REF-886 · Saunders et al., 2005 · Acta Physiologica Scandinavica, Vol. · Paper · PubMed

The effects of different air velocities on heat storage and body temperature in humans cycling in a hot, humid environment

Blimited evidenceMeasured on a mixed groupWritten before October 2026, being re-checked

Nine subjects cycled for 2 h at 33 °C and 59% relative humidity on five occasions: with facing air velocity of 0.2, 9.9, 33.3 or 50.1 km/h while replacing 58.8% of sweat losses, and once at 33.7 km/h while replacing 80.0% of sweat losses.

Heat storage, body temperature and perceived exertion were higher in still air and at 9.9 km/h than in every other condition; 50.1 km/h added nothing over 33.3 km/h, and drinking more at high airflow changed nothing.

The authors conclude that lab findings on the benefit of high fluid intake in windstill heat cannot be carried over to outdoor exercise, where facing airflow is seldom lower than the athlete's own speed.

Participants

n=9 ("nine subjects"). Sex, age and training status are not given in the abstract.

Protocol

Five occasions, each 2 h of cycling at 33.0 ± 0.4 °C and relative humidity 59 ± 3%.
Four airflow conditions with fluid replacing 58.8 ± 6.8% of sweat losses:
0 WS: 0.2 km/h
10 WS: 9.9 ± 0.3 km/h
100 WS: 33.3 ± 2.2 km/h
150 WS: 50.1 ± 3.2 km/h
Fifth condition (100.80 WS): 33.7 ± 2.2 km/h with fluid replacing 80.0 ± 6.8% of sweat losses.
Exercise intensity is not given in the abstract.

Key Results

Rating of perceived exertion, heat storage and body temperature were all higher in 0 WS and 10 WS than in any other condition.
No differences in any measured variable between 100 WS and 150 WS, or between 100 WS and 100.80 WS.
When air velocity was high, taking in fluid at a higher rate did not alter heat storage, body temperature or sweat rate.
The abstract gives no magnitudes (no temperature or heat-storage values).

Practical Application

In nine subjects cycling 2 h at 33 °C and 59% humidity, still air (0.2 km/h) and a light 9.9 km/h airflow left heat storage, body temperature and perceived exertion higher than facing airflow of 33.3 km/h or more; 50.1 km/h added nothing over 33.3 km/h, and replacing 80.0% rather than 58.8% of sweat losses changed nothing once airflow was high. To "does a fan really matter when I ride hot?": yes, and a weak breeze was not enough in this study; strong facing airflow was what reduced heat storage.
Inference, not tested here: a rider on an indoor trainer has no airflow from forward motion, so without a fan they are close to the 0 WS condition. The abstract does not say what fan setup would deliver 33 km/h at the rider.
The paper's own conclusion: fluid-intake studies done in windstill labs overstate what extra drinking does outdoors; with realistic airflow, drinking more than 58.8% of sweat losses did not improve thermoregulation here.
Not from this paper; the base's own practice: heat still costs performance even with realistic airflow (REF-461); drink to a plan built from your own sweat rate (REF-231); indoor session and trainer choices are in SYNTH-56.

Limitations

n=9; sex, training status and exercise intensity not given in the abstract.
No performance outcome; heat storage, body temperature, RPE and sweat rate only, and the abstract gives no values.
One environment (33 °C, 59% RH); only facing airflow was varied.
Written from the abstract and record metadata only (full text not read).
How this entry was checked and changed
Why this level

crossover, n=9