Under Pressure – Scuba Diving Risks

Natalie Gibb owns a dive shop in Mexico and is a PADI-certified open water scuba instructor and TDI-certified full cave diving instructor.

Scuba diver near the surface

Olga Melhiser Photography / Getty Images

How does pressure change underwater and how do pressure changes affect aspects of scuba diving such as equalization, buoyancy, bottom time, and the risk of decompression sickness? Review the fundamentals of pressure and scuba diving, and discover a concept no one told us during our open water course: that pressure changes more rapidly the closer a diver is to the surface.

The Basics

Air Has Weight

Yes, air actually has weight. The weight of air exerts pressure on your body—about 14.7 psi (pounds per a square inch). This amount of pressure is called one atmosphere of pressure because it is the amount of pressure the earth’s atmosphere exerts. Most pressure measurements in scuba diving are given in units of atmospheres or ATA.

Pressure Increases With Depth

The weight of the water above a diver exerts pressure on their body. The deeper a diver descends, the more water they have above them, and the more pressure it exerts on their body. The pressure a diver experiences at a certain depth is the sum of all the pressures above them, both from the water and the air.

Every 33 feet of salt water = 1 ATA of pressure

Pressure a diver experiences = water pressure + 1 ATA (from the atmosphere)

Total Pressure at Standard Depths*

Depth / Atmospheric Pressure + Water Pressure = Total Pressure

0 feet / 1 ATA + 0 ATA = 1 ATA

15 feet / 1 ATA + 0.45 ATA = 1 .45 ATA

33 feet / 1 ATA + 1 ATA = 2 ATA

40 feet / 1 ATA + 1.21 ATA = 2.2 ATA

66 feet / 1 ATA + 2 ATA = 3 ATA

99 feet / 1 ATA + 3 ATA = 4 ATA

*this is only for saltwater at sea level

Water Pressure Compresses Air

Air in a diver’s body air spaces and dive gear will compress as pressure increases (and expand as pressure decreases). Air compresses according to Boyle’s Law.

Not a math person? This means that the deeper you go, the more air compresses. To find out how much, make a fraction of 1 over the pressure. If the pressure is 2 ATA, then the volume of the compressed air is ½ of its original size at the surface.

Pressure Affects Many Aspects of Diving

Now that you understand the basics, let’s look at how pressure affects four basic aspects of diving.


As a diver descends, the pressure increase causes the air in their body’s air spaces to compress. The air spaces in their ears, mask, and lungs become like vacuums as the compressing air creates a negative pressure. Delicate membranes, like the ear drum, can get sucked into theses air spaces, causing pain and injury. This is one of the reasons that a diver must equalize their ears for scuba diving.

On ascent, the reverse happens. Decreasing pressure causes the air in a diver’s air spaces to expand. The air spaces in their ears and lungs experience a positive pressure as they become overfull of air, leading to pulmonary barotrauma or a reverse block. In a worst-case scenario, this could burst a diver’s lungs or eardrums.

To avoid a pressure-related injury (such as an ear barotrauma) a diver must equalize the pressure in their body’s air spaces with the pressure around them.

To equalize their air spaces on descent a diver adds air to their body airspaces to counteract the “vacuum” effect by

  • breathing normally, this adds air to their lungs every time they inhale
  • adding air to their mask by breathing out their nose
  • adding air to their ears and sinuses by using one of several ear equalization techniques

To equalize their air spaces on ascent a diver releases air from their body air spaces so that they do not become overfull by

  • breathing normally, this releases extra air from their lungs every time they exhale
  • ascending slowly and allowing the extra air in their ears, sinuses and mask to bubble out on its own


Divers control their buoyancy (whether they sink, float up, or remain “neutrally buoyant” without floating or sinking) by adjusting their lung volume and buoyancy compensator (BCD).

As a diver descends, the increased pressure causes the air in their BCD and wetsuit (there are small bubbles trapped in neoprene) to compress. They become negatively buoyant (sinks). As they sink, the air in their dive gear compresses more and they sink more quickly. If they do not add air to his BCD to compensate for their increasingly negative buoyancy, a diver can quickly find themselves fighting an uncontrolled descent.

