The evidence
All the evidence.
40 studies, reviews and regulatory notes — the encouraging ones and the ones that found less. Each is summarised, rated and linked to the original.
40 shown
Read before relying on a link. This list was compiled from search-engine summaries; each citation should be opened and checked against the original before you quote it. Entries marked “details to confirm” are ones where the summary was incomplete.
-
Air ions and mood outcomes: a review and meta-analysis
Perez V, Alexander DD, Bailey WH · BMC Psychiatry 13:29 · 2013
Systematic review & meta-analysis, 33 studies
High-density negative ionization was associated with lower depression ratings (5 studies), with a larger effect at higher exposure. No consistent effects on anxiety, mood, relaxation/sleep or comfort.
-
Controlled trial of naturalistic dawn simulation and negative air ionization for seasonal affective disorder
Terman M, Terman JS · American Journal of Psychiatry 163(12):2126–33 · 2006
Randomised controlled trial, n = 99
High-density ions gave about 48% symptom improvement against about 23% for low-density (placebo) ions. Bright light and dawn simulation did similarly well.
-
A controlled trial of timed bright light and negative air ionization for treatment of winter depression
Terman M, Terman JS, Ross DC · Archives of General Psychiatry 55:875–82 · 1998
Randomised controlled trial
High-density ions and bright light both acted as specific antidepressants; low-density ions did worse than all other groups.
-
Controlled trial of bright light and negative air ions for chronic depression
Goel N, Terman M, Terman JS, Macchi MM, Stewart JW · Psychological Medicine 35(7):945–55 · 2005
Randomised controlled trial, n = 34
Symptoms improved about 51% with high-density ions, 54% with bright light and 17% with low-density ions. Small sample.
-
The effects of negative air ions on cognitive function: an event-related potential (ERP) study
Chu C-H, et al. · International Journal of Biometeorology 63(10):1309–17 · 2019
Crossover, n = 39 young adults
Participants were faster and more accurate on a Stroop task under negative ions, with EEG changes. A single small, unreplicated study.
-
Effect of negative air ions on computer operation, anxiety and salivary chromogranin A-like immunoreactivity
Nakane H, Asami O, Yamada Y, Ohira H · International Journal of Psychophysiology 46:85–9 · 2002
Crossover, n = 12 female students
A stress marker rose less during a 40-minute computer task with negative ions, with slightly better task performance. Very small study.
-
Air ions and respiratory function outcomes: a comprehensive review
Alexander DD, Bailey WH, Perez V, Mitchell ME, Su S · Journal of Negative Results in Biomedicine 12:14 · 2013
Systematic review, 23 studies
The literature does not clearly support a benefit of negative ions for respiratory function or asthma symptoms.
-
Ionisers for chronic asthma
Blackhall K, Appleton S, Cates CJ · Cochrane Database of Systematic Reviews CD002986 · 2012
Cochrane review, 6 crossover trials, 106 participants
No significant benefit for lung function or symptoms from ionisers.
-
Ion-air purifier trial in workers (ISRCTN11050992)
Registered trial; manufacturer-reported results · ISRCTN registry · 2023
Double-blind RCT, n = 150
Only manufacturer press figures were found (33% vs 42% infection rate); no peer-reviewed paper. Do not treat as evidence.
-
Biological impact of small air ions
Krueger AP, Reed EJ · Science 193(4259):1209–13 · 1976
Narrative review proposing the serotonin hypothesis
Source of the “serotonin” idea. Mostly animal and in-vitro work and centred on positive ions. A hypothesis, not proof of a human benefit.
-
The effect of exposure to negative air ions on the recovery of physiological responses after moderate endurance exercise
Ryushi T, et al. · International Journal of Biometeorology 41(3):132–6 · 1998
Crossover, n = 10 men
Diastolic blood pressure recovered more slowly with ions present, and serotonin and dopamine were lower — the opposite of what marketing claims.
-
Effects of negative air ions on oxygen uptake kinetics, recovery and performance in exercise: a randomized, double-blinded study
Nimmerichter A, et al. · International Journal of Biometeorology 58(7):1503–12 · 2014
Randomised double-blind crossover, n = 14 trained men
Very high ion levels (about 220,000 per cm³) had no effect on oxygen uptake, hormones, lactate or sprint power.
-
Exposure of laboratory animals to small air ions: a systematic review of biological and behavioral studies
Bailey WH, Williams, Leonhard · BioMedical Engineering OnLine 17:72 · 2018
Systematic review, 62 animal studies
The highest-quality studies showed no effects and no dose–response across a five-log range of ion intensity, which undercuts the serotonin mechanism.
-
Evaluation of ionic air purifiers for reducing aerosol exposure in confined indoor spaces
Grinshpun SA, et al. · Indoor Air 15(4):235–45 · 2005
Chamber experiment, 5 devices
The strongest unit cleared particles from a small chamber within about 10–12 minutes. Particles are deposited on surfaces, not destroyed.
