​Demystifying Oxygen Tanks: Why Diving Tanks Aren’t Filled with Pure Oxygen

One of the most common misunderstandings for non-divers is the use of the term “oxygen tank (cylinder)”.” Typically with news reporting.

If a diver were to mistakenly refer to their tank as an “oxygen tank”…

It could lead to an embarrassing situation, so let’s clear up this confusion right away!

It’s important to note that the tank contains a mixture of gases, not just oxygen. Calling it an “oxygen tank” is a significant misconception.

However, what we truly need to sustain life is pure oxygen, not the combination of gases found in regular air. So, wouldn’t it be more logical to fill the tank with pure oxygen for extended dives?

With this in mind, let’s explore why the tank isn’t filled with pure oxygen.

Understanding Oxygen Toxicity: Even Oxygen Can Pose Risks

In straightforward terms, 100 % oxygen can become harmful. Under specific circumstances, inhaling oxygen can lead to a condition known as oxygen toxicity.

Some of you might think, ‘I’ve heard about this! Oxygen transforms into free radicals in the body, potentially affecting the skin or something of the sort…’. However, the reality is a bit different! 

Oxygen toxicity isn’t a minor issue like skin troubles. It initiates with sensations of dizziness, nausea, and headaches. It can also escalate to loss of consciousness and even seizures.

The prospect of losing consciousness underwater is undeniably alarming. However, it’s highly unlikely to encounter oxygen toxicity during standard diving with air in your tank.

I mentioned earlier that oxygen toxicity arises from ‘inhaling oxygen under specific conditions.’ These conditions involve breathing in oxygen at elevated pressures.

The air we breathe while diving has increased pressure due to the surrounding water pressure. While this might sound concerning, the ‘high pressure’ we’re referring to is an oxygen partial pressure of roughly 1.4 atmospheres. (It’s worth noting that effects start to manifest between pressures of 1.4 and 1.6 atmospheres).

With an oxygen partial pressure of 1.4 atmospheres, if we were to breathe in 100% oxygen underwater, oxygen toxicity would occur at a depth of approximately 13 feet (around 4 meters). For us divers, that’s very shallow and not practical.

This illustrates why using 100% oxygen for diving is not considered a safe practice.

Could Breathing Regular Air Lead to Oxygen Toxicity?

Some astute individuals might have considered the opposite scenario. If the oxygen pressure in regular air were to reach 1.5 atmospheres, would it result in oxygen toxicity?

The answer is yes.

For reference, air consists of roughly 79% nitrogen and about 21% oxygen at sea level, 1 atmosphere.

Furthermore, you can determine the pressure of a specific gas by multiplying its proportion in the mixture by the total pressure.

For example, at sea level, the air pressure is 1 atmosphere. In this case, the pressure solely from oxygen, or the oxygen partial pressure, is approximately 0.21 atmospheres (1 atmosphere × 21%).

Hence, when the surrounding pressure increases to 6.6 atmospheres, the oxygen partial pressure rises to 1.39 atmospheres. This equates to a depth of about 185 feet (56 meters) sea water.

It’s a realm it is beyond the limits of recreational diving! Hopefully this information will help you assuage any concerns about oxygen toxicity with recreational diving.

What is Nitrox?

While it might not seem crucial at first, understanding this knowledge is important for divers.  As you advance in diving, knowing about oxygen toxicity becomes essential. This knowledge remains important even though the risk is low when using regular air within normal recreational diving limits.

When you first get your diving certification, you might not be familiar with the idea of oxygen toxicity. However, as you advance to higher levels, understanding the associated risks becomes paramount. One such instance arises when you transition to using Nitrox, also known as Enriched Air.

In short, Nitrox is a specialized tank with a higher oxygen percentage than regular air. While regular air contains 21% oxygen, Nitrox offers concentrations like 32% or 36% up to 40%.

Chronic and Acute Oxygen Toxicity

Let’s take a deeper look at oxygen toxicity. Essentially, there are two main types:

One type affects the central nervous system (the brain). It is referred to as CNS oxygen toxicity. When people discuss oxygen toxicity in standard diving, they’re usually referring to CNS which is life threatening while diving due to convulsions.

To prevent CNS oxygen toxicity, regulations stipulate that the oxygen partial pressure should not exceed 1.4 atmospheres. 

In general, a partial pressure of 1.4 atmospheres, equal to diving to a depth of 185 feet (around 56 meters) with regular air, isn’t a cause for concern. However, with Enriched Air containing 32% oxygen (EAN32), you reach this pressure at a depth of 110 feet (33 meters). With 36% oxygen (EAN36), it’s at 92 feet (28 meters). So, vigilance is crucial.

By the way, the minimum oxygen concentration believed to be safe at a depth of 130 feet (40 meters) is 28%.

The other type of oxygen toxicity is known as chronic  or pulmonary oxygen toxicity and is not life threatening. It affects areas outside the central nervous system, primarily in the lungs. It can develop even with oxygen levels that don’t reach 1.4 atmospheres, if inhaled for extended periods. (Think Saturation Diving).

For different levels of oxygen partial pressure, there are set time limits for a single dive and for all dives within a day. Pretty much like dive tables. 

For example, when you dive down to 130 feet (40 meters) with regular air, the oxygen pressure is 1.05 atmospheres. Even with a slightly higher oxygen pressure of 1.1 atmospheres, it’s considered safe to dive for a maximum of 240 minutes at a time. It also should not extend over a total of 270 minutes within a day. This isn’t a significant cause for concern with recreational diving.

If you were to dive to 130 feet (40 meters) for a total of 270 minutes in one day, you’d be more likely to face decompression sickness than oxygen toxicity.

With the critical oxygen pressure of 1.4 atmospheres to avoid CNS oxygen toxicity, you’re limited to 150 minutes max per dive. It also should not extend over a total of 180 minutes within 24 hours.

You can do a maximum of three dives, each lasting 50 minutes. Still, decompression sickness becomes a concern before Chronic or Pulmonary oxygen toxicity.

It’s important to mention that even with a small increase in oxygen pressure at 0.6 atmospheres, which is like diving to 40 feet (12) meters with regular air, you may start to notice effects. However, with a maximum total dive time of 720 minutes (12 hours!) within 24 hours, there’s no significant cause for concern.

To sum up, there’s usually no reason to be concerned about Chronic or Pulmonary toxicity unless you’re involved in technical diving with decompression stops.

Concluding Thoughts

In recreational diving, concerns about Chronic or Pulmonary oxygen toxicity are generally unwarranted.

However, if you plan to use Nitrox, it’s prudent to exercise some caution. Remember, during your Nitrox certification course, you’ll receive thorough training. So, there’s no need for undue anxiety. Never exceed your PO2 of 1.4 and always know your MOD (Maximum Operating Depth) at a PO2 of 1.4

Although it’s not crucial to dive into oxygen toxicity when you first get certified, make sure you don’t bring the wrong tank at a Nitrox dive session! 

Above all, focus on safety!