Dear Surfing Medicine International community,

In the never ending SMI quest for unique knowledge and experience, we connect state of art sciences and surfing. This effort is a key step to the SMI product development program, based on medical and scientific knowledge.

The global surfers community has so many talented people. In this community we found unique characters, now supporting the SMI. We are calling these special persons “The SMI Scouts”. No, they are no Health Professionals (AHP), but professionals in other topics, like for instance physics.

It is our pleasure to introduce a colleague scientist, physicist and author Chris Woodford, who has worked out an interesting article for our scientific and non-scientific viewers explaining the secrets of wetsuits. In doing so he has touched a medical topic related to surfing, Hypothermia. Hypothermia has also been a topic at the #WCSM, leading to great discussion and knowledge exchange. The international scientific effort on Hypothermia is progressing; we have added some recent and very interesting literature references to round up the topic. Furthermore we are excited to hear more state of the art knowledge on this topic from our vast community in the coming world conferences.


The wonder of wetsuits

World sea temperatures

World sea temperatures

Ocean temperatures vary dramatically across the globe thanks to seasonal variations in solar heating, ocean circulation, and events such as El Niño. This image of world sea temperatures was produced by NASA’s Moderate-resolution Imaging Spectroradiometer (MODIS). Image courtesy of MODIS Ocean Group, NASA GSFC, and the University of Miami

How fantastic to live in Hawaii! Just imagine: waves the size of mountains and 25-30° of wonderful water all year round. If you’re like me, Pipeline is and always will be a “pipe dream”; I live in England and the reality of swimming and surfing here is being prepared to brave water that’s half my body temperature at best. I don’t think I’ve ever seen anyone surfing in England without a wetsuit—and there’s a very good reason for that: the real surfing happens in Autumn and Winter, when the water temperature dips from its modest summer peak of 17/18°C to a miserable 6°C or so before slowly creeping back up again. If you’re the kind of person who swims or surfs in the wintertime, you’ll be used to baffled looks from bystanders who think you’re crazy: if you’re standing on the beach shivering in five thick layers of clothing, it’s hard to comprehend what all those lunatics are doing in skin-tight rubber. Of course, if you happen to be one of those lunatics, and you’re having the best fun of your life charging the winter waves, you’ll know the answer already: your black rubber wetsuit is nothing short of magic. But have you ever stopped to think what a wetsuit is and how it actually works? It’s not magic, it’s science

How ordinary clothes work

I’m sitting in my house in November and, because I’m feeling reasonably eco-friendly,  I have the heating turned down and I’m wearing lots of clothes instead. It’s probably about 5°C outside and 12-13°C here indoors. My body temperature is round about 37°C, so there’s a massive disparity between what’s happening outside my skin and what’s going on inside it.

We all know instinctively that the rate of heat loss from an object depends on the difference between its own temperature and that of its surroundings. Any forensic scientists among you will know all about Newton’s law of cooling; among other things, it tells us that a dead body cools at an exponential rate, based on the difference between its own temperature and the ambient temperature—and it’s how all those world-weary TV pathologists calculate a reasonably accurate time of death at the crime scene. If we’re not actually dead and cooling, the alternative interpretation of Newton’s law is that, the colder it is, the harder our bodies have to work to keep us warm. I might not have the heating on in my freezing-cold home, but deep inside my skin, it’s always toasty: I have a very efficient internal “central heating system” working night and day, with warm blood pumping round my body in much the same way that the hot water pumps through the circuit of radiators in my home. There might be a 25°C difference between my body core and room temperature, but I don’t feel remotely cold.

Now by an interesting coincidence, I happen to know that the sea temperature here is also about 12-13°C at the moment. But if I were to down my laptop, walk to the beach, and stroll out into the sea, I’d feel freezingly, dangerously cold immediately. My teeth would start chattering and I’d be shivering in a minute as my body fought back hypothermia. And yet the temperature difference, 25°C, is exactly the same. So what’s different about sitting in a chair surrounded by air at 12-13°C and standing in water the same temperature.

Year-round ocean temperatures for Hawaii

Year-round ocean temperatures for Hawaii

Year-round ocean temperatures for Hawaii, USA and Newquay, UK compared to human body temperature. The grey area shows temperatures below 15°C, which is about my own cut-off point for wearing a wetsuit. While you can manage all year-round without a westuit in Hawaii, you get only three wetsuit-free months in the UK—precisely the months when there are relatively few rideable waves! It’s worth noting that even in Hawaii, the water temperature is significantly lower than body temperature. I’d always assumed hypothermia was a problem only in seas around cold places like the UK, but a quick look at The Ship Captain’s Medical Guide [] an official first-aid handbook for British sailors, tells a very different story: “Almost all seas in the world are at a temperature which can be classed as a cold environment, as heat loss will occur in water at temperatures below 35.5°C”. Theoretically, that means you could get hypothermia even in Hawaii!Air and water are, of course, very different things: air is a wispy-thin gas and water is a thick dense liquid. An empty litre bottle (in other words, one full of air) weighs next to nothing; fill it full of water and it’ll weigh a kilogram: that’s an astonishing difference. Where does all that weight come from? Molecules of water: in a single liter, there are something like 30 million billion billion of them. When you’re bobbing about in the sea, your body is surrounded by billions upon billions of water molecules—audacious little pickpockets happy to steal your body heat right in front of your eyes! Water conducts heat much more effectively than air and pulls it away from your body at least 25 times faster. If you were in a bath-tub surrounded by cold water, you’d cool fairly quickly, but the bath-water would at least warm up as you cooled down, limiting the cooling of your own body, albeit very slightly. In the ocean, you’re surrounded by (effectively) an infinite amount of cold water that will steal as much heat from your body as it possibly can without warming  in any way. All told, your body cools many times faster when you’re surrounded by water compared to air the same temperature—and the colder the water, obviously the quicker you cool. Why does that matter? It’s worth reminding ourselves that, where body temperature is concerned, there isn’t that much room for manoeuvre: hypothermia kicks in at 35°C and death follows at 30°C.

