Does drinking tea on a hot day cool you down? written 2 years ago

Summary

Using a simple model it is easy to come to the conclusion that drinking a hot cup of tea on a warm day will not cause any beneficial cooling. The human body however is a complicated machine, and using a more refined model we can show that although one specific part of the body may heat up, the majority of the body will cool down — below 37°c.

Introduction

“have a cup of tea, it’ll cool you down”

The first time I heard this was a million years ago at one of my primary school’s sports days. All the kids were getting small cups of incredibly sugar filled orange juice¹, whilst the teachers and parents were drinking cups of tea. I quizzed a random teacher about this apparent fallacy and was told that drinking a hot drink cools you down more than a cold one.

Leaving this statement unquestioned until my more senior years I began to ask colleagues and friends if they had heard of the same mystical properties of hot tea as I had all those years ago. All of them had heard this at some point, some questioned it, some were firm believers and some others thought it quite absurd.

The UK Tea Council even promotes this magical property of tea!

I therefore sat myself down and set myself the challenge of finding out once and for all the answer to the question:

Theory

1. Hot drinks cause your body to sweat, and by sweating it causes your body to cool down;

Assumptions

• Sweating occurs when skin temperature is above 37°c;

• The body is not already sweating.

  1. If the body is already sweating, then by the theory above there is no need to drink the hot tea in order to sweat. Drinking tea in this case could only heat the body up.

• The actual tea content is immaterial, it’s the hot liquid we are concerned with.

When we make these two points, the proof we need to answer our question is quite simple…

Proof that drinking hot tea does not cool you down on a hot day

1. As stated above the body sweats if and only if the temperature of the body is above 37°.

2. Once the hot drink has been consumed, the internal temperature will rise (above 37°), and sweating will begin.

3. As the person sweats, the temperature will be reduced, until it is at 37°.

4. At 37°, sweating will stop.

5. The temperature of the person will remain at this temperature.

The key point is that if you’re not already sweating, then your body is doing a good job of keeping you cool without needing to sweat — the energy being created inside you is being dispersed adequately by convection. Adding heat to your body in the form of tea worsens the situation, and forces the body to remove the new heat by sweating. Your body is clever however, and will only sweat enough to remove the additional heat added by the tea, and won’t just start sweating indefinitely².

Conclusion

Drinking a hot drink on a hot day is not going to cool you down. It will make you warmer for a while, then you’ll return to your normal 37°.

Further Investigations

For the proof above we’ve taken quite a simple model of the human body — Assuming that the heat from the tea is dispersed all over the body. In reality though, the heat will be localised to the body’s core — the important bit. The brain worries quite a lot if the core of the body gets too warm, and so, what if under this situation the brain goes nuts and demands that everyone sweats until the core is back at a proper temperature? This might just cause a net temperature loss throughout the entire body.

To explore this option, I invented some random guesstimates and came up with a new model with some more complexity:

• Average Human male Weight: 80kg
• Sweat rate: 1 l/hour. (this is the rate under moderate exercise)
• The entire specific heat capacity of the human body is about 3.5 cal/gram (water is about 4.2)

In this model we assume that the “core” (10kg) of the human gets all of the heat from the tea, and thus in the example given above will raise in temperature by 0.68°c. (Not much, but enough to get the body worried). This then causes the body to go into sweating mode, pumping 1l of water to the skin every hour. Assuming all of this gets evaporated, this will consume 9666 calories of body heat every minute.

This is where we differ from the above example though. Although only the “core” of the body is at a heightened temperature, the only way to dissipate this heat is to cool off the entire body, meaning that the 9666 calories of heat are removed from the entire body, and not just the core (since it’s hidden within 4 inches of blubber). This means that 0.0000036°c is removed from the entire body (including the core) every minute. Thus, since the majority of the body was already at a reasonable temperature, it will actually start to feel cooler, as it will have dropped below it’s optimum 37°c.

Body/Core temperature After drinking a hot cup of tea on a Sunny day

Conclusion 2

So, assuming that the above assumptions are correct, it seems that there might be a shred of truth to the tale that drinking a hot drink will cool you down on a warm day.

Links

• UK Tea Council
• Summer Drinks
• Perspiration cooling of body
• Cooling of the human body

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¹ The kind of orange juice that can only be served at schools, from gigantic beaten up aluminium jugs.

² This is a Good Thing, since you would probably die of dehydration otherwise.

³ Read into this what you will.

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