As we are entering a hot summer I thought it was worth posting this article from the Mornington Peninsula Landcare Network December Newsletter as a separate entry.
It’s Hot. Turn on the Fan? Maybe!
We have discussed the deadly combination of heat and humidity in two previous newsletter articles – Heat Stroke – The Sly Death Trap, and Diary of a Heatstroke Person.
In both of these, the humidity was the real killer. In essence, we are hot-blooded animals with an internal fire (metabolism) that is cooled in hot conditions by sweating. When the temperature and humidity reach a certain level, our sweat cannot evaporate and we gradually stew from the inside out.
This danger level is embodied by the wet bulb temperature (Tw), which depends on both dry temperature and humidity. The survivable limit for many occurs at Tw=30C (e.g. 40C and 50% humidity), and when Tw exceeds 34C (40C and 65%) no one can survive without an external cooling aid.
Does turning on the fan provide such an external aid? Maybe, maybe not. A recent study tested the levels of heart stress for a cross-section of people under laboratory conditions, with surprising results.
Condition 1: Temperature 38C, Humidity 60% (Tw=31.2C)
This condition is well past the danger level but turning on the fan did have an beneficial effect. Heart stress was reduced substantially, even more when the subjects were spritzed with a water spray.
The physical reason is that in still air the sweat soon saturated the nearby air, and was no longer capable of providing a cooling effect. By providing a continuous source of drier air, the fan enabled a continuing cooling.
Condition 2: Temperature 45C and 45% Humidity (Tw=35C)
This condition is above the extreme danger level, and paradoxically turning on the fan made it even worse, – heart stresses rapidly reached a level that caused the experiment to be terminated.
Here, the physics appear to be that turning on the fan exposed the subjects to a continuous source of hot, humid air and sweating could not keep up.
The message is clear: when temperatures and humidity are high, the only solution is to be artificially cooled – for example by an air conditioner or bathing in cool water.
– Article by Greg Holland, references as indicated, image from here.