In the opposite scenario, as a diver ascends, the air in their BCD and wetsuit expands. The expanding air makes the diver positively buoyant, and they begin to float up. As they float towards the surface, the ambient pressure decreases and the air in their dive gear continues to expand. A diver must continuously vent air from their BCD during ascent or they risk an uncontrolled, rapid ascent (one of the most dangerous things a diver can do).

A diver must add air to their BCD as they descend and release air from their BCD as they ascend. This may seem counterintuitive until a diver understands how pressure changes affect buoyancy.

Bottom Times

Bottom time refers to the amount of time a diver can stay underwater before beginning their ascent. Ambient pressure affects bottom time in two important ways.

Increased Air Consumption Reduces Bottom Times

The air that a diver breathes is compressed by the surrounding pressure. If a diver descends to 33 feet, or 2 ATA of pressure, the air they breathe is compressed to half of its original volume. Each time the diver inhales, it takes twice as much air to fill their lungs than it does at the surface. This diver will use their air up twice as quickly (or in half the time) as they would at the surface. A diver will use up their available air more quickly the deeper they go.

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Increased Nitrogen Absorption Reduces Bottom Times

The greater the ambient pressure, the more rapidly a diver’s body tissues will absorb nitrogen. Without getting into specifics, a diver can only allow their tissues a certain amount of nitrogen absorption before they begin their ascent, or they run an unacceptable risk of decompression illness without mandatory decompression stops. The deeper a diver goes, the less time they have before their tissues absorb the maximum allowable amount of nitrogen.

Because pressure becomes greater with depth, both air consumption rates and nitrogen absorption increase the deeper a diver goes. One of these two factors will limit a diver’s bottom time.

Rapid Pressure Changes Can Cause Decompression Sickness (the Bends)

Increased pressure underwater causes a diver’s body tissues to absorb more nitrogen gas than they would normally contain at the surface. If a diver ascends slowly, this nitrogen gas expands bit by bit and the excess nitrogen is safely eliminated from the diver’s tissues and blood and released from their body when they exhale.

However, the body can only eliminate nitrogen so quickly. The faster a diver ascends, the faster nitrogen expands and must be removed from their tissues. If a diver goes through too great of pressure change too quickly, their body cannot eliminate all of the expanding nitrogen and the excess nitrogen forms bubbles in their tissues and blood.

These nitrogen bubbles can cause decompression sickness (DCS) by blocking blood flow to various parts of the body, causing strokes, paralysis, and other life-threatening problems. Rapid pressure changes are one of the most common causes of DCS.

The Greatest Pressure Changes Are Closest to the Surface.

The closer a diver is to the surface, the more rapidly the pressure changes.

Depth Change / Pressure Change / Pressure Increase

66 to 99 feet / 3 ATA to 4 ATA / x 1.33

33 to 66 feet / 2 ATA to 3 ATA / x 1.5

0 to 33 feet / 1 ATA to 2 ATA / x 2.0

Look at what happens really close to the surface:

10 to 15 feet / 1.30 ATA to 1.45 ATA / x 1.12

5 to 10 feet / 1.15 ATA to 1.30 ATA / x 1.13

0 to 5 feet / 1.00 ATA to 1.15 ATA / x 1.15

A diver must compensate for the changing pressure more frequently the closer they are to the surface. The more shallow their depth:

Divers must take special care during the last portion of the ascent. Never, never, shoot straight to the surface after a safety stop. The last 15 feet are the greatest pressure change and need to be taken more slowly than the rest of the ascent.

Most beginner dives are conducted in the first 40 feet of water for safety purposes and to minimize nitrogen absorption and the risk of DCS. This is as it should be. However, keep in mind that it is more difficult for a diver to control their buoyancy and equalize in shallow water than in deeper water because the pressure changes are more extreme!

Heat Loss and Hypothermia When Diving

Heat Loss and Hypothermia

T hermal challenges are among the most common issues that many divers encounter during or in between the dives. The reason for this is that our core temperatures have to stay within a certain, narrow range for our bodies to function properly. Although, getting a bit too hot from waiting in the sunshine while wearing a wetsuit or starting to shiver ever so slightly when underwater may seem insignificant, ignoring these signals can lead to serious health problems. Hypothermia, hyperthermia and heat stroke are some of the heat challenges that can result from prolonged exposure to extreme temperatures. In this particular article we are going to focus on hypothermia, so read on, if you want to learn how to prevent, recognize and deal with it.

