"Are Hot Tubs safe for TTC?" and other heat related questions

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ahcadvocacy

New member
We get a lot of questions about hot tubs and their safety while TTC. I am not a doctor, but I sure have read a bunch on the topic. We also get questions about saunas, jacuzzis, hot yoga, fevers, hot baths or showers, and I’m going to attempt to get to the nitty gritty so you can make the best decisions for yourself weighing the benefits vs the risks.

What’s the big worry with Hot Tubs?​


…For people with testes:​


Testes function better below human core body temperature - they actually get raised or lowered to maintain a temperature of about 35 °C. (wikipedia) If you submerge them in hot water, they can’t self-regulate.

So, hot tubs are not great for sperm. How bad are they? (Well, taking a dip every 3 months is not a valid form of birth control - right?). In a fairly small study, infertile testes-havers that spent more than 30 minutes in a hot tub per week were asked to abstain and those who did saw great improvement. (Shefi et al 2007). Not hot tubs, but testes-havers who worked in bakeries were found to have 22.7% prevalence of infertility over 3% in the control group. (Al-Otaibi 2018). Anything further is beyond my scope - please enjoy this lovely review paper. Figure 1 is great - hot tubs and professional baking are definitely “toxic exogenous factors”.

If you’ve had a bad semen analysis, please talk to your doctor about laptops, tight pants, cats etc. Way way beyond my scope.

… For uterus havers​


There is lots of evidence that high core body temperature (above 39° C or so) can act as a teratogen (causes an increases in abnormalities with embryos or fetuses). (Edwards 1986, Edwards 2006. Higher core body temperature can be caused by fever, excessive exercise, or you guessed it, hot tubs. Most of the research available focuses on the effects on pregnancy, not on conception, just a heads up if you’re clicking on links.

Organization of Teratology Information Services (OTIS) says keep it below 101º F.
ACOG says below 102.2º F (39° C).

Why this is complicated:​


Because homeostasis and physics.
  1. Humans like a narrow range of temperature, and we are constantly working to regulate that in many ways. We sweat, our blood vessels dilate, and we have behavioural responses - drinking cool water, seeking cooler environments.
  2. Let’s talk physics. You’re a human that weighs 150lbs. You’re roughly cylindrical, and you have the density of a human. You are sitting in a hot tub that is heated to maintain a temperature of 40 °C. Your body temperature averages out to 37 °C. How long can you sit in the hot tub before your core body temperature reaches the same temperature as the hot tub?
The system is much more complex in reality:
  • You have a surface area, and your skin temperature is lower than your internal temperature. A hot tub overwhelms most of your methods of losing heat, but some portion of you is still above water, breathing air and losing heat through your head.
  • Is the hot tub outside? Is it snowing? Did you go skiing? Did other people get in as well? Did you have a cover on it? How’s the insulation, does it only have a 120-volt heater? There may be many things that are going to be working against it boiling you like a lobster.
Look, I’m not a physics major, either.

If you hooked a human up directly to 240-volt hot tub heater, and it dumped 6000 watts into them… it would take 2 minutes. And they’d be dead. Not great.

If we rolled you up in a magic perfectly insulating blanket that didn’t allow you lose any heat, assuming you run at about 90 watts? An hour and a half. Maybe two hours, if we cut you an air hole.

The answer is somewhere between those two things. Probably about 30 minutes is a bad idea.

It’s important to note that humans have a fairly high specific heat capacity. It takes a lot of energy to change our temperature. If you’ve ever found yourself too hot or cold via environmental exposure, I’m sure you remember how long and how much effort it took to get back to normal.

Why none of this matters so much:​


Because you feel hot before you get hot. Your skin is designed to protect you from environmental stuff, and you will start to feel sweaty, hot, tired, dizzy, nauseous, and uncomfortable. Most people will want to get out before your core body temperature gets into the danger zone. General safety recommendations are that you should get out after 15-30 minutes, and that you not be under the influence of drugs or alcohol - because falling asleep is the way most people get dangerously overheated.