-
Negative air ions and their effects on human health and air quality improvement
Jiang S-Y, Ma A, Ramachandran S · International Journal of Molecular Sciences 19(10):2966 · 2018
Narrative review
Negative ions can remove particulate matter, but there is no consistent, reliable evidence of therapeutic effects and antimicrobial claims are contested. Parts read as promotional.
-
Waterfall ion measurements (Alpine waterfalls)
Kolarž P, et al. · Atmospheric Chemistry and Physics 12:3687 · 2012
Three-year field campaign, five waterfalls
Negative ion concentrations were steeply elevated near waterfalls; positive ions rose only moderately.
-
Lenard effect and waterfall ionisation
Laakso L, et al. · Atmospheric Chemistry and Physics 7:2271 · 2007
Atmospheric physics paper
Restates the finding that air near waterfalls becomes negatively charged when moving water strikes an obstacle.
-
Taiwan forest, waterfall and city air-ion survey
National Taiwan University repository entry · NTU Scholars
Field survey (details not retrieved)
Reported values (secondary): forests about 2,800–22,000 ions/cm³, downtown 130–970, waterfalls 9,430–44,130. Instruments and methods vary between studies.
-
The physiological effects of Shinrin-yoku (taking in the forest atmosphere) in 24 forests across Japan
Park BJ, Tsunetsugu Y, Kasetani T, Kagawa T, Miyazaki Y · Environmental Health and Preventive Medicine 15:18–26 · 2010
Crossover field experiments, n = 280 young men
Forest walks gave lower cortisol, pulse and blood pressure than city walks. The study did not isolate ions: greenery, quiet, walking and scenery are all confounded.
-
Blood pressure-lowering effect of Shinrin-yoku (forest bathing): a systematic review and meta-analysis
Ideno Y, et al. · BMC Complementary and Alternative Medicine 17:409 · 2017
Meta-analysis, 20 trials, n = 732
Systolic and diastolic blood pressure were lower in forest settings. Designs were mixed and not all randomised; ions are not shown to be the cause.
-
Ozone generators that are sold as air cleaners
US Environmental Protection Agency · EPA guidance
Regulatory guidance
At safe concentrations ozone has little ability to remove indoor pollutants. Some units exceed health standards even when used as directed.
-
Air cleaning devices for the home; hazardous ozone-generating air purifiers
California Air Resources Board · CARB fact sheet
Regulatory guidance
Ozone causes coughing, chest tightness and asthma aggravation and reacts indoors to form formaldehyde and ultrafine particles. California limits air-cleaner ozone emissions to 0.050 ppm.
-
The effect of an ion generator on indoor air quality in a residential room
Waring MS, Siegel JA · Indoor Air 21(4):267–76 · 2011
Experiment in one 27 m³ room
Ozone, formaldehyde and nonanal increased, and ultrafine particles rose when a terpene air freshener was present. The authors suggested limiting indoor use until safety is shown.
-
Treatment of seasonal affective disorder with a high-output negative ionizer
Terman M, Terman JS · Journal of Alternative and Complementary Medicine 1(1):87–92 · 1995
Double-blind controlled trial, n = 25 (home use, 30 minutes each morning for 20 days)
58% of people responded to high-density ions versus 15% with low-density ions. Symptoms returned in responders when treatment stopped, and no side effects were attributed to the treatment.
-
A randomized, placebo-controlled trial of bright light and high-density negative air ions for treatment of seasonal affective disorder
Flory R, Ametepe J, Bowers B · Psychiatry Research 177(1–2):101–8 · 2010
Randomised trial in women with seasonal affective disorder (73 women recruited, per the 2013 meta-analysis)
Compared bright light with high-density negative ions against a low-density placebo. A summary of its results is still to be added.
-
Controlled trial evaluation of exposure duration to negative air ions for the treatment of seasonal affective disorder
Bowers B, Flory R, Ametepe J, et al. · Psychiatry Research 259:7–14 · 2018
Controlled trial of how long people are exposed
Tested whether the length of daily exposure changes the effect. A summary of its results is still to be added.
-
Positive and negative ions potently inhibit the viability of airborne Gram-positive and Gram-negative bacteria
Comini S, et al. · Microbiology Spectrum 9(3):e00651-21 · 2021
Laboratory study (S. aureus and E. coli on plates and trapped in filters)
Ion exposure reduced bacterial viability in every condition tested: about 95% for S. aureus and 70% for E. coli after 12 hours on plates, and effects within 3 hours on bacteria trapped in filters. These are laboratory results, not infection rates in people.
-
Bactericidal effects of negative and positive ions generated in nitrogen on starved Pseudomonas veronii
Noyce JO, Hughes JF · Journal of Electrostatics 57(1):49–58 · 2003
Laboratory study
Tested bacterial kill by ions generated in nitrogen, an approach used to look at ion effects without ozone. A summary of its results is still to be added.
-
Bactericidal action of positive and negative ions in air
Fletcher LA, Gaunt LF, Beggs CB, et al. · BMC Microbiology 7:32 · 2007
Laboratory study, 7 bacterial species; separated the effects of ions, electric field and ozone
For six of the seven species the main cause of death was ozone, with electroporation second. The authors suggest the bactericidal action attributed to air ions in earlier work may have been overestimated.