Of course, I can’t give my body all the credit for keeping me warm as I sit here writing about body heat: I’m also wearing several layers of fairly thick clothes. Thinking back to school science, I know that each layer of clothing traps a thin layer of air between itself and the layers either side—which is why wearing several thin layers is always more effective than wearing one thick layer. We tend to think of heat insulation as meaning “keeping out the cold” or “keeping in the heat”, but it’s often just as much about trapping air in strategic places. That’s because air is a reasonably good heat insulator (not brilliant by any means, but very good, very abundant, and completely free). When I was younger, and living in a draughty house on top of a hill, I used to think our thick curtains kept out the cold. Although there’s some truth in that, the real explanation is that good, heavy curtains work by trapping maybe 10cm or so of air between themselves and your windows—and it’s this air that acts as heat insulation rather than the fabric of the curtains themselves. An air gap of about 10cm has an R value (a measurement of heat insulating ability) of 1.0, which is better than most hardwoods, brick, concrete, or ordinary glass, though worse than good DIY home insulating materials such as polyisocyanurate, polystyrene, or rock wool.

But what’s all this got to do with wetsuits?

How does a wetsuit work?

Winter wetsuit

Winter wetsuit

My winter wetsuit has a body core area lined with 5mm of neoprene. If you look closely at the photo, you can see the nylon inner lining and the abrasion-resistant outer layer. With luck, you can’t see all the sand I should have cleaned out of the zip.

When it comes to wetsuits, both factors—layers and trapped air—play a part. First, we have multiple layers of material spread across a temperature gradient between our bodies (at 37°C) and the sea (at 6-15°C). When I was in the sea the other day, I was wearing a wetsuit, a rash vest, some board shorts, gloves, and boots. Now that sounds like only one or two layers, but, in fact, even the simplest wetsuit is likely to be a laminate (made of multiple bonded layers): there’s some kind of nylon (or a nylon-lycra blend) inside to protect your skin, possibly a thin metallic heat-reflective layer (often based on titanium or copper—some say added as a marketing gimmick), a thick layer of neoprene (typically 5mm in a winter “steamer” wetsuit), and some kind of abrasion-resistant outer coating. As most surfers and swimmers are aware, wetsuits are called wet-suits because you get wet wearing them; even though they’re made of waterproof neoprene and cunningly stitched and taped at the seams, they still let water in and, unlike dry suits, don’t keep you dry.

It’s commonly believed that wetsuits work by trapping a layer of water between your body and the wetsuit itself. According to this theory, your body warms this water, which is locked in place, so you’re surfing inside a kind of snug-fitting hot-water bottle, with semi-warm water completely surrounding your body and reducing its heat loss. Although there’s some truth in this, it’s certainly not the whole story. The reality is that a well-fitting wetsuit with tight wrists and ankles reduces the flow of cold water in and out. It’s not so much that trapped water keeps you warm, but that if that water weren’t trapped, much colder water would be constantly “flushing” in and out and cooling your body more rapidly. That’s the sense in which the water trapped in a wetsuit keeps you warm.

So if a wetsuit’s warmth doesn’t come from the trapped water, where does it come from? Trapped air! Now you might be thinking “How on Earth can air be trapped in a wetsuit when you’re under the water?” And the answer, if you do any swimming or diving, will be perfectly obvious from the amazing extra buoyancy your wetsuit gives you: the air is trapped inside the wetsuit itself—and specifically inside the neoprene from which it’s made. Ordinary wetsuits are made from a type of neoprene called closed-cell, in which bubbles of nitrogen gas (the major constituent of air) are purposely trapped during manufacture. It’s these trapped gas bubbles that give neoprene it’s excellent heat-insulating and buoyancy properties.

Layers in wetsuit

Layers in wetsuit


A typical wetsuit works by creating multiple layers of insulation between your body and the sea. Although most people assume the trapped water is key, the nitrogen trapped inside the neoprene is actually much more important. A wetsuit works despite the water trapped inside it, not because of it.

