Does the mat produce negative ions when it is not plugged in? If so, how does it compare to when it is plugged in?

All mats produce negative ions by amethyst, tourmaline, and other stones and special anion layers inside the mat. 

Crystals have the natural ability to generate ions. When they are heated, they generate more ions. 

The crystals and negative ion layers can create up to 500-1500 ions per cc of air upon heat distortion from the mat’s heating system or even by using your body warmth when laying on an unpowered mat. 

If there is nobody on the mat, it may emit 200-300 ions per cc.

Ion measuring devices available in the market are used to test ion generators used for air cleaning, producing many more ions in one small spot to catch dust and then dissipate ions in the air. 

But these ions can be absorbed by the body only through breathing, and even then, only a small portion. 

The main difference with MediCrystal mats is that in addition to ion emission into the air (which may be measured), they conduct ions through the skin directly to the body when one lays directly on the mat. 

It is a much more efficient method, and the surface of the mat is millions of times larger than the ion generating nozzle of an air ion generator.