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Fans harmful in heatwaves with dry heat, helpful in humid heat

Using fans may be harmful in heat waves with very hot and dry conditions but helpful in hot and humid conditions, according to a research report published in the Annals of Internal Medicine.

The findings suggest that using heat index values to recommend fan use, as the U.S. Environment Protection Agency does, may be problematic, according to researchers.

“The [heat index], a unified weather metric used widely in summer forecasts, expresses the combined effects of temperature and humidity, but its utility for recommending fan use may be low,” Nathan B. Morris, PhD, of the Thermal Ergonomics Laboratory at the University of Sydney, Australia, and colleagues wrote. “A very hot day with low humidity may yield a lower [heat index] than a slightly cooler but much more humid day.”

Researchers recruited volunteers to be exposed to 2-hour exposures to two different simulated conditions of a heat wave in a randomized crossover study. One exposure was very hot and dry (47°C [116.6°F]; relative humidity, 10%; heat index, 46°C [114.8°F]), whereas the other had a lower temperature but was more humid and had a higher heat index (40°C [104 °F]; relative humidity, 50%; heat index, 56°C [132.8°F]).

A total of 12 male volunteers underwent four 2-hour experimental trials where they sat in a climate chamber that simulated both heat wave conditions. Each condition was tested with and without an electric fan in the room.

For each condition, thermal strain was measured through rectal temperature and cardiovascular strain was assessed through measurements of heart rate, systolic blood pressure and rate pressure product. Dehydration was measured by change in body mass before and after the trial to determine the whole-body sweat rate.

Compared with no fan use in very hot and dry conditions, fan use was tied to increased rectal temperature, rate pressure product, heart rate and whole-body sweat rate in addition to decreased thermal comfort.

Fan use in hot, humid conditions improved all measures except whole-body sweat rate compared with no fan use.

Systolic blood pressure did not change with fan use in either simulated weather condition.

Researchers noted fans should not be recommended for heat waves in very hot, dry regions such as the South Central United States, South Australia and the Middle East. However, they felt fans could still be helpful during heatwaves in areas where temperatures rarely go higher than 40°C (104°F) and are accompanied by moderately high humidity (>30% to 40%).

“In hot, humid conditions with an [heat index] of 56 °C [132.8°F], fans reduced core temperature and cardiovascular strain and improved thermal comfort,” Morris and colleagues wrote. “Yet, fans were detrimental for all measures in very hot, dry conditions despite a lower [heat index] of 46 °C [114.8°F]. These findings highlight issues that may arise when [heat index] values are used to recommend fan use during heat waves.” – by Erin Michael

Disclosures: Morris reports no relevant financial disclosures. Please see study for all other authors’ relevant financial disclosures.