By definition, hypothermia is the condition in which the core temperature drops below that, required for normal metabolism and body functions. When the body loses heat faster than it can produce heat, hypothermia is said to set in.

The first thing you need to do in order to be able to deal with thermal challenges and hypothermia in particular is understand how the heat transfer works.

There are four ways in which the heat energy can be transferred:

  • Conduction – the transfer of heat between two surfaces that are in direct contact. The heat flows from the warmer object to the cooler one, until they are both the same temperature. Solids are better conductors than liquids and liquids are better conductors than gases. As you probably know, water conducts heat about 25 times faster than air, this is why we can get cold even in seemingly warm water.
  • Convection – heat transfer between liquids and gasses during which warmer areas of a liquid or gas rise to cooler areas in that liquid or gas. Cooler liquid or gas then takes the place of the warmer areas which have risen higher. This process is ongoing, which means that as the diver’s body heats the water around it, this warm layer moves away to be replaced with cooler water that the body must also warm.
  • Radiation – the process by which a warm body emanates heat energy into its environment without any contact between the heat source and the heated object.
  • Evaporation – the physical reaction of a liquid turning into a gas with the help of heat energy. For instance, if the moisture is present on the skin, it will evaporate, using heat from the body.

In the context of diving and hypothermia, radiation is basically irrelevant; so is evaporation, unless you’re wearing a still-wet wetsuit between the dives. lt’s conduction and convection that we primarily have to worry about. Upon immersion, a diver begins losing heat via conduction. The rate of loss is proportional to the temperature difference between the points of contact and depends on the type of exposure protection used. Then the diver faces convective heat loss due to the movement of water around him/her. In fact, convective heat loss is much more significant than the conductive one, and can be 1000 times greater than that in air.

Another factor that contributes to greater heat loss when diving is breathing. The air we inhale is around the ambient water temperature, and our lungs warm it, expanding more heat. For this reason, breathing generally accounts for about a quarter of our body’s heat loss in the form of exhaled warm air. What’s more, as the diver goes deeper, the density of the breathing gas increases, which means that the heat loss from exhaling increases as well.

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One final issue worth addressing is the common belief that women are more susceptible to hypothermia than men. The logic behind this is that females supposedly have a larger surface area-to-mass ratio than men. In reality, this isn’t necessarily true. The surface area-to-mass ratio differs very little between the sexes. The mechanism of how male and female bodies respond to heat loss, however, does appear to vary. As a rule, women can preserve more heat in their core, while men normally produce more heat to offset the loss.

Preventing Hypothermia

So what can you do to prevent hypothermia? First and foremost, you should always wear an adequate exposure suit . Even when diving in warm waters, it is recommended that you wear a thin (1-3mm) wetsuit. It will help you to prevent rapid heat loss. Thicker wetsuits are available for colder waters and deeper dives. Such accessories as hoods, gloves and booties are also crucial for good thermal protection when diving in colder waters. If you dive in a drysuit, choose good undergarments to keep you warm.

Don’t forget to bring sufficient clothing for when you get out of the water . Make sure it’s warm and windproof. It is recommended that you change right after the dive, as the damp wetsuit will only contribute to heat loss in the air (remember evaporation). In case you plan to stay in your wetsuit between the dives, cover it with a windproof jacket.

Other factors that can make you move vulnerable to hypothermia include dehydration, work overload, poor physical condition and the influence of alcohol or drugs. So, staying in shape, drinking enough water and avoiding alcohol before diving should help you prevent hypothermia .

Finally, if you feel cold, start shivering or see your dive buddy shivering while underwater, make sure that you end the dive and surface . Remember, while shivering helps the body rewarm on land, it only promotes further heat loss in the water.

Signs of Hypothermia

The severity of hypothermia can be classified into three stages based on the core temperature or presenting symptoms, when it is not possible to determine the core temperature accurately.

Stage 1. Mild Hypothermia – Body Temperature 32-35 C (89.6-95.0 F) – This stage is marked by shivering, decline in motor function, apathy and lethargy. Because of the loss of dexterity and possible limb numbness, the victim of mild hypothermia may be unable to perform such tasks as zipping up or unzipping a wetsuit, tying boots, etc. While the person will still be alert, he/she may neglect to take steps to help themselves and minimize further heat loss, since hypothermia can hinder one’s ability to think clearly.

