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Technical Diving Instructor Mark Powell looks at some of the risks of dehydration


dehydration & diving

It’s difficult to avoid alcohol completely on dive trips

There are many controversial aspects to technical diving. Whether it be equipment configuration, gas selection, decompression planning or any of a hundred other topics it is easy to find widely varying opinions. However, there is one topic on which everyone agrees and that is the subject of hydration. When I learnt to dive in 1987 I was told about the importance of hydration and how being dehydrated was one of the biggest risk factors for DCS. Since becoming an instructor in 1994 I have passed on this view to hundreds of my students. In addition I have seen a number of cases of DCS over the years and, in my non-medical opinion, dehydration has almost always played a part. As a result, you can imagine my surprise when I found, while researching my book Deco for Divers, that there was essentially no scientific proof for this point of view. As recently as 2005 there was very little in the way of any scientific research into this area. It is only in the last few years that a small number of studies have tried to definitively prove whether hydration has an impact on DCS. While the evidence does seem to suggest that the traditional view is in fact true, there is still no strong body of scientific evidence to justify stating it as a fact.

Despite this I still believe that one of the easiest things that technical divers, indeed any divers, can do to reduce their risk and increase their enjoyment while diving is to maintain proper hydration and avoid dehydration. In addition, unless the diver goes to extremes and vastly over hydrates, there is really no down side to being well hydrated. Especially if a pee-valve is fitted to avoid any potential discomfort from a full bladder.

Dehydration is the excessive loss of water from the body, as from illness, fluid deprivation, exercise, sweating, nausea or vomiting. Anyone who exercises on a regular basis is susceptible as well as anyone who is in a hot climate. Water is the body’s most important nutrient and so its’ importance cannot be underestimated. Athletes have long understood the importance of proper hydration to sports performance. Climbers and skiers also understand that proper hydration is essential for safety, in addition to helping them perform at their best. Loosing just a few percent of the water content of our bodies can result in significant symptoms which can produce noticeable results on our dives. The table below summarises the possible symptoms at certain levels of dehydration.

% Body Water Lost


1 %

Few symptoms or signs of any thirst present.


Beginning to feel thirsty; loss of endurance capacity and appetite.


Dry mouth; performance impaired.


Increased effort for exercise, impatience, apathy, vague discomfort, loss of appetite.


Difficulty concentrating, increased pulse and breathing, slowing of pace.


Further impairment of temperature regulation, higher pulse and breathing, flushed skin, sleepiness, tingling, stumbling, headache.


Dizziness, labored breathing, mental confusion, further weakness.


Muscle spasms, loss of balance, swelling of tongue.
11% Heat Exhaustion, delirium, stroke, difficulty swallowing; death can occur.

It’s interesting to note that by the time we are thirsty we have already lost 2% of our body water which can cause loss of endurance capacity. This means that we should drink before we get thirsty.

Roughly half of our body weight is water with 75% of muscle tissue and 25% of fat tissue being made up of water. It is the primary transport and reactive medium of the body and is fundamentally important for a wide range of functions within the body. Dehydration can be caused by a number of factors. Flying, sweating, excess consumption of diuretics such as alcohol, as well as diarrhoea or nausea can all cause dehydration. When these are combined, for example when you arrive off the plane at the start of your holiday in a hot climate and spend the first evening in the bar then it is likely that you will be heavily dehydrated for the first dive of your trip.

Diving itself can cause dehydration. If you are wearing a thick undersuit under a drysuit and kitting up in the sun then you are likely to sweat heavily. The air from your cylinders is very dry and you are continually moistening the air you breathe with moisture from the mouth and throat. Breathing dry air alone will double the rate of loss of moisture from the body. In addition, on dry land the force of gravity means that much of our available blood is pooled in our legs. When we enter the water and the force of gravity is reduced this blood redistributes itself around the rest of the body. Our body’s reaction to this is to try and reduce the volume of blood in circulation by removing some of the water and transferring it to the bladder. This is one of the reasons why you often get that urge to go just after you have entered the water. The cold has the same effect as it causes peripheral vasoconstriction which drives fluid back into the core increasing the blood pressure and further stimulating the removal of water from the circulation and into the bladder.

Suffering from dehydration, a lack of water, while diving and completely immersed in water is an ironic situation. Luckily dehydration is one factor that is under our control and is relatively easy to eliminate. Proper hydration is essential when diving. This doesn’t just mean a cup of tea or coffee the morning before the dive or a swig of water just before getting into your dive gear but should begin at least 24 hours before the dive. It takes some time for the body to absorb the liquid it needs when you are dehydrated and simply drinking a large volume of water the morning of the dive is likely to result in you just passing most of it straight through.