So, what could you do to decrease risk?​

  • Decrease exposure time - 10 minutes or less.
  • Decrease temperature - can’t go above 102 if the water is below 102
  • Sit with your just your legs in.
  • Take regular breaks
  • Have a timer or clock to keep track of time
  • Have somebody else with you to make sure you don’t fall asleep
  • Take a thermometer with you. For science.

What about steam rooms, saunas, jacuzzis?​


They’re in the same club at hot tubs - when you’re submerged and can’t thermoregulate in many of your normal ways, you’re at higher risk for overheating. Consider the above risk management techniques where possible.

What about hot baths/showers?​


You’re good. Both of these things are constantly losing heat to the environment, it would be very challenging to raise your core body temperature this way.

Hot yoga?​


Water is a better conductor of heat than air, so it would take longer to affect your core body temperature than a hot tub. I cannot pinpoint the duration of exposure that would be problematic, but it’s the same as hot tubs- if you’re feeling too hot (dizzy, nauseous, tired), go cool down.

Fever?​


Take some acetaminophen, drink water, rest. Treating the fever reduces the risk.

Exercise?​


Yeah, don’t exercise until you overheat. It’s not easy to do, though.

Microwaved heat pack for cramps?​


Please enjoy.

Electric heating pad for cramps?​


It’s fine, don’t fall asleep on it.

This doesn’t feel worth the risk.​


Cool! Do whatever feels right to you.

(CW: Pregnancy)
So, there was a really interesting review paper (Ravelli et al 2018) on pregnant women and activities that are generally considered higher risk for raising core body temperature. Very good, thorough paper. Their conclusion:

Pregnant women may safely engage in (…) sitting in hot baths (40°C) or hot/dry saunas (70°C; 15% RH) for up to 20 min, irrespective of pregnancy stage, without reaching a core temperature exceeding the teratogenic threshold.
Click to expand...

Admittedly, that’s the ACOG level (102.2°F or 39°C), not the OTIS recommendation. OTIS says anything higher than 101 is a concern, it’s very easy to get to 102 and you might not feel uncomfortable - I believe these are very cautious recommendations.

Let’s all be cool​


So, this is my opinion based on my best review of the available evidence. I personally agonized over sauna use, and ended up doing a lot of homework because of it. I hope it enables other people to make a more informed risk-benefit analysis when it comes to things that bring us comfort, help us cope, or make us feel good. Remember that while a particular activity may not worth the risk to you, for others the benefits may outweigh the risks.

Edited: Formatting
 
@ahcadvocacy TW: Dark Humor

If you hooked a human up directly to 240-volt hot tub heater, and it dumped 6000 watts into them… it would take 2 minutes. And they’d be dead. Not great.
Click to expand...

Do you have a reproducible study as a reference for this one? I'm especially curious about the being dead not being great for TTC. /s 🤣
 
@ahcadvocacy Yeah that makes sense, since there would be no temps or OPKs that you could do to time intercourse, you'd be flying as blind as you would be using an app to predict ovulation, and that assumes the sperm-having partner even thinks to use an app.

It sure would be a lot less stressful to just be able to lay there and do nothing though.

All jokes aside, for anyone that's in the boat of they or a loved one are currently considering self-harm or death, please reach out for help. Even though I don't know who you are, know that if you have read this post then I love you and I wouldn't want to lose you.
 
@ahcadvocacy In theory wouldn't frozen eggs or a frozen blastocyst intended for IVF also be a possibility for folks with a uterus? It would just require a third party surrogate uterus, right?
 
@heyyoo 🤷 I never said it couldn't be done, just that it was harder. I would argue that insemination is easier than finding a third party surrogate.

It also depends on if you define conception as fertilization or implantation.
 
