“Ions” are charged particles. They are floating in the air around us all of the time and have either negative or positive charges. Ions that are negatively charged are called “negative ions” or "anions". Ions that are positively charged are called “positive ions” or “cations”. The beneficial effect of negative ions is first known to human since more than one hundred years ago. However, actual implementation of negative ions in clinical medicine is only about 10 years, and the use of negative ions in daily healthcare products becomes popular only in the recent years.

Of all the gaseous molecules that make up the air, oxygen and carbon dioxide molecules have more power in gaining the electrons. In addition to that, oxygen occupies larger proportion in the air, resulting in formation of oxygen anions as the free electrons are captured by oxygen molecules. As such, the so called “negative ions” actually means cluster of negatively charged oxygen molecules. This cluster of molecules usually comprised of not more than 12 gaseous molecules surrounding the charged oxygen molecule. Water molecule is also one of the most easily ionized molecules. In environment with abundant water molecules such as waterfall, negative ion exists as negatively charged water molecules (with hydroxyl ion attached to water molecules).

Ions in the air can be divided into three categories :  small, medium and large ions. Only the small ions are biological active as they are small enough and able to penetrate and enter human body to result in health effect.