The physics half of the claim is real. Splashing and breaking water does separate charge and generate air ions (the Lenard or "waterfall" effect), and this has been measured at waterfalls and in sea spray. But the atmospheric literature is more complicated than the claim suggests. The Alpine waterfall work notes that in sea spray, larger droplets above about 2.5 micrometres carry positive charge while smaller, more numerous particles are negatively charged, and it explicitly records that multiple researchers measuring over the Atlantic and Pacific found a positive space charge and positive electric field, the opposite polarity to what the waterfall model predicts. So "breaking surf reliably produces net negative ion enrichment where a person stands" is not a settled finding.

The biology half is much weaker than presented. The strongest evidence on the human mood question is the 2013 BMC Psychiatry meta-analysis by Perez, Alexander and Bailey, which reviewed 33 human experimental studies from 1957 to 2012. Its conclusion was that consistent ionization effects were not observed for anxiety, mood, relaxation and sleep, or personal comfort. The one signal it did find was that negative ionization was associated with lower depression scores, particularly at the highest exposure levels, and the authors stated that future research is needed to evaluate the biological plausibility of that finding. A later trial paper also notes that Perez et al. found no dose-response relationship between total negative ion exposure time and post-treatment depression severity in seasonal affective disorder patients.

On cognition, the closest direct evidence found is a single small study of 39 young adults using a Stroop task and event-related potentials, which reported shorter reaction times, higher accuracy and larger N2 amplitudes in the negative air ion session. That is one small laboratory study using an artificial ion generator, not a beach.

On oxidative stress, the studies located are in vitro (human keratinocyte cell line exposed to particulate matter) and animal (rats, mice). The 2023 literature review in Environmental Science and Pollution Research also reports the opposite direction in at least one line of work, noting evidence that high concentrations of negative air ions may cause tracheal tissue damage and oxidative stress.

On serotonin, the claim runs against the direction of the historical literature it appears to be drawing from. The classic Krueger and Sulman work, reproduced on ionizer retail sites, describes positive ions raising blood serotonin and negative air ionization bringing elevated serotonin back down toward normal. Background knowledge, not a retrieved primary source: this is the basis of the "serotonin irritation syndrome" hypothesis, in which negative ions were proposed to accelerate serotonin oxidation, that is, to lower it. The popular wellness claim that negative ions "increase serotonin" is a later inversion that circulates on lifestyle and product pages rather than in the source literature.

No source was found, at any quality level, for a 50 metre distance threshold.