Negative air ionization therapy
For SAD, a randomized controlled trial (RCT) comparing high (4.5x1014 ions/second) and low (1.7x1011 ions/second) flow rate negative air ionization with bright light therapy found that posttreatment improvement results were 57.1% for bright light (10,000 lux) compared with high-density ions, 47.9%; and low-density ions, 22.7%. An older RCT conducted by the same authors also found air ionization effective for SAD at 2.7x106 ions/cm3. A 2007 review considers this therapy "under investigation", and suggests that it may be a helpful treatment for SAD.
A RCT comparing the short-term effects of bright light, an auditory stimulus, and high- and low-density negative ions on mood and alertness in mildly depressed and non-depressed adults found that the three active stimuli, but not the low-density placebo, reduced depression on the Beck Depression Inventory scale; the auditory stimulus, bright light and high-density ions all produced rapid mood changes—with small to medium effect sizes—in depressed and non-depressed subjects.
Michael Terman, professor of clinical psychology in the Department of Psychiatry at Columbia University, who conducted the two studies on SAD, suggests that the mechanism responsible for the effect of this therapy on SAD is that the negative ion machines used in his studies are designed to mimic summer-like conditions by supplementing the sparse winter ion supply. He stresses however that although some air purifiers utilize negative ion technology, the dose of ions delivered by a typical air purifier is far too low for it to have an antidepressant effect. As of 2009, the negative ion generators used are still undergoing multicenter phase II clinical trials.
A separate randomized placebo-controlled study published in May 2010, found that high-density ion therapy was more effective than placebo (dim red light and low-density ions) but that the results were not statistically significant. The conclusion of this study was that bright white light therapy was significantly more effective than negative ion therapy for treating Seasonal Affective Disorder (SAD). 
- Terman, M.; Terman, J. S. (2006). "Controlled Trial of Naturalistic Dawn Simulation and Negative Air Ionization for Seasonal Affective Disorder". American Journal of Psychiatry 163 (12): 2126. DOI:10.1176/appi.ajp.163.12.2126. PMID 17151164.
- Terman, M.; Terman, J.; Ross, D. (1998). "A Controlled Trial of Timed Bright Light and Negative Air Ionization for Treatment of Winter Depression". Archives of General Psychiatry 55 (10): 875. DOI:10.1001/archpsyc.55.10.875. PMID 9783557.
- Westrin, ÅS.; Lam, R. (2007). "Seasonal Affective Disorder: A Clinical Update". Annals of Clinical Psychiatry 19 (4): 239. DOI:10.1080/10401230701653476. PMID 18058281.
- Goel, N.; Etwaroo, G. R. (2006). "Bright light, negative air ions and auditory stimuli produce rapid mood changes in a student population: a placebo-controlled study". Psychological Medicine 36 (9): 1253. DOI:10.1017/S0033291706008002. PMID 16756690.
- Michael Terman. "I haven’t heard of negative air ionization therapy before. What is it?". Columbia University Medical Center. http://asp.cumc.columbia.edu/psych/asktheexperts/ask_the_experts_inquiry.asp?SI=102. Retrieved 2009-03-09.
- "A Trial of Negative Ion Generation Versus Light-Emitting Diode Phototherapy for Seasonal Affective Disorder (SAD)". clincaltrials.gov. http://clinicaltrial.gov/ct2/show/NCT00809523?term=negative+ion+generator&rank=1. Retrieved 2009-03-09.
- Flory R, Ametepe J, Bowers B., A randomized, placebo-controlled trial of bright light and high-density negative air ions for treatment of Seasonal Affective Disorder, Psychiatry Res. 2010 May 5;177(1-2):101-8. Epub April 9, 2010.