Why your ‘thermoneutral zone’ is important

How hot is too hot? That’s perhaps not a question most Australians, apart from those living in the top end, would expect in July.

But when it comes to human limitations, this is a question scientists have been grappling with for a long time. Now, a new study has provided evidence of an upper limit to what is known as our ‘thermoneutral zone’ (TMZ).

These findings were presented at the annual conference of the Society for Experimental Biology in Edinburgh, Scotland. Those who know the Scottish climate well will appreciate the irony here. Scotland is not exactly known for its heat.

In presenting the findings, the University of Roehampton’s Professor Lewis Halsey explained that the lower limit of the human thermoneutral zone is well-established. That limit is around 28° Celsius.

Remarkably, though, there is no scientific consensus regarding the upper limit. Past estimates have ranged from 32°C to 40°C.

Prof .Halsey’s findings suggest that the higher end of that range is the likeliest answer.

What is the thermoneutral zone and why is it important?

Although this research may sound somewhat esoteric, defining the upper limit of our TMZ could have important implications. And with the planet continuing to warm, that importance is likely to increase.

Most of you will know that ideal human core temperature is around 37°C (98.6°F). The thermoneutral zone is the temperature range at which the body doesn’t have to increase its metabolic rate or exert more energy to maintain this temperature.

Outside of this range, your body needs to expend energy to keep your core at 37°C. If the ambient temperature drops to a certain level, your body expends more energy to maintain its ideal temperature. One of the common ways it does so is by making you shiver. Shivering is the involuntary contraction of key muscle groups that produce heat.

At the other end of the spectrum, your body will sweat to keep you core temperature down. It will also invoke a process known as ‘vasodilation’. Vasodilation is a widening of the blood vessels within the body, occurring naturally in response to increases in body temperature. Its purpose is to increase blood flow and oxygen delivery to parts of the body that need it most.

What did the study reveal?

Prof. Halsey’s team measured 13 participants who rested while exposed for an hour to five different temperatures and humidity levels. The combinations tested were:

• 28℃ (82.4F) and 50 per cent relative air humidity (RAH)
• 40℃ (104F) and 25 per cent RAH
• 40℃ (104F) and 50 per cent RAH
• 50℃ (122F) and 25 per cent RAH
• 50℃ (122F) and 50 per cent RAH

For each condition, participants’ measurements were recorded. These included metabolic rate; core and skin temperature; blood pressure, sweat, heart and breathing rate; minute ventilation and ‘fidgeting’ levels.

The researchers found that participants’ metabolic rate increased by 35 per cent when exposed to 40℃ and 25 per cent RAH. At 40℃ and 50 per cent RAH, the increase was 48 per cent.

Crucially, they found increased metabolic rate at the 40℃-25 per cent RAH condition wasn’t accompanied by an increased core temperature. But participants in the 50℃-50 per cent RAH condition experienced a rise in core temperature of 1℃.

These findings suggest that the body is able to dissipate heat at 40℃, but not at 50℃. In turn this suggests the upper limit of our thermoneutral zone is at least 40°C but possibly higher.

“Our study shows clear evidence that resting metabolic rate increases in people at high temperatures,” said Prof. Halsey. “There is an upper limit to the human TNZ, at least as low as 40°C.”

As to mechanisms setting that limit, Prof. Halsey freely admits to not knowing what they are. In concluding his presentation abstract, he wrote: “But what explains this upregulation? We don’t yet know.”

Further research will provide answers that may have implications for things such as working conditions, sports, medication, and international travel.

In the meantime, now that you know why you feel so uncomfortable when it’s hot and humid hot, stay cool.

How do you cope when it gets too hot or too cold? What are your comfortable upper and lower limits when it comes to temperature? Let us know in the comments section below.

Also read: How to support your body’s biggest organ

Health disclaimer: This article contains general information about health issues and is not advice. For health advice, consult your medical practitioner.