I heard that negative ion generators generate negative ions and despite their pejorative appelation, negative ions are good for you. However, I have had one negative ion generator or another running at night for years, and I haven’t felt any different. They are supposed to give you the feeling you have after a rainfall of fresh air and energy, since raindrops strip away electrons as they fall from the atmospheric molecules, thus constituting a gigantic negative ion generator. First, how do you tell whether a n.i. machine is really generating n.i.'s, or is it all just a hoax? Second supposing they have been generating n.i.'s, why don’t I feel any better? (I often wonder this about vitamins and minerals as well).
Uhh, just a wild guess here, but maybe you’re not feeling any better because, uhhh, negative ions do jack squat?
Nah, it’s gotta be something else entirely. 
Friedo’s tip for feeling better: Eat good, excercise, and sleep.
It should be appellations.
Assuming they have a health benefit, which is a huge assumption, is there a specific quantity of them you need to notice such benefit? Assuming you are talking about air, the only ions of any significant quantity would be oxygen and nitrogen. Lightning could create a whole lot of these ions. Small ion generators probably create significantly less, although they have a much longer run time, so they would be sucking up the juice. Negative ions will decay to a stable state, so the effect of the generator is only temporary. The half-life of -1 oxygen is whopping 0.122 seconds, and -2 nitrogen is even shorter. I don’t see how you could get these negative ions into your lungs, or absorb them through your skin, or whatever you are doing with them in any significant quantity.
Maybe if you got a whole bunch of the ion generators, and put them all around you, you would see an effect. Of course it could just be the EMF from all the wires, which is an imaginary detrimental effect, and would probably cancel out the imaginary positive effect of the negative ions. Bummer, I think you have to wait for a decent lightning storm.
Thunderstorms produce ozone in the lower atmosphere (i.e. where it can affect us) and ozone is, I believe, what gives the air that characteristic post-thunderstorm smell. It can also be a component in smog pollution.
Ozone, which is a singly-charged negative ion consisting of three atoms of oxygen, is longer-lived than O-, but it is not a good thing to breath too much of this.
Ozone is not an ion; it is a neutral molecule with no formal charge.
And he didn’t say it was. I think he was just commenting on don willard’s mention of post-thunderstorm fresh air smells.
Granted, it could also be because the air is lower in particulates then…
Hm. If you negatively charged a whoooole bunch of air, then wouldn’t some of the particles in the air gain that charge, and start clinging to stuff, cleaning the air?
Sorta like how those filters work, but using the whole room. 
Glad that mystery has finally been solved. Now we can get the research team busy figuring out what ice machines make.
Kyberneticist: Thank you. I was commenting on the post-thunderstorm smell, but Dr Lao is correct. Ozone is a neutral, though highly reactive, molecule. It can, however, become an ion. Since I work with ion-molecule reactions, I tend to think in terms of stable ions rather than molecules.
opus You should’ve titled that “From the Department of Redundancy Department”.
I used to run a neg. ion generator in my bedroom some time ago. I did think, subjective I know, that the air smelled fresher. But an objective effect was the dust and dirt stuck to the wall behind the generator. Better on the wall than in my lungs. We just vacuumed the wall each week.
I say objective effect because my wife objected to the dirt on the wall!
I’d have to ask then, GaryM, why didn’t you just use an air filter? That way your walls stay clean and the air stays clean.
They do work on smells and particles. If you have a pet, stick it next to the cage or the litter box. Pot growers use them to kill the smell of the plants. They sell dual air filters with both Neg. Ions and normal filters. But other than hat, I don’t think they help your health.
Kyberneticist:
At the time, maybe 15 years ago, I thought there might be some benefit to the negative ions. Now, apparently, most folks seem to think that this is not the case. No cites, but reading this thread, that’s my impression.
A benefit I did notice was that if the dust is on the wall, it must not be floating in the air for me to breathe. I don’t recall there being many ionic? air filters around. Today an electrostatic furnace filter would do the job and not use the wall as it’s collector.
Air filters make noise too. That’s not a big issue though, A lot of folks seem to like the sound.
If all you want to do is remove small particles from the air, there are a lot of good ways to do it. Electrostatic filters are more available than before, but not neccesarily all that effective, and can cause problems. A good filter, perhaps even a HEPA filter in the furnace can do the job. The fact is, though, that even as tight as houses are getting, there is going to be stuff in the air. Besides normal inside sources, such as people or animals, outside stuff will stick to clothing and anything you carry into the house. I have seen people go nuts with the filters and air cleaning things, but it just doesn’t help all that much, and can, as I said above cause problems. The most common problem is a significant pressure drop across the filter, which restricts air flow and can cause the ducts to leak a whole lot, or the filter to get blown through. Even if it works when the filter is new, as it gets clogged the pressure increases and usually the air finds a new path around the filter. Houses just aren’t going to be like industrial clean rooms, unless you tear down and rebuild it from scratch.
The electrostatic filters (real ones that require a power source) can get dirty, especially if they get greasy air (from cooking usually) on them, and then arc and cause fires. If you have one, keep it clean. Also, most of the work on benefits of the electrostatic filters is really suspect and mostly unconfirmed.
All I see in the magic air cleaning boxes is electric energy going to waste, first in the manufacturing process, then in the operation of the damn things, and finally in the heat load it adds to people A/C’s. If you need a plastic bubble then get one. The other stuff seems to be mostly half-ass patches, and without other fairly extensive modifications to the home, they appear totally ineffective.
Just my experience.