Quote- "Ions are charged particles in the air that are formed in nature when enough energy acts upon a molecule such as carbon dioxide, oxygen, water, or nitrogen 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. It is the negative ion of oxygen that affects us the most."
http://www.comtech-pcs.com/ions/whatareions.html
http://fy.chalmers.se/f3a/negion/
http://www.negativeiongenerators.com/negativeions.html
And in relation to being beneficial to plants:
"Scientists at the University of California grew barley, oats, lettuce, and peas with a total of only sixty positive ions and negative ions and found that growth was stunted and the plants were diseased. The same experiment in air with more than double the natural number of ions produced accelerated growth."
"In the 1960s one U.S. Department of Agriculture scientist grew seedlings in ion-enriched air and produced cucumbers eighteen inches longer than normal."
"Photosynthesis could not take place without ions in the atmosphere".
"In a Faraday cage, where the outside electrical fields are excluded, plants grow only about half the size they would if rooted in the garden."
http://www.comtech-pcs.com/ions/toc.htm
I repeat, I would not recommend ionisers for the control of lab smells.