How hot before someone gets heat exhaustion while running?

Question

I myself have gotten heat exhaustion once but that was while sitting still in a sauna. I knew to get out when I was dizzy. I gently collapsed onto the floor, still conscious. I noticed something odd happened. Well first off I repeatedly blinked slowly which I expected. But also, at first, even lifting my head an inch would make me dizzy. But as time passed, I could lift myself up further until an hour after collapse, I was back to normal.

But I was wondering, if someone is running to exercise and it is hot outside, how hot does it have to be before heat exhaustion sets in and they need to stop running? For me in the sauna, it took a temperature over 120 degrees Fahrenheit to make me dizzy and even then, the amount of time it took was 45 minutes between getting in the sauna and getting dizzy. But I know that exercise would lower the temperature and time needed. I think it would be closer to 100 degrees ambient temperature before it sets in and here is my hypothesis as to why:

Hypothesis

So, assuming the person in question does not have a condition that prevents him/her from sweating and is hydrated and has snacks or has eaten a meal not all that long before he/she started running so that dehydration and hunger aren't a factor here, the sweating and temperature play a role.

Sweating helps initially because it cools the body. But while you are running, you emit heat from your muscles working. Running involves most of the muscles of the body so that is a lot of heat. This heat goes into the cooler air, up to a point.

At some point the ambient temperature right around you will be in equilibrium to the amount of heat from the muscles. This is when the heat will start building up. At this point, sweating isn't sufficient to cool down much, if at all. Once sweating becomes insufficient, it is only time before that person gets dizzy.

If that person gets dizzy and collapses in consciousness, that person has heat exhaustion. Becomes unconscious and body temperature rises, heatstroke. In either case, cooling down and more importantly getting out of the heat is a priority but heat exhaustion is less urgent than heatstroke is.

So am I right about how someone would get heat exhaustion while running and also, am I right that near 100 degrees Fahrenheit would be the air temperature at which this will start happening?

Answer 1

Getting dizzy and collapse in the sauna or during running is not enough to say that this is from heat exhaustion. Such symptoms can also occur in vasovagal syncope due to heat and the associated stress resulting in a drop in the blood pressure.

Symptoms of vasovagal syncope and heat exhaustion can be very similar (dizziness, pale and clammy skin), but in heat exhaustion your body temperature will rise to 38-40 °C (100.4-104 °F), while in syncope it won't.

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The following study shows that heat exhaustion can occur during running at ambient temperature as low as 22 °C [71.6 °F]:

Environmental Conditions and the Occurrence of Exertional Heat Illnesses and Exertional Heat Stroke at the Falmouth Road Race (PubMed Central, 2014)

The study reveals the incidence of heat illness (heat exhaustion and heat stroke) and the ambient temperature and humidity from 18 races, 11.3-km [7-mile] each.

For example, in the 2003 race, at the ambient temperature 27.7 °C [81.9 °F], the incidence of heat illness was 7.6 per 1,000 runners (Table 1 and 2).

Warning signs of heat exhaustion that could make someone stop running before collapsing could be nausea, fatigue, headache and dizziness.

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According to a study from Japan Heat Stroke and the Thermal Environment, 2013:

Patients developed heat stroke when the maximum [ambiental] temperature reached approximately 28°C, and the incidence increased rapidly when the temperature exceeded 31°C.

Answer 2

What matters is the so-called wet-bulb temperature :

The wet-bulb temperature is the temperature read by a thermometer covered in water-soaked cloth (wet-bulb thermometer) over which air is passed.

If the wet-bulb temperature exceeds 35 C, then that's going to be fatal on the long run, as the human body can then no longer dump all the heat it produces into the environment. At rest we produce about 100 Watts of heat, we need a wet-bulb temperatures of just about 2 C lower than the body temperature to get rid of that heat.

A well trained athlete will produce a power of 250 Watt or more, with a typical efficiency of 25%, this means a total energy production of about 1 KW. That's ten times more than at rest and that would roughly require a ten times larger difference between the body temperature and the wet-bulb temperature. So, the maximum wet-bulb temperature for someone running quite fast would be about 19 C (note that body temperature typically rises when running, typically about 39 C), at 50% humidity this corresponds to a temperature of about 26 C.

This does not mean that running at 26 C and 50% humidity will cause people to drop dead, but running performance will be affected at such conditions, and you can expect that some people may pass out while running under these conditions.