Fighting Exposure – Is Helium a “cold” gas?

by: João Neves & Christian Thomas

I was amazed to recently find out that among some diving communities the classic notion that Helium is a “cold” gas to breathe still lingers…” So I thought that this subject should be revisited.

So is Helium “cold”? Well, it depends on how it is used. I hope that by reading this small text (small for such a big and complex subject) the reader will be able to make an educated decision.

Let’s start by presenting some definitions that will help us along the way:

Thermal Regulation

Birds and mammals are homoiothermic, this means they have a physiological ability to maintain a constant body core temperature that is typical for each species. In man, typical core temperature is around 38ºC. To achieve constant core temperature our body uses thermoregulation mechanisms triggered by nerve temperature sensors. Skin sensors will trig fast and dramatic physiological response and for temperatures above 45ºC and bellow 10ºC, it will be perceived as pain. Other sensors are located deep, in the hypothalamus region of the brain and on the spinal cord. It triggers response only when core temperature has dropped significantly.

When core temperature tends to rise (hyperthermia), blood vessels in the skin dilate to increase heat exchange surface with the exterior, acting much like a car radiator. Sweat will occur and water evaporation will further cool skin surface, helping to drain excessive heat. There will be an increase in breathing rate to further eliminate heat through lungs.

When core temperature tends to drop (hypothermia), vasoconstriction occurs to reduce heat loss through the skin. Vasoconstriction will shift fluids to central regions of the body. This will have the adverse effect of increasing diuresis and will lead to dehydration. The body concentrates on heating the central organs and limits heat exchange with the outside. Metabolic activity and breathing rate will increase to generate more heat to compensate for thermal losses. It takes 15 to 20 minutes for thermogenesis to reach its peak.  After this, activity will start declining as energy stocks become depleted and in two hours energy production will be half.

Two types of exposure to cold can be defined:

Sudden violent exposure to cold, will involve neural “cold” sensors in the skin and immediate response of thermoregulator mechanisms like hyperventilation, extreme shivering, and reflex vasoconstriction. In very low temperatures it can be followed by heart syncope and respiratory failure. It is a strong neurological reaction that is triggered even if core temperature remains within normal values.

Slow long cooling of the body, on the other hand, is the type of exposure divers experience in deep diving. Nerve reactions are reduced and go sometimes unnoticed. Periphery vasoconstriction will develop from the beginning, in an attempt to reduce thermal exchanges through the skin and keep core temperature. Other physiological reactions and extra energy production are slow to respond.

When thermogenesis can no longer cope with thermal losses core temperature will start to drop. The diver is unaware of the slow onset of dangerous hypothermia. When it reaches 36ºC, the diver may no longer be able to think and act properly to safely conduct the dive. Also, the radical physiological changes will prevent a “normal” elimination of inert gases from tissues, leading to increased DCS probability.

Heat Loss

As we have seen above, keeping body temperature is vital in deep trimix diving. As an Extended Range Diver, you should know by now that anything that interferes with normal blood circulation may lead to a DCS increased probability. Hypothermia leads to blood vessel constriction that will limit the amount of blood flowing through peripherical tissues and thus the capability of draining inert gas. Controlling exposure should, therefore, be a top priority for the trimix diver as heat loss increases dramatically with depth and a lot longer hang times. In this section you will learn that heat loss occurs in several ways:

Respiratory Heat loss (through lungs)

–    Heating inspired gas

–    Humidifying inspired gas (Latent heat)

Body heat loss (through skin)

–    Radiation

–    Conduction

–    Convection

Body heat loss can be copped with by carefully selecting correct procedures, equipment, and thermal protection. However respiratory heat loss cannot be easily avoided or controlled. This becomes a major problem in long deep dives and should be addressed and taken into account.

Respiratory Heat Loss: Lungs

Gases undergo dramatic depressurization when flowing through SCUBA. At a depth of 80 m at the beginning of your bottom time, your bottom mix goes from 200 bar to around 19 bar, and from 19 to 9 bar with each breath. The sudden decrease in pressure cools the gas to a very low temperature sometimes bellow 0ºC. The final temperature of the inspired gas depends on water temperature and volume/mass of gas that is decompressed. The deeper you go the colder the water and more volume of gas needs to be decompressed, the cooler the inspired gas will be. We cannot “detect” this low inspired temperature, as we do not possess “thermal sensors” in the respiratory tract (trachea and lungs). Body reaction to limit this thermal loss will not be activated until late in a hypothermic state.

