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 as well as by special anion layers inside the mat. 

Crystals have 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 coming from the mat heating system or even using your body warmth when you lie on 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 which produce much more ions in one small spot to catch the dust and then ions dissipate in the air. But these ions can be absorbed only by breathing and only a small part. 

The main difference of Ereada® mats is that in addition to ion emission into the air (which may be measured) they conduct ions directly to tissues when you lie on the mat. 

It is a much more efficient because the surface of the mat is million times larger than the ion generating nozzle of the air ion generator. 

If your mat has a NEGATIVE ION or N.P. button, it means that the function activating electrical discharge of negative ions (E.D.N.I.) through the silver fiber layer in your mat is installed. It converts your mat into the bath of ions and surrounds it with Negatively Charged -600V High Potential electro static field.