Health & Safety

The Dangers of Heat Stress

Nice Weather

We have been enjoying a relatively warm, dry spell in some parts of the UK recently. For many of us summer has finally arrived after a rather disappointing spring. On my local patch the farmers are making hay while the sun shines.

But every silver lining has a cloud and for some people that cloud is heat stress.

Heat Stress

Heat stress is what happens when your core body temperature starts to creep up above the normal 37oC. The human body has complex thermoregulatory mechanisms to keep core temperature at or close to 37oC. If these mechanisms fail to work effectively then very significant health effects will start to occur as a consequence of relatively minor temperature fluctuations. At 40oC it becomes a life-threatening medical emergency. At 43oC irreversible tissue and organ damage will occur and most people will die as a consequence. That’s just a 6oC increase (which seems quite trivial relative to the temperature fluctuations of the external environment where you might be at 12oC on a rather chilly summer’s morning rising to 26oC by lunch time).


Foremost amongst the thermoregulatory mechanisms is sweating as a way of losing heat. Blood supply to the skin is increased by vaso-dilation (the diameter of the blood vessels in the skin increases), sweat glands situated in the dermal layer of the skin extract water and salts (electrolytes) from the blood and this liquid is excreted onto the surface of the skin. As the water changes state from a liquid to a vapour it takes energy from the skin in the form of heat. This heat loss cools the skin, which in turn cools the blood and so cooler blood can be returned to the core to collect more heat that is then returned to the skin, etc. In this way a cooling cycle is established where heat from the core is transported to the surface and then away by sweating.

Of course heat is lost from the surface of the body by radiation and conduction as well. But radiation and conduction only work when the temperature of the surroundings are less than the temperature of the body. Once air temperature reaches 35oC heat won’t radiate or conduct away at all and sweating becomes the only effective cooling mechanism. Unless you are able to put yourself in contact with something cool. Such as immersing yourself in cold water, which will cool you down very rapidly by conductive heat loss. This is why splashing cold water on your face, neck or forearms on a hot summer’s day is such a pleasant experience.


Whilst sweating is a very important cooling mechanism it can also be rather dangerous. When you sweat you lose water and so you will be dehydrating. Your blood gets thicker and more difficult to pump around the body. And whilst you are losing valuable salts that are essential for normal body functioning you are also concentrating the remaining salts in the blood due to water loss. Significantly, one of these salts is sodium which plays an important role in the workings of nerve and muscle cells. Having an excessively high sodium concentration in the blood causes many ill-health effects, some of which are characteristic of extreme heat stress.

If heat stress is not controlled it leads, at the extreme, to heat exhaustion and heat stroke. Heat stroke is where thermoregulatory control has broken down completely leading to the core temperature rapidly spiralling out of control. End result; delirium, convulsions, coma and death. The deaths of three reservists on an SAS selection march in the Brecon Beacons in 2013 illustrates the risks all too well (see here).

Yellow image of the sun with clouds in background

It Not Just About Air Temperature

So when air temperature is high there is the potential for heat stress. But other factors play a part in the thermoregulatory process. Such as the humidity, wind speed, radiant heat gain, how hard you are working and the clothing that you are wearing. A part of the HSE website is dedicated to these various factors and their management (see here).

To single out just one of these other factors; relative humidity is a measure of the amount of water vapour in the air expressed as a percentage of the maximum possible water vapour content of air at that same temperature. So if relative humidity is 100% it means that it is not possible to get any more water vapour into the air. It is saturated. To be effective as a cooling mechanism sweating only works if the sweat can evaporate. At 100% relative humidity sweat can’t evaporate. If sweat can’t evaporate then all that happens is that you lose water and salts from your bloodstream and your clothes get wet. Not pleasant. And completely ineffective as a cooling mechanism. 

Stay Hydrated

Managing the risk associated with heat stress can be a big challenge. For some workplaces, such as foundries and mines, this is a year-round problem that requires constant attention. Of course, for outdoor workers it’s normally a seasonal problem.

One of the key control measures to avoid the worst ill-health effects associated with heat stress is to keep hydrated. This normally means drinking small amounts of water routinely. The HSE recommend that workers who are working hard in a heat stress environment drink 250 ml water every 15 minutes (see here). If practicalities prevent this sort of intake then 500 ml an hour before work starts and 500 ml during breaks.

One way of checking to see if you are hydrated/ dehydrated is to look at the colour of your urine. Pale yellow indicates well hydrated. The darker and browner the colour the more significant the dehydration is. You can get colour charts to put up on the walls of your welfare facilities to help people check their hydration levels (see here).

But Don’t Overdo It

The recommendation to stay hydrated by frequently drinking small amounts of water is sound advice.

Unsurprisingly one of the symptoms of heat stress and dehydration caused by sweating is feeling thirsty. This thirst is symptomatic of high sodium concentrations in the bloodstream due to the water loss. But thirst is not a great symptom to rely on to tell you to drink because once you feel thirsty it means you are already becoming dehydrated. It’s a late symptom rather than an early one. And unfortunately if you quench a raging thirst by drinking too much water too quickly it can have catastrophic effects. Excessive amounts of water enter the bloodstream, sodium concentrations fall to dangerously low levels (a condition called hyponatraemia) and tissue swelling can cause irreversible damage and even death (see here). You can literally kill yourself by drinking too much water.

So the trick is to stay hydrated but don’t overdo it.

Dr Jim Phelpstead BSc, PhD, CMIOSH

RRC Consultant Tutor                                   

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