@ahcadvocacy This reads like a homework assignment from my undergrad heat transfer class. Especially the part about the 150 lb cylindrical human. Good thing I actually liked heat transfer, the flashbacks aren't too unpleasant haha! Thanks for this!
 
We did a problem about how much power a dolphin uses in swimming. In case you're curious, 1 dolphin power is about the same as 1 horsepower (assuming the dolphin is a cylinder with a low-medium drag coefficient).
 
@ahcadvocacy If it were me, I'd ask the question as: How fast would your body need to reject heat to maintain 98.6F if the hot tub is at 110F or whatever. Then it's a simple Q=cm(delT) that's probably pretty close. Not sure if it's tabulated somewhere how much you can cool off by breathing or by other means. I'll check my handbook lol.

Edit: the reason I think this way makes more sense is that your body tries very hard not to change temperature, so really there would just be a static temperature gradient across your skin, your body wouldn't heat up uniformly ever. That gradient will be maintained by your body rejecting heat by your head sweating and by breathing, and how much heat you lose there depends on the vapor pressure of wherever you are and the temperature there. So, it's a very complicated question indeed! But you could see how cold it would need to be to reject X amount of heat and compare that to the amount of heat you'd have to reject to maintain your whole body at a particular temperature, like the heat duty on your body.
 
@ahcadvocacy I can't figure a way to attach things on Reddit, but if you want it you can DM me somewhere to send the PDF.

The main conclusion reads: " We conclude that healthy women of childbearing age can remain in a hot tub at 39.0°C for at least 15 minutes and at 41. I °C for at least 10 minutes without risk of their core temperature reaching a level that might cause a problem for a developing embryo or fetus (Fig. 4). This is a decidedly conservative estimate. Only one subject's body temperature reached 38.9°C within these times. If we omit the data from her two experiences, the earliest risk of hyperthermia would be 25 minutes in the 39.0°C tub and 15 minutes in the 41.1 °C tub. Furthermore, the study did not allow for the behavioural thermoregulation that would be normal when hot tubs are used informally. For example, our subjects had to be reminded to keep their hands and arms immersed; there is a tendency to expose the arms to the air by resting them on the rim of the tub, and this practice slows heating, as we showed with one subject."

They go on to talk about retrospective studies that looked for harm caused by hot tubs. There was some evidence that developing embryos exposed to hyperthermia had higher rates of poor muscle tone, neural tube defects, and facial deformities, especially if exposed before 8 weeks. This includes hyperthermia not related to hot tubs, though, like fever, but there were cases that seemed to be linked to the use of a hot tub or sauna. They note this will be a very rare outcome, because dangerous conditions are also very uncomfortable for most people. They note that societies that have long traditions of use only use hot tubs or saunas for 6-12 minutes, and shorter for pregnant people.

Another good paragraph: " The results of this study do not contraindicate sauna and hot-tub use during pregnancy. They do, however, suggest the wisdom of avoiding lengthy use, even when interrupted by short cooling-off periods, and indicate guidelines for the length of time a pregnant woman can remain in a heated environment before her temperature reaches a level that might be harmful to the embryo or fetus. "
 
@johnpawloski Aha, I think I've had enough of a time hitting my head against the physics modelling of the problem! :)

For example, our subjects had to be reminded to keep their hands and arms immersed
Click to expand...

That behavioural element!

Without knowing the temperature of a tub, 10 minutes seems like a decent conservative limit! Retrospective studies are so tough to try and evaluate for validity, particularly with early pregnancy exposures - at minimum these things happened over 9 months ago. Blerg.
 
@johnpawloski Physics! Science! Math! Embrace your inner dorkiness, you are not alone! 💖🧪 I'm a software engineer focused on avionics data processing plus an uncompleted 3 years of a chemistry undergrad before I switched majors to CompSci & Math (got bored with running test tubes through machines). I married a Mechanical Engineer that focuses on modelling HVAC systems. I'll ask him later tonight for his thoughts on this whole equation.
 
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