Oh, those summery days of spring! The sun is warming up for its main gig in a month or two, beads of sweat are forming on your forehead, and you'd like nothing better than to cool off somehow. How about taking a dip?
Be very careful if you do, warns Dr Stefan Linsler, the German Lifesaving Society's (DLRG) federal commissioner for diving medicine and a neurosurgeon at Bayreuth Hospital. Why? Because a heated-up body and cold water - large bodies of water in particular can still be quite cold in spring and early summer - is a dangerous combination.
It puts a strain on the cardiovascular system, explains Linsler. Your undoing can be a natural biological mechanism known as the mammalian diving reflex, which enhances survival during submersion and vestiges of which persist in humans.
"We all have it when we enter water, especially when we submerge our face," he says. "Our heart rate slows to save oxygen so that our body can stay longer under water."
If your body has been heated up by summer-like temperatures on land, however, the reflex can overshoot the mark. At worst, cardiac arrest can result. Or, Linsler says, your blood pressure can momentarily plummet, causing you to briefly pass out.
"This isn't so critical on land - you collapse and then regain consciousness. But if you're in the water, you can drown."
Be it a lake or the ocean, if you enter cold water in spring your body is likely to react by shivering - the rapid, involuntary contractions of skeletal muscles produce heat. Sometimes your hands will also get red and your limbs tingle "like they do when they 'fall asleep' - very unpleasant," Linsler says.
While you might think it's a good sign if these reactions abate after a while in the water, it's not - on the contrary. "Your body then takes a different strategy and falls into exhaustion stage, so to speak. Instead of trying to generate warmth, it merely ticks over in an effort to survive."
Then, at the latest, it's time to get out of the water and warm yourself up. Otherwise you're risking your life, Linsler warns.
He recommends four precautions for anyone who takes a dip in cold water, the first being to cool yourself down before you do. If you're lucky, the bathing site will have an outdoor shower. Alternatively, Linsler suggests splashing yourself with some cold water.
Tip number two is to resist the temptation to dive into the water. A temperature reading of, say, 16 degrees Celsius for a body of water doesn't mean that all of the water is that warm. "Water temperature is usually measured at a depth of about a metre," notes Linsler.
You need to take into account the thermocline, the abrupt temperature gradient between warmer surface water and much colder deep water a metre and more below. It can be huge, especially in lakes with little water movement. Deep water may be in the single digits when surface water is 15 degrees or higher.
"If you haven't cooled yourself down and just dive right in, you'll instantly be hit by very cold water," Linsler says. "This can cause cardiac arrest."
Another precaution is not to stay in the water long. How quickly you become hypothermic depends on factors including your build, amount of fatty tissue and how often you bathe or swim in cold water. To be on the safe side, don't stay in the water longer than five to 10 minutes, advises Linsler - and heed your body's signals, of course.
Lastly, warm yourself up - both externally and internally - after getting out of the water. Any breeze blowing across your wet swimsuit and skin will have a further cooling effect, "so you've got to get out of your wet clothing and put on something warm," Linsler says.
Ideally, you should warm yourself up on the inside too, and a cup of hot tea is a much better choice than alcohol. "It's a misconception that alcohol warms you up. Actually, it tends to cool you down by causing blood vessels to dilate [increasing blood flow to the skin] and more heat to dissipate."
It's also a good idea, Linsler adds, to eat or drink something with sugar in it. Your body has consumed a lot of energy in the cold water and now needs some to warm up.