The air we breathe contains water vapor; this is essentially present due to the sun activity that let evaporate huge amount of water from seas and oceans, introducing in the atmosphere variable contents of water at vapor state. That is why commonly we speak about humid air. Humidity is a basic parameter in various sectors of human activities; but it is also an invisible element who can bring serious damages on processes and products. The control of humidity, allows the reduction of moulds and consequently healthier environments, reduces corrosion problems; increases reliability of electric and electronic components, increases preservation time of foodstuffs, reduces several type of physical troubles in people, improves environmental comfort and remove bad smells.