Stage 2. Moderate Hypothermia – Body Temperature 28-32 C (82.4-89.6 F) – If hypothermia is not addressed in the mild stage, the victim will begin to experience uncontrollable shivering and a decline in gross motor skills. The person will still be alert, but his/her coordination will be impaired. Surface blood vessels will contract further, and the victim can become pale, with his/her lips, ears, fingers, and toes blue.

Stage 3. Severe Hypothermia – Body Temperature 20-28 C (68.0-82.4 F) – During the severe stage of hypothermia the shivering will stop and many basic body functions will slow down due to diminished core temperature. The victim may have difficulty speaking, show signs of amnesia and abnormal behavior. If the person is unconscious, it is important to take great care when assessing the signs of life, as pulse and respiration rate may be slow and faint. Chest compressions are appropriate only when signs of life are truly absent. Rescue breaths can be given freely, if needed.

Dealing With Hypothermia

When treating hypothermic people, there are three main goals:

  • preventing further heat loss;
  • re-warming the victim;
  • getting professional medical help as needed.

In order to prevent further heat loss it is necessary to minimize the victim’s physical exertion and get him/her out of the water. Next, gently remove wet clothing and cover the person with dry clothing or blankets. It is important to protect the victim from wind and avoid re-exposure to cold.

Rewarming can be divided into passive external rewarming, active external rewarming, and active core rewarming. The first two methods can be accomplished in the field, while the third one should only be performed by medical personnel.

Passive external rewarming involves the use of a person’s own ability to generate heat. All you need to do is provide properly insulated dry clothing and move a victim to a warm environment. Unheated blankets can also be used. This method is recommended for those with mild hypothermia.

Active external rewarming involves applying warming devices externally. Heating blankets or hot water bottles can be applied to skin. If the victim is fully alert and can swallow, you can give them warm sweetened liquids. Alcohol and caffeine beverages such as coffee and tea should be avoided. Such techniques as placing the victim in a hot tub and massaging their arms and legs are also not recommended. These measures can cause blood to be directed to the skin, causing a fall in blood pressure to vital organs, potentially resulting in death.

Active core rewarming should only be performed by medical personnel and involves instillation of warm fluids into the veins; placement of warm fluids through a tube into the victim’s stomach; inhalation of warm respiratory gases and dialysis.

Important note: handle the victims of hypothermia gently. Internal organs are sensitive to physical shocks. Violent movement can cause arrhythmia and other potentially fatal issues. The victim should remain inactive to prevent blood from their cold extremities reaching their core too quickly.

Remember, preventing hypothermia is much easier and safer than dealing with it. Wear adequate exposure protection, listen to the way your body is responding to the environment and don’t be afraid of calling the dive if you feel that something is wrong.

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Factors Affecting Visibility When Scuba Diving

Scuba Diving in Cenote, Mexico

Natalie Gibb owns a dive shop in Mexico and is a PADI-certified open water scuba instructor and TDI-certified full cave diving instructor.

Put simply, in diving terms, visibility is an estimation of water clarity and is defined as the distance a diver can see horizontally. Many divers abbreviate visibility with the slang term “viz.” Visibility is given in units of distance, such as “50 feet of viz.”

What Are Factors That Affect Visibility Underwater?

PADI’s review questions from the open water course review several main factors that affect visibility underwater: weather, suspended particles, and water movement. These seem like only one factor to me, as weather causes water to move, which causes particles to float into the water. Here is my list of five common factors that can disturb visibility underwater.

1. Particles in the Water

Suspended particles of sand, mud, clay, or other bottom sediments affect the visibility underwater in much the same way as fog effects visibility on land – distant shapes become colorless, poorly-defined shadows. Visibility reduction caused by suspended particles may be slight or severe depending upon the density, type, and amount of sediment suspended in the water. As an example, clay sediment will become suspended easily, will reduce the visibility to nearly zero feet in a few moments, and will remain in suspension for many hours. In contrast, sand does not become suspended as easily as clay, rarely reduces the visibility to zero, and will fall out of suspension in a matter of minutes.