The lungs at alveoli level are perfect heat exchangers due to their very large exchange surface. They heat and humidify almost instantly the inspired gases, bringing them to a core temperature of 38ºC, regardless of the inspired temperature. On the other hand, bronchioli and trachea are bad heat exchangers. Also, they represent the respiratory residual volume. The residual gas volume is only partially re-heated. When resting, the tidal volume is about 0.5 liter. In moderate effort, tidal volume may be around 1.5 liters. Residual volume is about 0.15 liter. The tidal volume is therefore not all brought to core temperature. A reasonable assumption could be considered, that only about 80% of the tidal volume is heated up to 38ºC.  At 80 m the volume to be decompressed is 9 times bigger than at the surface. This means that at depth, you would lose heat via lungs 9 times faster then you would on a freezing cold day at the surface.  We will be using these figures later on for some interesting calculations as all of this warm gas is exhaled with each breath and with it, precious calories go to waste.

Deep commercial divers, be it saturation or bounce diving, are well aware of the danger of this thermal loss so they are supplied with pre-heated gas when inside the saturation facility or via the supply umbilical when outside. In case of interruption of gas supply, ruptured umbilical or similar incident, they need a bailout capability. Sudden exposure to dense, very cold inspired gas may result in a critical nervous reaction with choke due to contraction of the glottis. Open circuit bailout is therefore not advised. In depths of 150 m or more, they need moist and warm CCR bailout gas.

Portugal Rui-Guerra 2

Heat Loss by evaporation – Latent Heat

Most of the gas cooling takes place at the regulator first stage high-pressure valve seat. As we breathe, this is repeated; inspiration after inspiration creating a “freezer effect” around the valve seat area that will get cooler and cooler. This area will be drawing heat from the regulator body that in turn receives heat from the surrounding water. Eventually, the temperature of the valve seat area will reach a steady state, but it will stay well below freezing. Any moisture in the gas will turn into ice and build up at this place, sticking the valve assembly in open mode. Once this happens there is no stopping the flow of gas. Intermediate pressure will increase and open the second stage valve into free flow. You better be quick closing that cylinder valve; you only have a few seconds until the pressure gauge pointer slams into the zero limiting pin! In order to prevent this well know phenomena from occurring the breathing gas must be fairly dry. This is why compressors are equipped with filtering systems and condensate drains for water and there are standards for water content in scuba breathing gases. Remember that the deeper you go the cooler the gas gets, and the more likely a regulator is to freeze. So, watch out those filter elements on your compressor when filling trimix!

Moisturized gas simply cannot be used deep with scuba, so we must breathe very dry gas. As we stated above, gases are also humidified at the lungs. This moisturizing is achieved by evaporation of water at the alveoli internal walls surface. Evaporation implies heat transfer, as energy is needed to turn liquid water into water vapor, and thus further temperature drop. This transferred energy is called “latent heat”. Latent Heat is independent of depth and is approximately 10 W for an RMV of 20 liters/minute. Humidifying the inspired gas does not only drain energy from our lungs it also is a major contribution to dehydration – also a major player in the onset of DCS. More about this later.

Tip: breathing a two-meter long hose regulator will dramatically reduce respiratory thermal loss. In fact, it has been shown that inspired gas temperature will be several degrees centigrade higher than in a regular length hose, depending on water temperature. The time gas remains in the long hose before it is inhaled is enough for gas to draw some heat from surrounding water. It would be a good idea for independent doubles divers to consider using 2 m long hoses in each of their bottom regulators!

Total Respiratory Thermal Energy Loss

How much does respiratory loss contribute to the total thermal loss during a deep dive? Due to its well-known high heat conductivity, does helium in the breathing mix aggravates this loss, as is so often seen stated? Let’s do some calculations and quantify it:

The energy used to heat the gas is proportional to gas mass and depends on gas properties. The deeper the diver goes the higher the mass of gas to be heated.

The formula is:

Thermal loss = Cp x 0.8 ( 38 – IT ) x Q x P / 60 + LV


IT – Inspired gas Temperature
Q – Inspired gas Volume (liters/minute)
P – Absolute Pressure
Cp – gas heat Capacity
LV – Latent Heat

A gas with a high Cp will require more energy to reach a given temperature and conversely will give away more energy to cool down, and take longer to do it. It will, therefore, be a better insulator. A low Cp means gas “heats up quickly” and “cool down fast”, it gives energy away easily so it is a poor insulator.

For nitrogen and oxygen, the Cp is very similar and is approximately 1.31 W/1ºC
For helium, Cp = 0.93 W/1ºC

The heat capacity of a mix is calculated considering the fractions of the gases in the mix.

Let’s look at three examples; one with air as a bottom mix, another with heliox, being radical to prove our point and the last with trimix.

Example 1: A diver has an RMV of 20 liters per minute. He is breathing air at 2ºC and at 50-meter depth. What is his lung thermal loss?

Thermal loss = 1.31 x 0.8 (38 – 2) x 20 x (6/60) + 10 = 85.5 W

Example 2: A diver is diving to 80 m with a 10/90 heliox mix at 2ºC. The mix heat capacity is (0.10 x 1.31) + (0.90 x 0.93) = 0.97 W/1ºC. What is thermal loss?

