September 6, 2016

expert reaction to study looking at electric fan use in the elderly during extreme heatwaves

A study published in JAMA has looked at electric fan use by nine elderly people and examined heart rate and core temperature. The paper reported that fan use resulted in greater heart rate and core temperature, although differences were small.

Prof. Mike Tipton, Professor of Human & Applied Physiology, University of Portsmouth, said:

“The researchers are reputable. The findings follow from first principles: if you place people in a room temperature above skin temperature they will gain heat by convection, radiation and conduction and lose it by evaporation. If evaporation is impaired by age (sweat production) or humidity (sweat evaporation), thermal balance shifts towards heat gain rather than loss. This gain will be greater from convection if a fan is increasing heat exchange at the surface of the skin. However, as the authors say, this finding is only relevant in 42°C (and above) i.e. much higher than we see in the UK where air temperate tends to remain below skin temperature during heat waves. Also, even if in very high temperatures, evaporative heat loss can be augmented by the spraying of mist on the skin.

“The important message is therefore: do not generalise these finding to the idea that fanning to cool people in the heat is bad.”

Prof. Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“It’s important not to read too much into these results. There are several reasons for that. The study involved only nine people; that’s not really enough to establish how variable the responses to a use of a fan are across the whole elderly population. The charts in the report do indicate that these nine people did vary quite a bit in their response. The results are quite complicated to interpret, arguably. And as the authors clearly point out in their report, they looked only at the use of a fan at a temperature of 42 degrees C (108 F), and that’s at the high end of the range usually found in a heat wave. The study has provided information we didn’t have before, but it hasn’t definitively settled the question of whether using a fan when it’s very hot is good for you, bad for you, or neither.

“The reason for doing a study such as this is that existing health advice on whether to use a fan in a heatwave is not clear. The World Health Organization advise that using a fan at temperatures above 35 C “may not prevent health related illness”, but note that they use the word “may”. And a review in 2012 of published studies did not find any data that, in its authors’ view, provided adequate evidence on whether using a fan in a heatwave might be helpful or harmful. A study published in 2015, described by its authors as “preliminary”, involved eight young men, and found that using a fan did significantly reduce the adverse effects of heat and high humidity, but eight people is rather few, and the authors pointed out that the results might not apply in other populations (e.g. older people). Hence this new study.

“The press release says, correctly, that the new study “examined whether electric fan use would delay elevations in heart rate and core temperature of elderly adults exposed to extreme heat and humidity.” But it’s worth looking at what’s meant by “elevations”. Both this study and the previous one in young men involved sitting the subjects in a hot chamber, with or without a fan, and gradually turning up the humidity. What generally happens is that, when the humidity gets high enough, the person’s body temperature and heart rate begin to increase more rapidly, because the body cannot keep itself cool enough any more. In the study with young men, the humidity at which this elevation in temperature and heart rate occurred was significantly higher when a fan was being used – that is, the elevation was delayed by the fan. But with the elderly people, it wasn’t delayed. However, it wasn’t speeded up either – there was no significant difference. So in terms of these elevations, the new study didn’t find a helpful effect of the fan as with the young men, but it also didn’t find a harmful effect of using a fan, as the World Health Organization advice indicates.

“The new study did find, in statistical jargon, a “significant condition x relative humidity interaction” for heart rate and for body temperature. What this means is that, as they increased the humidity in the chamber, the pattern of increase in heart rate was different, depending on whether the fan was running or not, and this difference was too big to be attributed only to chance. The same was true for core body temperature. This isn’t the same as saying that heart rate or body temperature were uniformly higher (or indeed lower) when the fan was running – as the authors point out, the difference became less as humidity increased, and their charts show that the difference was greatest in the middle levels of humidity. So quite a complicated pattern of results.

“The authors make it clear that the differences in heart rate and body temperature are rather small, and again it’s worth remembering that this is all based on data from just nine people. This study, and the previous one using young men, have added to what’s known about whether it’s healthy, dangerous, or neither to use an electric fan in a heatwave, but they certainly haven’t settled the question in any definitive way.”

‘Cardiac and thermal strain of elderly adults exposed to extreme heat and humidity with and without electric fan use’ by Daniel Gagnon et al. published in JAMA on Tuesday 6 September 2016. 

Declared interests

Prof. Mike Tipton and Prof. Kevin McConway declare that they have no relevant interests to declare.