Negative air ionization therapy
Negative air ionization therapy (NAIs) uses air ionisers as a non-pharmaceutical treatment for respiratory disease, allergy, or stress-related health conditions. The mainstream scientific community considers many applications of NAIs to be pseudoscience.[dubious ] Many negative ion products release ozone, a chemical known to cause lung damage. 
For Seasonal Affective Disorder (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 the post-treatment improvement percentage was 57.1% for bright light, 47.9% for high-density ions and 22.7% for low-density ions. An older RCT conducted by the same authors also found air ionization effective for SAD. A 2007 review considers this therapy "under investigation" and suggests that it may be a helpful treatment for SAD.
An 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 first (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.
A 2008 clinical trial showed that negative ion generators produced a smaller percentage of change on SIGH SAD compared to LED photo-therapy. A separate randomized placebo-controlled study published in May 2010 found that the difference between high-density ion therapy and placebo (dim red light and low-density ions) was not statistically significant. The study concluded that bright white light therapy was significantly more effective than negative ion therapy for treating SAD.
Researchers have continued to cite a dearth of evidence about the effects of negative air ionization. "The presence of NAIs is credited for increasing psychological health, productivity, and overall well-being but without consistent or reliable evidence in therapeutic effects and with controversy in anti-microorganisms," researchers wrote in a 2018 article published in the International Journal of Molecular Sciences.
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- Jiang, Shu-Ye; Ma, Ali; Ramachandran, Srinivasan (October 2018). "Negative Air Ions and Their Effects on Human Health and Air Quality Improvement". International Journal of Molecular Sciences. 19 (10): 2966. doi:10.3390/ijms19102966. PMC 6213340. PMID 30274196.