride. The myth of salt being highly hygroscopic dates from those times when people were using mostly sea salt and rock salt. Particularly sea salt is considerably more hygroscopic than a pure vacuum salt with a sodium chloride content of above 99 per cent, due to the high percentage of the other minerals contained in sea salt.
In a nutshell, sodium chloride is not a very effective air dehumidifier. A pure salt with a high sodium chloride content, such as a food-grade vacuum salt, will work as a dehumidifier only from 75 per cent of relative humidity on, due to its very low hygroscopicity. Substances with a significantly higher hygroscopicity are, as already mentioned, magnesium chloride as well as calcium chloride. The latter comes with the highest hygroscopicity and is therefore found in the majority of standard products end users can buy in hardware stores for the dehumidification of their cellars.
