A fan helps in a heatwave — until you are dehydrated, a small trial finds

Most people who die in extreme heat do not have air conditioning, but many do own an electric fan [s1]. That makes the question of when a fan helps and when it hurts a genuinely consequential one, and a randomised crossover trial published on 13 August adds a variable that earlier work had not tested: what happens when the person using the fan is dehydrated [s1].

The short version of the finding is that dehydration changed the answer. In the trial's conditions, fan use worsened cardiovascular strain when participants were dehydrated, and the comfort benefit that fans provided when participants were well hydrated disappeared [s1].

What the trial did

Twenty participants each completed four three-hour sessions on separate days in a climatic chamber held at 39.2 ± 0.7 °C with 49% ± 4 relative humidity, wearing shorts and a cotton vest [s1]. Those conditions were chosen deliberately: they represent the environmental threshold at which fans have been modelled to become detrimental for optimally hydrated healthy adults [s1].

The four sessions crossed two variables. In two sessions participants were euhydrated — they drank to daily recommendations for 24 hours beforehand and drank a prescribed volume of roughly 37 °C water during the exposure [s1]. In the other two they were dehydrated: no fluids and no food containing more than 75% water for 24 hours beforehand, and nothing to drink during the three hours [s1]. Each hydration state was tested once with a 44.5 cm fan placed 1.5 metres away and facing the participant at an air velocity of 2.5 m/s, and once without [s1].

Primary outcomes were rectal temperature, heart rate and whole-body sweat rate, the last calculated from the change in body mass over 180 minutes corrected for fluid consumed [s1]. Secondary outcomes were thermal discomfort, thermal sensation and thirst, each rated on visual analogue scales [s1]. Participants established a five-day hydration baseline before the first session by measuring morning body mass and urine specific gravity [s1].

What it found

There was a statistically significant interaction between fan use and hydration status for end-of-trial heart rate (P = .02), thermal comfort (P = .006) and thermal sensation (P = .007) [s1]. In plain terms: whether the fan helped depended on whether the person was hydrated.

Three findings did not depend on hydration. The effect of fan use on core temperature, whole-body sweat rate and thirst was similar in both hydration states [s1]. And regardless of hydration, the fan increased whole-body sweat rate — in the dehydrated condition, by a mean difference of 125 g per hour (95% CI, 108 to 142; P = .005) comparing fan with no fan [s1]. The authors describe the fan as accelerating sweat losses by approximately 60% [s1].

The largest effect in the study was not about fans at all. Irrespective of fan use, dehydration drastically worsened every marker of thermal, cardiovascular and subjective strain — heart rate without a fan was a mean of 17 beats per minute higher when dehydrated than when euhydrated (95% CI, 13 to 20; P < .001) [s1]. Heat-related increases in heart rate and core temperature were 2.0 to 2.5 times greater when dehydrated [s1]. No adverse events were reported [s1].

How this sits against the existing evidence

Public health guidance on fans has been revised in recent years partly because of biophysical modelling work from the same research group. A 2021 modelling study in The Lancet Planetary Health argued that international organisations had been discouraging fan use above 35 °C on thin evidence, and proposed simplified thresholds instead: fan use at temperatures below 39.0 °C for healthy young adults, below 38.0 °C for healthy older adults, and below 37.0 °C for older adults taking anticholinergic medication [s2]. Applying those thresholds across 108 populous cities and 13 years of hourly weather data, fans would have been recommended on 29.6 of 31 hot-weather days for healthy young adults, versus 27.2 days under the 35 °C rule [s2].

The new trial does not overturn that framework, but it adds a condition the model did not carry. The thresholds were derived for optimally hydrated people. This trial ran at the top of that range and found that when hydration slips — which the authors note is common during heatwaves — the calculation changes [s1].

The authors also note that the absence of a fan effect in the euhydrated condition differed from their own previous study, which is worth stating plainly rather than smoothing over [s1].

Limits worth holding onto

This is a 20-person laboratory trial in young healthy participants under a single set of environmental conditions, and the authors say so [s1]. The people most likely to die in a heatwave are older, often on medications that affect sweating, and frequently living with chronic illness — a group this study did not include. The authors identify testing older participants with varying health and medication status across different conditions as the needed next step, along with testing fans combined with self-dousing [s1].

Nothing here is advice about what to do in a heatwave; that depends on age, health, medication and the actual conditions outside. What the trial supplies is a specific, tested observation: at around 39 °C and moderate humidity, hydration status changed whether a fan made the physiological picture better or worse, and dehydration itself was by a wide margin the bigger problem.

Sources

• Electric Fan Use With Dehydration in Extreme Heat and Humidity: A Randomized Crossover Trial — JAMA Network Open, 2025-08-13
• Electric fan use for cooling during hot weather: a biophysical modelling study — The Lancet Planetary Health, 2021-06-01