"in unindustrialized verdant areas and near large bodies of water, the air is negatively charged, that is, there are electrons floating in the air unattached to atoms or molecules. In industrialized areas or very dry regions, the air is positively charged; there are atoms and molecules missing electrons.
Some researchers claim that the air's electrical charge affects plant growth (and also animal behavior). They claim that plants in a positively charged environment grow slower than those in a negatively charged area. Regardless of the controversy regarding growth and the air's electrical charge, the presence of negative ions creates some readily observable effects. Odors are characteristic of positively charged particles floating in the air. A surplus of negative ions causes the particles to precipitate so that there are no odors. With enough negative ions, a room filled with pungent, flowering sinsemilla is odorless. Spaces with a "surplus" negative ion charge have clean, fresh smelling air. Falling water, which generates negative ions, characteristically creates refreshing air. Dust particles are precipitated so that there are fewer bacteria and fungus spores floating in the air, as well as much less dust in general. This lowers the chance of infection. Many firms manufacture "Negative Ion Generators", "Ionizers", and "Ion Fountains", which disperse large quantities of negative ions into the atmosphere. These units are inexpensive, safe and recommended for all growing areas. Ion generators precipitate particles floating in the air. With most generators, the precipitating particles land within a radius of two feet of the point of dispersal, collecting quickly and developing into a thick film of grime. Newspaper is placed around the unit so that the space does not get soiled. Some newer units have a precipitator which collects dust on a charged plate instead of the other surrounding surfaces. This plate can be rougly simulated by grounding a sheet a aluminum foil. To ground foil, either attach it directly to a metal plumbing line or grounding box; for convenience, the foil can be held with an alligator clip attacked to the electrical wire, which is attached to the grounding source. As the foil gets soiled, it is replaced."

-Marijuana Grower's Handbook