Sediment particles become suspended when they are disturbed by water movement or by divers. Natural causes of water movement that forces particles into suspension include currents, wave action, choppy seas, runoff, and rough weather. A diver can stir up bottom sediments and reduce visibility by using improper kicking techniques, by swimming with his hands, or by landing on the bottom (one of the many reasons these actions are discouraged).

2. Salinity Gradients (Haloclines)

Water of different salinities forms distinct layers in a manner similar to that of olive oil and vinegar. The interface between the two layers is called a “halocline” (halo = salt, cline = gradient). When viewed from above, an undisturbed halocline resembles a shimmering underwater lake or river (an effect caused by the variation of refractive properties with salinity). However, when water of different salinities is mixed, the visibility becomes very blurry. Divers have compared the visual effect of swimming in a disturbed halocline to having lost one contact lens, to being inebriated and unable to focus, and (my favorite) to swimming in Vaseline. The loss of visibility in a halocline may be extreme; a diver can see light but cannot distinguish shapes. In some cases, a diver in a halocline may even have difficulty reading his gauges!

Haloclines are encountered in estuaries, at springs that empty into the ocean, and at inland caves and caverns. A diver may also observe the blurry effect of mixing fresh and salt water near the surface of the ocean during a rainstorm, as the fresh rainwater mixes with the ocean’s salt water.

To avoid the visual disturbance caused by a halocline, a diver must swim above or below the depth where water of different salinities mixes. Once a diver leaves this mixing region, the visibility clears immediately. If ascending or descending to escape the halocline is impossible, a diver can minimize visual disturbances by swimming to the side of (but never behind) other divers, as their kicks will mix the water and make the visual disturbance worse.

3. Temperature Gradients (Thermoclines)

The term “thermocline” signifies a temperature gradient (thermo = temperature and cline = gradient), or a level at which water of two different temperatures meets. Water of different temperatures layers similarly to water of different salinities, although the effect is not as pronounced. Colder water is denser than warmer water and sinks below it. Therefore, divers will typically encounter increasingly cold layers as they descend. When the temperature difference between two water layers is extreme, the interface between the two layers looks “oily” (similar to a halocline). In general, the visual disturbance created by different water temperatures is not great, and a diver quickly passes through the thermocline region as he ascends or descends, hopefully enjoying the pretty visual effect.

4. Organic Particles

Bacteria or algal blooms can disturb the visibility in a very dramatic way. A typical place to encounter this sort of visual disturbance is a body of fresh water with little or no circulation. Algae and bacteria usually require very specific conditions of temperature, salinity, and light, and may be present only seasonally. An example is Cenote Carwash in Mexico’s Yucatan Peninsula, where an algal bloom is present only during the warmer months. The algal bloom forms an opaque, greenish cloud extending from the surface to about 5 feet. Divers must descend through the cloud in near zero visibility before reaching the crystal-clear spring water of the cenote. The presence of organic particles may also be indicative of pollution.

5. Hydrogen Sulfide

Unless he is diving in a cave or cavern, a diver is unlikely to encounter hydrogen sulfide. Hydrogen sulfide is most commonly found in fresh water with little circulation where decaying organic matter is present. Large quantities of hydrogen sulfide tend to form a dense, foggy layer, as in Cenote Angelita in Mexico. When only a small amount of hydrogen sulfide is present, it forms thin, smoke-like wisps. Inside a cloud of hydrogen sulfide, the visibility is almost zero. Hydrogen sulfide is worth mentioning because the visual effect is fascinating.

The Take-Home Message About Visibility

Water clarity, or visibility, is affected by a variety of factors. Identifying the cause of a visual disturbance will allow a diver to manage it correctly. Keep in mind that visual disturbances may be caused by factors other than water clarity, such as foggy masks, reduction of ambient light, nitrogen narcosis and oxygen toxicity. The cause of any reduction in visibility or visual disturbance should be identified by the diver, and proper judgment should be used when deciding whether to continue with the dive or not.

Source https://www.tripsavvy.com/depth-and-pressure-scuba-diving-2963200

Source https://dipndive.com/blogs/scuba-health-and-safety/heat-loss-and-hypothermia-when-diving

Source https://www.liveabout.com/factors-that-affect-visibility-underwater-2963268

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