The science of fresh air
Negative ions:
what researchers are finding.
Stand by a waterfall and the air feels different. Scientists have measured why — and tested what ions might do for mood, air and focus, and looked at early signs on inflammation. A friendly, honest guide to what looks promising and where the questions are still open.
40 studies, each summarised and linked to the original
Four areas of research
Where ions have been put to the test.
Each area has encouraging findings — and a balancing view, so you can see the whole picture.
Winter depression
58% vs 15% responded to high-density versus low-density ions in a double-blind trial of seasonal depression. A 2013 meta-analysis of 33 studies found lower depression ratings at high ion density (from 5 depression studies).
Balancing view Specific to depression; no consistent effect on anxiety, sleep or relaxation.
Read the research →
Particles and microbes
~95% drop in the viability of S. aureus after 12 hours of ion exposure in a 2021 lab study, with E. coli down about 70%.
Balancing view A 2007 study found ozone was the main cause of bacterial death for most species.
Read the research →
Attention and memory
1 hour after adding ions, 40 students scored higher on reasoning and memory tasks in stuffy, high-CO₂ air.
Balancing view The gains had gone by three hours, and an older study found no benefit.
Read the research →
Early lab findings
In cells and animals negative ions lowered inflammatory signals in skin cells and eased lung injury in rats, with antioxidant effects reported.
Balancing view No human trial that we found has shown this, and one sham-controlled study found no net benefit.
Read the research →
Where ions come from
Nature makes them. So do devices.
Air ions are counted per cubic centimetre. Waterfalls and forests carry far more than city air; the trials of ionizers for depression used higher levels again.
Logarithmic scale: each gridline is ten times the last. Figures come from different instruments and methods and are only indicative. The clothing figure is one manufacturer’s own chamber test of a friction-charged fibre; we found no independent measurement from a worn garment.
Nature, and why it feels so good
Scientists are still mapping the ingredients — and air ions are one of the things they measure.
Explore the nature research →Keep exploring
Where to next.
Forests & waterfalls
Why the air by moving water is different, and what we know about nature and wellbeing.
Read →Choosing a device
Ozone, certification and what to look for.
Read →Clothing & wearables
An open question: what has and hasn’t been tested.
Read →All the evidence
Every study, rated, with links to the originals.
Read →Thinking of getting an ionizer?
Choose one that doesn’t make ozone, a lung irritant. California certifies air cleaners to a 0.050 ppm ozone limit — a good thing to look for. Choosing safely →
Honest about the evidence
We include the studies that found less, too. The strongest results are for winter depression with high-density ionization; the rest are early. How we rate →