Closed-cell neoprene is waterproof because water droplets (which are surprisingly big things—containing something like 1,000 billion billion water molecules) can’t squeeze through the gaps the neoprene’s long-chain polymers. (GORE-TEX® is waterproof and breathable for broadly the same reason: big water drops can’t get in, but steam—in the form of smaller water molecules—can get out.) That may be a revelation for anyone who wonders why a wet wetsuit seems to weigh about twice as much as a dry one, but there’s really no contradiction: when your wetsuit gets wet, the water is absorbed not by the main, closed-cell neoprene but by all the other fabrics and layers in the suit. If the whole thing absorbed water, it would be much heavier when you took it off.

Who invented the wetsuit?

“Neoprene” is one of those slightly baffling, generic trade-names that tell us little or nothing about science; in that respect (as I’m sure surfing doctors will appreciate) chemicals are much like pharmaceuticals. Neoprene is actually nothing more, nothing less than a synthetic rubber—boiled and piped through a chemical “plant” instead of squeezed and sliced from the trunk of a real rubber plant. It’s proper name is polychloroprene, where the word “poly” is short-hand for polymer: a plastic whose molecules are long-chains built from endlessly repeating units, much like trains are built from coupled coal trucks or long lines of carriages. (It’s well worth remembering that your rubber wetsuit is a plastic one. Like most plastics, neoprene photodegrades in ultraviolet light—sunlight, to you and me. Dry your wetsuit slowly in the shade, not quickly in direct sunlight, if you want it to last.)

Neoprene, which was originally called DuPrene, was accidentally discovered on April 17, 1930 by an industrial chemist named Arnold Collins, working for the giant industrial chemical company DuPont. He was playing around with a relatively new technique called condensation polymerization (making a polymer by fusing large molecules together so they give off water) and happened to produce this strange new material, polychloroprene, which he found he could bounce off his workbench. Collins deserves only part of the credit, however: his work was being directed by Wallace Carothers, one of the pioneers of condensation polymerization, but better known to most of us as the inventor of nylon. Carothers, the real brain behind neoprene, was the genius whose chemistry gave us nylon stockings and disposable toothbrushes, wipe-clean washing-up bowls, nylon umbrellas, and drip-dry boardshorts. Unfortunately for him, he was also depressive and alcoholic and hadn’t the slightest idea of the plastics revolution his inventions would help to unleash; for various reasons, including his depressive temperament, a failed love affair, and a growing lack of belief in the value of his work, he committed suicide in 1937 at the age of just 41. All of us have a reason to thank Wallace Carothers every time we brush our teeth, but surfers owe this tragic genius a double debt: you can surf or swim in chill winter water because neoprene is a great heat insulator that keeps you warm.

Wallace Carothers gave us nylon and neoprene, but he didn’t invent the wetsuit that uses both of these materials to such great effect. That honor belongs to a Scripps Institution of Oceanography physicist named Hugh Bradner (1915-2008). While working for the US Navy in the 1950s, Bradner realized that the trapped nitrogen bubbles in cellular neoprene make it a perfect material for a thermal diving suit. Plenty of other people had invented diving suits before that (mostly dry suits that pilots and sailors could wear to help them survive accidental exposure to the sea), but it was Bradner’s insight to use neoprene as a heat insulating wetsuit fabric.

Returning to the myth that wetsuits work by trapping and warming water, it turns out that this is almost exactly the opposite of how Bradner conceived them. According to Carolyn Rainey, who has studied his papers in the Scripps archives: “In a letter… dated June 21, 1951, Bradner wrote that suits do not need to be watertight if thermal insulation is obtained by air entrapped in the material of the suit. The diver does not have to be dry to stay warm.” In other words, it’s not that the trapped water is keeping you warm, but that the trapped air keeps you warm even though water gets inside your suit and tries to cool you down.

Astonishingly, Bradner thought there was little market for wetsuits—maybe a few hundred would be sold at most—never patented his invention and, as a government employee at the time, was pretty much against commercial development. How could he possibly have imagined that his modest, military invention would enable a massive growth in cold-water sports like surfing, diving, and swimming? Others developed the idea instead, but although they made millions, they were mere tradesmen; Bradner was the true inventor—the man history will remember.

So, next time you’re bobbing about in the line-up, waiting for a cold-water wave, spare a thought or two for water temperature and body temperature, and why you’re not shivering and shuddering, and thank two heroes of 20th-century science—Wallace Carothers and Hugh Bradner—for keeping hypothermia at bay.

C. Woodford.

November 2012

Further reading and references

l  “Wet Suit Pursuit: Hugh Bradner’s Development of the First Wet Suit” by Carolyn Rainey. Archives of the Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093-0219. November 1998.

l  Obituary Notice: Renowned Physicist and Inventor of Wetsuit: Hugh Bradner. Scripps Institution of Oceanography, University of California. News release: Thursday, May 8, 2008.

l  The Skin They’re In: US Navy Diving Suits: An online exhibition of military diving suits and the creatures that helped to inspire them from the US Navy’s Naval Undersea Museum.’reIn/skin1.htm

Enough for One Lifetime: Wallace Carothers: Inventor of Nylon. by Matthew Hermes. Chemical Heritage Foundation, 1996.


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