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First of all I'm not a medical specialist at all, but still I hope my question is precise enough to be answered.

One day I asked a General Practitioner if a father could go outside with his baby / new born while the weather was hot (let's say around 30-35°C), carrying it skin to skin against the father's chest in a carrying scarf. The GP's answer was "obviously the father shouldn't". Unfortunately the GP could not explain me why the father should not.

Disclaimer : This question intent is only theoretical, I don't plan nor recommend to go outside with a new born when it's hot, since I know it could be dangerous for the baby.

So I wanted to know if in a carrying scarf (skin to skin against the father's chest) the adult's body could help the baby's one to regulate its temperature, throwing away the exceeding heating from the baby. Or put another way can the adult's body act as a heat exchanger to keep the baby's internal temperature around the adult's body temperature (~ 37°C) ? Or would it be worse and would the baby's internal temperature increases up to risky values ?

Thanks in advance to whoever that can shed me some lights!

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Your GP clearly has no understanding of Physics, nor of Anthropology. Firstly as the other answer points out, millions of people carry their babies like this in tropical climates and have done for hundreds of thousands of years. It's fairly insulting to presume that they must have been so stupid as to suffer great harm coming to their babies without doing anything to alter their habits.

As to the physics, according to the second law of thermodynamics (Clausius's statement), heat will always pass from an area of higher temperature to an area of lower temperature. Consider that the only way in which baby might be in danger of getting too hot in either case, is if the outside temperature were high enough that it's ability to lose heat to the outside was compromised. That means that it's degree of contact with the outside air is irrelevant as that is causing the problem in the first place. Because of the second law of thermodynamics, when you put two bodies of different temperature together, they will homogenise to the same temperature, essentially becoming one unit. We have biological mechanisms to maintain homoeostasis. Presuming a healthy father, the adult's mechanisms will be more efficient than the baby's (they have a larger surface area to loose heat over and their heat loss system will be fully developed). By putting the two bodies together, therefore, the baby is becoming part of a system which is overall more efficient at losing temperature than it was on it's own.

Another way of looking at the second law is that the baby can only pick up heat from the adult if it started off cooler than the adult, why would it have done that if both are in the same environment?

Of course if the sling were made out of an insulating material, then heat could get trapped in that part of the adult's body which would comprise the entirety of the baby's body and so could cause some harm. Charitably, I suppose that's what the GP might have been thinking about, but you just need a less insulating sling material.

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    Thanks Isaacson for your answer, it makes lots of sense! – HelloWorld Jun 18 '17 at 13:20
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    @HelloWorld Or at least the answer you wanted to hear. – Carey Gregory Jun 19 '17 at 13:22
  • @CareyGregory you may be right ;-) – HelloWorld Jun 19 '17 at 14:00
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This is basically a physics question, actually.

Let's assume father and infant both start out at 37C core temperatures.

The baby is placed against father's chest. Due to their size differences, about 40% of the baby's skin surface is in contact with the father while only about 10% of the father's body is in contact with the baby (estimated using Rule of Nines).

When they're both the same temperature nothing happens. But let's suppose one of them starts getting hotter than the other. What happens then?

If it's dad, he becomes a big heat source for baby since he's in contact with 40% of baby's skin surface and a much larger body. He can only make it harder for baby to stay cool.

If it's baby, baby can't radiate heat from 40% of his body surface, making cooling much harder.

So I think the answer is your GP was right.

(Meanwhile, millions of people around the world in tropical climates strap their infants to their body and go work in the fields all day long with no apparent harmful effects.)

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    Thanks Carey for your answer. I tend to agree with your first case. Yet in the second case (baby hotter than adult) why would part of the baby's heat radiations be blocked (the adult chest is not made of tin foil) ? If Dad's chest acts as a heat exchanger then it will help the baby to cool won't it ? – HelloWorld Jun 18 '17 at 13:30
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    @HelloWorld Dad's body is probably going to be much warmer than ambient air, but even if it's not, sweat can't evaporate from 40% of baby's body. Reducing the body's primary cooling mechanism by 40% can only result in less cooling. Whatever cooling he offers will be minimal and vastly less than free flowing air would be. – Carey Gregory Jun 18 '17 at 13:49
  • You're right the cooling through free flowing air would be cut. So as Dad's body is mostly made of water and water is much more efficient to transfer heat than the air is, then baby's body would be at Dad's temperature (~37°C). Would it make sense to you ? – HelloWorld Jun 18 '17 at 13:57
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    @HelloWorld No. First, the epidermis is the only part of their bodies touching and it is dead tissue with very little water content. Second, there's just no way you're cooling better pressed against a 37C body than with 25C circulating air evaporating sweat. Sweat evaporation is a far more efficient heat radiator than a human body pressed against you. If you doubt that, try it yourself. Go out, get all hot and sweaty, then find someone who will let you hug them. All that will do is make you both hotter than you were. – Carey Gregory Jun 18 '17 at 17:47
  • 1. Unless either baby or father have a defective physiology, neither will "get hotter than the other" they may feel subjectively hotter, but they will both actually be 37C (32-34C at skin) because that's the temperature our bodies maintain, so father's not going to be a heat source any more than the baby's own metabolism is (about 90 watts), his skin is going to be 34C, so is baby's. – Isaacson Jun 19 '17 at 6:45

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