It's sometimes possible, in this age of ubiquitous air conditioning, to imagine that summer is something that happens only to other people. What's the big deal about a heat wave when the forecast for your office, home, and car remains comfortably chilly?
In reality, of course, we still go places – outside, for example – that aren't air conditioned, and many people don't have access to cooling at all. The overuse of power-hungry air conditioners is also an increasing burden on maxed-out power grids. But going without can have severe consequences: the heat wave that swept across Europe in 2003 caused an estimated 30,000 deaths, mostly poor and elderly people without air conditioning in their homes.
Could the humble electric fan provide a cheap, efficient and accessible cooling option in heat waves?
The conventional answer is no, because you simply blow more hot air at yourself. But as a new study in the journal Medicine & Science in Sports & Exercise reveals, the physiology of fan use in heat waves – as well as in summer sporting events like the notoriously muggy U.S. Open tennis tournament – is a surprisingly complex physiological puzzler, with answers that vary depending on who you are and how hot and humid it is.
Under normal circumstances, you're constantly giving off heat to the air around you, typically at a rate of about 100 watts, like an old, incandescent light bulb. But the flow reverses when the air gets hotter than your skin, which is typically around 35 degrees Celsius.
At that point, you're gaining heat from the air instead of losing it. Using a fan to blow more of this hot air past you will actually accelerate this heat gain – which is why many public health agencies, including the World Health Organization and the U.S. Centers for Disease Control and Prevention, advise against fan use during a heat wave.
But fans have another, more subtle benefit: they can help accelerate the evaporation of sweat from your skin, which provides a powerful cooling boost. Over the past few years, thermal physiologist Ollie Jay, formerly of the University of Ottawa and now at the University of Sydney, has used theoretical calculations and a series of experiments to argue that fans may provide a crucial cooling advantage even at temperatures well above the supposed "danger zone" of 35 Celsius.
In a 2015 study in the Journal of the American Medical Association, Jay and his colleagues showed that sitting in front of a fan staved off heat-related elevations of heart rate and core temperature even at temperatures as high as 42 Celsius with 50- per-cent humidity.
The new study, from researchers at the University of Texas Southwestern Medical Center in Dallas (along with Jay as a co-author), addresses a further wrinkle: the older you get, the less you're able to sweat. That's one of the reasons the elderly are among the most vulnerable in heat waves, and it also means that the enhanced sweat evaporation promoted by fan use might be less useful for older adults.
The researchers had two groups of volunteers, one with an average age of 26 and the other with an average age of 68, sit in a heat chamber at 42 Celsius for 130 minutes with the humidity gradually increasing from 30 to 70 per cent. They repeated this protocol once without a fan and once sitting a metre away from a 40-centimetre fan.
For the younger group, the fan resulted in a slightly lower heart rate, by two beats per minute (bpm) on average, and negligible changes in core temperature. For the older group, in contrast, the fan resulted in a five bpm increase in heart rate, as well as a 0.2-degree Celsius increase in core temperature, indicating an increase in physiological strain that could, in theory, have negative effects in a heat wave.
So does this mean fans are dangerous for older people? Not so fast, cautions Craig Crandall, the University of Texas physiologist who led the study. "The conditions we used were intentionally extreme," he notes.
While last month's heat wave in the southwestern U.S. is an exception, most heat waves in North America and Europe top out around 38 or 39 Celsius. In that range, Jay says, fan use is definitely protective for younger people, and at worst neutral for older people.
"The reason I think these considerations are important is because the most vulnerable may have very limited options," he says. "If they are being told that fans should be turned off, without any attention to the environmental conditions, then this in itself could be dangerous."
Moreover, there's a simple way to supercharge the evaporative benefits provided by a fan: wet your skin, for example with a damp sponge.
Jay and his colleagues have published two recent studies simulating the conditions at the U.S. Open and Australian Open tennis tournaments, both of which have been disrupted by heat in recent years. Compared to the usual cumbersome between-game cooling protocols, which involve draping specially prepared ice-filled towels on the neck and damp towels on the head and thighs, simply sitting in front of a fan while dampening the skin with a sponge proved to be just as effective.
In theory, that same technique should work for anyone relying on a fan for cooling – and since the "sweat" is provided externally, "the age-related differences in the efficacy of fan use should disappear," Jay says. "We are working on this now!"
Alex Hutchinson's latest book is Which Comes First, Cardio or Weights?