Dehydration & diving

One of the easiest ways to check on your hydration levels is to check the colour of your urine

One of the easiest ways to check on your hydration levels is to check the colour of your urine. Clear or very pale yellow generally means that you are well hydrated; darker yellow means that you are dehydrated and should be drinking more water. Many people are permanently dehydrated and the general recommendation is to drink at least 8-10 glasses of water a day.

It is relatively easy to maintain proper hydration, just drink plenty of fluids. Ordinary water is the best way to rehydrate for the majority of people. Replacing electrolytes is only important for those who are loosing a lot of fluid through sweating, either as a result of very vigorous exercise or extremes of temperature. As the sweat contains a number of mineral these need to be replaced but for those just suffering from mld dehydration it is the fluid replacement that is the most important.  Sports drinks are not necessary for most people and soft drinks and fruit juices, whilst they will help, will not be as effective in rehydrating you as pure water. Tea and coffee are not ideal but is often better than nothing. Some sources of advice advocate avoiding tea and coffee completely as caffeinated products act as a diuretic, which means they increase urine output and so raises the amount of fluid loss. However, this should be balanced with the fact that the drinks themselves can contain a significant amount of fluid. An Expresso and a Latte both contain the same amount of coffee and so the same amount of caffeine but obviously a Latte has a much higher liquid content and would be more beneficial in terms of hydration than an Expresso. Similarly tea has less caffeine than coffee for the same amount of fluid and so is less of a problem. Whilst it is true that water is a better source of hydration than tea or coffee, a big mug of tea or coffee is probably better than nothing. Many people also drink energy drinks to provide an extra boost of energy. These are often high in caffeine whilst containing relatively little fluid content and so should be avoided.

Alcohol should also be avoided as it is a strong diuretic and can significantly increase fluid loss. Although very few people would consume alcohol just before diving it is very common for diving trips and holidays to feature a significant amount of drinking in the evenings. While a couple of beers are not going to do any harm if combined with appropriate rehydration, excessive alcohol consumption the night before a dive can result in significant levels of dehydration which are then much more difficult to make up prior to a dive. The level of alcohol I would consume the night before a dive would feature as part of my own personal risk assessment. For an easy shallow dive I would quite happily have a few beers the night before but if I was planning a 100m plus dive then I would avoid alcohol completely the night before. Diving holidays in tropical locations can be double risky as the temptation to over do it on the holiday alcohol combined with warmer temperatures and increased rates of sweating can lead to a potentially dangerous situation.

Dehydration is widely believed to be one of the more common factors affecting DCS hits. Certainly the majority of DCS incidents I have witnessed have, in my opionion been either caused or exacerbated by dehydration. As mentioned above, there is frustratingly little research that has been done on this area and so we cannot state this as a scientific fact but the majority of divers and researchers agree that this is a significant factor. Before 2005 there was almost no research in this area however research by the US Navy in 2006 seemed to show that dehydration could result in more severe cases of DCS whilst a DDRC study in 2007 showed that even relatively short recreational dives can result in dehydration. In 2008 a study by the French Institute of Naval Medicine showed that proper pre-dive hydration can significantly decrease the risk of venous gas embolism. However researchers are always careful to point out that bubbles in the venous system do not necessarily mean an increased risk of DCS although there is some correlation.

Despite the widespread belief and anecdotal evidence of dehydration causing DCS it is not really known why, or how, dehydration contributes to the risk of DCS. One common theory is that it’s because we need a good level of fluid in the circulatory system in order to off-gas the increased levels of inert gas in the body. If we are dehydrated then gas transport in the blood will be less efficient due to the reduction in plasma.

As bubbles grow and combine to form larger bubbles, they affect the surrounding tissue, making it more permeable to liquid. This results in a loss of fluid from the circulation and an increased level of dehydration. This means that dehydration is thought to be a cause of DCS but is also a result of DCS. This is why hydration is one of the key aspects of first aid treatment for DCS.  It is also the reason why we should continue to drink water after the dive.

Dehydration & Diving

Hydration should be part of your dive planning

In addition to the risk of DCS dehydration can also have other effects on us whilst diving. Dehydration can increase susceptibility to cold and so when diving in cold climates as well as in warm climates it is important to ensure correct hydration. Dehydration can also affect muscle performance and can lead to muscle cramps. It can also affect our levels of concentration. Finally it can also cause loss of endurance capacity and fatigue. Both of these could be dangerous when dealing with an emergency or rescue situation.

Whilst the scientist are still cautious about stating that dehydration definitively increases the risk of DCS or conversely that proper hydration can reduce your risk of DCS the evidence that there is combined with the empirical evidence means that I am still confident in the advice I have been giving which is that it is important to maintain proper hydration and I will certainly continue to make sure I am properly hydrated before every dive.

A special thank you to Mark Powell from Dive Tech for letting us use this article.

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