Thermal loss = 0.97 x 0.8 (38 – 2) x 20 x (9/60) +10 = 93.6 W
This same example using air would yield a result of 123.2 W.

Example 3: A diver is diving to 80 m with a 16/40 Trimix at 2ºC. The mix heat capacity is (0.16 x 1.31) + (0.40 x 0.93) + (0.44 x 1.31) = 1.16 W/1ºC. What is thermal loss?

Thermal loss = 1.16 x 0.8 (38 – 2) x 20 x (9/60) +10 = 110 W

What conclusions would you draw?
The first, and obvious, is that helium does not cool you more than nitrogen when breathing, much on the contrary. Surprised? Helium will, however “feel colder” in your mouth as it draws heat faster, and mouth and throat have plenty of “temperature sensors”. But it requires less energy to warm up then denser gases do, like oxygen and nitrogen. On the other hand, it will cool you a lot if used inside your suit, as it passes on the heat it receives from body to the surrounding water faster. Divers in a saturation habitat will also cool fast if the gas inside the chamber is not supplied at the correct temperature due to the high thermal conductivity of He.
The second is not so clear; just how much is 110 W? Is it much?

In order to answer this, we need to look at some more data.

Thermogenesis – Metabolic Energy Production

The typical value for the metabolic thermal energy produced by the body at rest is about 60 W, and may be broken as follows:
Different organs: 30 W
Nervous System: 12 W
Respiratory muscles: 6 W
Different muscles: 12 W

Total: 60 W

Thermal energy produced under moderate workload can go up to 300 W. This would be the case for continuous sustained fin swimming – the same as a runner at 8 km/h. This energy production is due mainly to increased muscle activity. Notice the raised value for respiratory muscles.

Different organs: 30 W
Nervous System: 10 W
Respiratory muscles: 60 W
Different muscles: 200 W

Total: 300 W

This thermal energy output cannot be sustained for long. After two hours of continuous exercise, the energy output will fall to 125 W only. It would be hard however to find an actual example of such extraneous dive. Gas consumption, or RMV, gives a far more useful indication of thermogenesis levels:

8 L/min (at rest) 100 W
20 L/min (bottom) 250 W
10 L/min (deco stops) 125 W

Now, if you recall, our heliox dive above represented a respiratory heat loss of 110 W. If we would consider an RMV of 20L/min, producing 250 W, we would still have a positive balance of 140W!
But are we not forgetting about body heat loss through the skin?

Coetaneous Thermal Loss

Skin will heat up gas inside suit by radiation and conduction. Convection inside the suit will transport heat to where it exits faster through suit. This makes it very difficult to generalize calculations as no two thermal protection configurations are the same, and no two persons are alike as to weight to body surface ratio or insulating body fat. However one can attempt to make a rough approach that can assist in proper planning for exposure protection.

Conduction has the more important role in heat transfer through a suit in diving. Fourier’s Law states that the amount of thermal energy passing from hot body on to a cold one, separated by an insulating layer, is proportional to the difference in temperature between the two bodies. The proportionality coefficient depends on the surface, nature and geometry of the layer, and is called Thermal Conductivity (W/ºC). The lower the value for thermal conductivity, the more insulating is the layer. If we consider our body to be the hot body and the outside water to be the cold one, we will have:

Coetaneous thermal loss (Q) = (T1 – T2) / (1/H1+1/H2+1/H3+…)


Q: Amount of heat
T1: Temperature of hot body (ours)
T2: Temperature of cold body (water)
H1: Thermal Conductivity of layer 1
H2: Thermal Conductivity of layer 2
H3: Thermal Conductivity of layer 3

Some examples of thermal conductivity values for different insulating layers are:

Air boundary layer 8
Periphery body fat (big guy) 15
Periphery body fat (regular guy) 30
Periphery body fat (thin guy) 50
Water boundary layer (almost still) 70
Water boundary layer (moderate current) 300
Polypropylene mountain 1st layer 400
Polar fleece 10mm thick 7
Polar fleece 7,5mm thick 10
Polar fleece 5mm thick 14
Polar fleece 10mm + Argon 4
Polar fleece 10mm + He 30
Neoprene 7 mm 20
Neoprene 5 mm 30
Neoprene 4 mm compressed 40
Neoprene 2 mm crashed 80
Trilaminate 300

Convection is defined as heat transport by fluid movement and can be used in our model as being a boundary layer of very high conductivity. It will be more important in a loose membrane suit than in a tight-fitting neoprene dry suit. Wetsuits can be treated the same as an extra “body fat” layer as they have similar values for heat conductivity, as long as they are tight fitting. Certain fiber underwear will allow for excessive convection and thus have high conductivity properties. The more gas circulation is restricted the better the insulation properties. Drawing sweat moisture away from the body will prevent that moisture from evaporating in contact with skin and thus reducing further cooling. Some fibers can accomplish this, however keep in mind that outer intermediate layers must allow water vapor to reach the inner wall of the suit and condensate there. Some “insulating underwear” have impermeable outer shells, that prevent this from happening and the inner fibers will become saturated with condensed moisture reducing insulation properties.