-
Effect of negative ion atmospheric loading on cognitive performance in human volunteers
Andrade AC, Fernandes C, Verghese L, Andrade C · Indian Journal of Psychiatry 34(3):253–9 · 1992
Human volunteer study (design and size to confirm)
Looked at cognitive performance under negative-ion “loading”. A summary of its results is still to be added.
-
Effects of increasing indoor negative air ions on cognitive performance and health of high pure CO₂ level-exposed college students
Guo, et al. · Indoor Air · 2023
Experiment, 40 college students, four conditions (500 or 2,500 ppm CO₂, with and without negative ions)
After one hour, adding negative ions was linked to better reasoning, short-term memory and verbal scores, including at 2,500 ppm CO₂. There was no significant effect after three hours.
-
Do negative air ions affect human mood and performance?
Hedge A, Collis MD · Annals of Occupational Hygiene 31(3):285–90 · 1987
Crossover field experiment, n = 28 women
No beneficial effects of negative ions on mood or on performance (colour-naming and Stroop tasks) were found.
-
Negative air ions alleviate particulate matter-induced inflammation and oxidative stress in the human keratinocyte cell line HaCaT
Kim M, Jeong GJ, Hong JY, Park KY, Lee M-K, Seo SJ · Annals of Dermatology 33(2):116–21 · 2021
Cell culture (human skin cells exposed to particulate matter)
Negative ions lowered particulate-induced pro-inflammatory cytokines (at mRNA and protein level), suppressed intracellular reactive oxygen species, and reduced p38 MAPK and AP-1 activation.
-
Negative air ions through the action of antioxidation, anti-inflammation, anti-apoptosis and angiogenesis ameliorate lipopolysaccharide induced acute lung injury and promote diabetic wound healing in rat
Cheng Y-H, Li H-K, Yao C-A, Huang J-Y, Sung Y-T, Chung S-D, Chien C-T · PLoS ONE 17(10):e0275748 · 2022
Rat study (lipopolysaccharide-induced lung injury; diabetic wound model)
The authors report antioxidant, anti-inflammatory, anti-apoptotic and angiogenic effects, less lung injury, and faster wound closure in treated diabetic rats. They note the mechanism is not fully established.
-
Negative air ion exposure ameliorates depression-like behaviors induced by chronic mild stress in mice
Hu Y-Q, Niu T-T, Xu J-M, Peng L, Sun Q-H, Huang Y, Zhou J, Ding Y-Q · Environmental Science and Pollution Research 29(41):62626–36 · 2022
Mouse study, 30 days of exposure under chronic mild stress
Ion exposure restored sucrose preference, reduced tail-suspension immobility, prevented the stress-induced rise in corticosterone and restored the balance of Th1/Th2 cytokines.
-
Negative air ions attenuate nicotine-induced vascular endothelial dysfunction by suppressing AP1-mediated FN1 and SPP1
Authors to confirm · Rat study; journal to confirm
Rat study (spontaneously hypertensive rats)
Per its summary, negative ions attenuated nicotine-induced damage to the lining of blood vessels, with antioxidant effects including higher SOD activity.
-
The stimulatory effect of negative air ions and hydrogen peroxide on the activity of superoxide dismutase
Kosenko EA, Kaminsky YG, Stavrovskaya IG, Sirota TV, Kondrashova MN · FEBS Letters 410:309–12 · 1997
Laboratory (red blood cells from rat, cow, human and duck)
Exposure to ion-generator air increased the activity of the antioxidant enzyme superoxide dismutase (SOD); the authors suggested micromolar hydrogen peroxide drives the effect.
-
Associations of forest negative air ions exposure with cardiac autonomic nervous function and the related metabolic linkages: a repeated-measure panel study
Liu S, Li C, Chu MT, et al. · Science of the Total Environment 850:158019 · 2022
Observational panel study, 31 healthy adults, 3 days and 2 nights in a forest park
Short-term exposure to forest negative ions was linked to better heart-rate variability, especially parasympathetic activity, along with metabolic changes the authors interpret as reflecting lower inflammation. It is observational, so it cannot show cause.
-
Negative ions offset cardiorespiratory benefits of PM2.5 reduction from residential use of negative ion air purifiers
Liu, et al. · Indoor Air 31(1):220–8 · 2021
Randomised, sham-controlled crossover, 56 college students, week-long interventions
The purifiers lowered PM2.5 but raised negative-ion exposure. Urinary malondialdehyde, an oxidative-stress marker, rose with both PM2.5 and ion exposure, so the net effect was null. The authors concluded such purifiers should not be used as a health-based way to cut PM2.5 exposure.
-
Biological effects of negative air ions on human health and integrated multiomics to identify biomarkers: a literature review
Xiao, et al. · Environmental Science and Pollution Research · 2023
Literature review
Reviews how negative ions are generated and how they have been linked to health, and proposes multiomics approaches to find biomarkers, with anti-inflammation among its themes.