Monday, February 8, 2016

How Cold Is It?

After a few months of swimming from Alki with the Notorious Alki Swimmers, I've become fascinated by the issues around getting cold and warming up again. I've watched other swimmers spend 15-20 min post swim in a teeth-chattering shiver, which can't be good.  My goal has been to avoid ever getting THAT cold.

I've been reading up on hypothermia and cold water, to try to get a sense of where the limits are, how long I can stay in the water and be ok.  One thing I've noticed is that all the writing is about people ACCIDENTALLY ending up in cold water. Not those who intentionally do so!

My initial reading on hypothermia took me to the Mayo Clinic site and their list of symptoms.  What became clear, as I was reading, was that it can be hard for the person with hypothermia to recognize it, because one of the symptoms is fuzzy headedness or confusion. This is scary, so I can get this thing, and not even know I have it!

All this made me slightly anxious that I was taking a foolish risk with my swimming.  At the same time, I'm one of about 20 people who do it weekly, and many of the others stay in the water a LOT longer than I do.

So where is the line?  How will I know if I'm getting too cold?  Can I know when I'm IN the water, or will I only know with hindsight? Most websites on the topic says that hypothermia sets in when the body temperature is 95* (F) or lower. (There is mild, moderate and severe hypothermia, but I'll talk about those some other time.

My research took me to the LoneSwimmer blog, where the author has asked these questions and attempted to answer them with science.   The chart below is from that blog and provided me some comfort.

Alki is usually around 10* C. So this gives me a safe zone of just under an hour in the water, and a marginal zone of three hours.  That would expand my swim range significantly, from the current 20-25 min.

The author of LoneSwimmer also pointed me to research that shows that there is no long-term negative effect of MILD hypothermia (the author has an in depth discussion of this topic as well). This may give me permission to push a bit further.

But the question remains, am I getting hypothermia on my current swims?

So I took a thermometer with me to Alki last week.  Here's what I learned:

Before the swim (at home) temp 97.3 (yes, this is normal for me, I'm always a bit low)
Before the swim (at the beach) temp 96.8. 
After the swim (at the beach) temp 95.3.
After the swim (at home, about 45 min later) temp 97.3.

What does this all mean?  

First, I lost about a degree and a half in the water for 20 min or so (I swam to the second set of stairs, and there was little to no current). With a low "normal" body temperature, I'm not sure hypothermia, for me, will be at the precise 95 degrees. I'm guessing that it has more to do with lowering the body temp 3.6 degrees (from 98.6 to 95) then a set 95. But this is just a guess, I'd love to find research that supports it. If I could lose up to 3.5 (for rounding sake) degrees and still be ok, then a body temp as low as 93.8 would put me into the hypothermic zone. 

Second, I'm losing a half a degree just between home and the beach.  We often spend a bit of time hanging out waiting for people to gather and working up the nerve to get cold.  My body is cooling in that time. I may want to work even harder to stay warm in that phase.

Lastly, I rewarmed by the time I finished the drive home.  With no adverse effect.

For this to go from anecdote to data, I'll need to add some data points. Will continue doing the temp checks, before and after swimming for a few weeks. Stay tuned for updates. 


  1. Hi Rebecca, very true about loosing heat before you hit the water. I learned that once you hit the water your body turns on all the features to keep your core heat up. For example, reducing circulation to the extremities. So the warmer you can be as you hit the water the warmer you will be throughout the swim. extremities will go numb. I blast the heat in my car and drink coffee or tea until i hit the water. For standing around I would recommend those swim parkas.

  2. The coldest part of a swim is about 10-30 minutes AFTER you get out of the water. That is because your skin perceives that it's being warmed up again, and so vasoconstriction ends and vasodilation begins. While you are swimming, vasoconstriction shunts the blood away from your skin in order to preserve your core temperature. After your swim, your skin perceives that it's in warmer conditions (not cold water) so vasodilation begins. So this blood get cooled by your cold skin and then circulates around your heart and lowers your core temperature even more. The best thing to do is to do jumping jacks and squats while you are attempting to get dressed after a swim, this causes your body to continue to create heat due to exercise, as opposed to a steep drop off in Thermogenesis due to no longer exercising.

  3. One reason we NAS warm up in front of the fireplace at Tully's after the swim is that the delirium of mild hypothermia is not a good condition to be driving in. It's impaired driving. (I have to say I'm speaking from experience.) But it doesn't hurt a body to shiver during that warm-up period, as long as one is taking measures to warm up: getting dressed, going indoors, getting a warm beverage, etc. Shivering burns several hundred calories an hour. I consider it part of the workout. You've noticed by now that NAS members have a number of different warm-up routines. I've adopted Wayne's trick of dumping a gallon of hot water on myself as soon as I get out, but I only find a need for that in the winter. It only warms up the skin, not the core, but that is enough to make getting dressed on the beach a little easier. I've had trouble in the past with snaps and belt buckles when my fingers don't work.

  4. What a great blog, Rebecca. I hope many read it!