We can now calculate the thermal loss for the following example.

Example 1: A diver is 1,75m tall and weights 72 kg. He uses a trilaminate dry suit and a 7.5 mm thick overall polar fleece underwear. He uses air as suit inflation gas. The water temperature is 10ºC and there is a moderate current of 1.5 knots. What would be the expected through skin thermal loss?

Q = (38 –10) / ((1/50) + (1/300) + (1/10) + (1/300) = 221 W

Let’s summarize our findings

Respiratory Thermal loss = 110 W
Coetaneous Thermal loss = 221 W
Total Thermal loss = 331 W

Thermogenesis – Total Thermal loss = Thermal Balance

+ 250 W – 331 W = – 81 W

OK, now we have what we need to draw some conclusions:

1st: during a dive to 80 m, under the conditions given, the diver is loosing 331 W
2nd: during the same period he is only generating 250 W
3rd: this means that the balance is a deficit of 81 W

You can work out more precisely the thermal balance by calculating for the different phases of a dive on a specific profile and decompression schedule.

What are the consequences of our findings?

Research data shows that decrease in core temperature in degrees centigrade per hour can be calculated:

Cooling (ºC/h) = (Thermogenesis – Thermal losses) / Body Weight

So a diver with a body weight of 72 kg would experience a decrease in core temperature of 1ºC in only 53 minutes. A typical total dive time in trimix dives will last longer, this is a serious hypothermic condition where a diver can start suffering from poor judgment, uncoordinated movements, apathy, shivering and inefficient breathing pattern. Although this is a plausible scenario and it does occur, it is also true that the organism will react to the heat loss by raising metabolic production of energy – thermogenesis – to maintain core temperature at 38ºC. But we also know that it cannot do it for long, maybe for one more hour. On the other hand, the calculations above are just a rough approach to reality.
Thermal losses are difficult to quantify and are generally higher than the values found. It does not matter whether you dive in cold water in a drysuit or in tropical waters in skins, the heat deficit is still very much a risk in deep diving. During deco stops water may be warmer and RMV lower but you are breathing heavier gas, so the loss is still significant. You will tend to achieve thermal comfort for the temperature of the water you dive in, but respiratory thermal losses will often be overlooked. Also, consider that hypothermia is cumulative and that repetitive multi-day exposures will eventually overwhelm your body’s ability to cope.
As a last remark, think of the consequences of a ruptured dry zipper, neck or wrist seal at the end of your bottom time. If you use a membrane suit in cold waters you are in for trouble. Your insulation will simply be nil in a flooded suit. If you use a thick, tight-fitting neoprene dry suit you will be better off as, in case of flooding, it will function much as a regular wetsuit. Comfort for safety trade-offs can kill you. Proper maintenance of your exposure protection equipment is a must.

So, now you should understand why it does not deserve to be called a “cold gas” when inhaled from a scuba regulator as compared to air. These are good news.
Better news still, is that with CCR you lose even less heat through lungs. But that is another future subject to write about.

Dive warm, dive safe!

Portugal Rui-Guerra 1

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4 replies
  1. Vladimir
    Vladimir says:

    “Latent Heat is independent of depth and is approximately 10 W for an RMV of 20 liters/minute”

    Where did this estimation (10 W) come from?
    It seems well underestimated with ~30 W being a more reasonable estimate at 20 L/min.

  2. Vladimir
    Vladimir says:

    “…bronchioli and trachea… Residual volume… only about 80% of the tidal volume is heated up to 38ºC.”
    Do bronchioles and trachea change their volume?
    Yes, they do create a “dead” residual volume. This is a different issue, however.
    However, the entire RMV is due to the change of the alveoli volume only, i.e. RMV=20 L/min means a production of 20 L/min of humidified air. There is no “0.8” correction required in this case, imho.

    “A low Cp means gas “heats up quickly” and “cool down fast”, it gives energy away easily so it is a poor insulator.”
    It is a wrong statement. “Heat capacity” has about nothing to do with “thermal conductivity”.
    – Ar and He have identical “heat capacity” and very different “thermal conductivity”.
    – Ar has smaller heat capacity than air, but it is a better “insulator” because of smaller thermal conductivity.

    • J
      J says:

      Well, heat capacity may actually appear in a calculation for thermal conductivity, at least in solids. Anyway, I mostly agree with you. You don’t worry about heat capacity when it comes to calculating how fast heat flows through a gas.
      You only need thermal conductivity.


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