Don't let the word negative fool you into thinking that there is anything bad with negative ions. It is just the opposite actually. 

If we are feeling lazy, depressed, easily irritated, then we just might need a big dose of negative ions.  We are probably too full of positive ions and need to have our balance of ions adjusted.  Kind of like, recharging our batteries.

What are ions?  The air all around us is comprised of these electrically charged ions or particles. The air we breathe is charged with both positive and negative ions.  They are atoms that have an electrical charge by either gaining or losing an electron.  

There's nothing good about positive ions. One of the many places they are found is around TV screens or computer monitors. We have so much indoor pollution even our metal and concrete building absorb positive ions and release them in the air. The heavy doses of positive ions are found in our most polluted cities where there is an abundance of car exhausts, tire dust, heating issues, soot, plastic etc. Metal ducts covering heating and a/c units, synthetic materials steel and concrete buildings all absorb the positive ions.  Even our energy saving fluorescent lighting is in fact generating an abundance of positive ions.   So in a normal interior environment a negative ion count may be under 100 per cubic centimeter, we really need a minimum of 1000 per cubic centimeter.

Other experts have reported that the influx of positive ions takes away our good sense and imposes bad dispositions while the negative ions help change our internal switch and they stimulate our good moods, even help make plants grow. 

Technically Speaking:

Ions are charged particles in the air that are formed in nature in different ways. One way is when enough energy acts upon a molecule such as carbon dioxide, oxygen, or water to eject an electron from the molecule leaving a positively charged ion. The displaced electron attaches itself to a nearby molecule, which then becomes a negatively charged ion.