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The benefits of diving nitrox have long been know, which is why today it is the breathing gas of choice for most divers. During all nitrox courses one of the biggest topics is elevated PO2 and why not to exceed the single and 24 hour exposure limits. When discussing PO2 limits for divers, it is common knowledge that a 1.6 is the maximum and divers should reduce the PO2 based on environmental conditions, repetitive dives, workload of dive and decompression obligations. But what is not commonly discussed is what to do in the unfortunate situation of a diving incident when a diver has been using nitrox.

Diving incidents are something that no one wants to deal with but, just like anything else, the more you do it the more likely you are to be involved in an incident. When incidents happen, there is a lot to think of and to deal with when it comes to divers using nitrox. There is an added piece of information that is needed: their PO2 exposure. For the sake of ease we will break this down into two types of diving:

1. Multiple days of single tank nitrox diving or the use of semi-closed circuit rebreathers (SCR)
2. Technical diving involving decompression or the use of closed circuit rebreathers (CCR).

Before we get into that, a reminder of the two types of oxygen toxicity is important. The type that affects recreational (sport and technical) divers the most is Central Nervous System (CNS) Oxygen Toxicity. This is generally caused by a high dose (high PO2) with short term exposure. The second type, which is general only seen in commercial diving or medical treatment, is Pulmonary Oxygen Toxicity; this is low dose (low PO2), long term exposure.

For divers doing multiple days of single tank nitrox diving or using an SCR, they generally do not come close to the single exposure limits, mostly because of air consumption rates, but they can be right on the edge of the 24 hour limits. It can be very difficult to ascertain exactly what their long term exposure has been unless they have programmed all mixes into their dive computer, or have logged all their dives, including the mixes they used, in their log books.

For divers conducting decompression or CCR dives, the tracking of dives becomes a little easier because it is either one or two dives per day or, in the case of a CCR, they operate on a fixed PO2. In both situations, the information is usually right at hand, as either the cylinders are labeled, th decompression schedule has been written down as a back-up, a multi gas computer has been used or the CCR’s built in dive computer has stored all the information (if using a eCCR).

No accident is exempt from the confusion that surrounds it but it is very important to collect the information regarding PO2 exposure and send it along with the diver if their treatment is going to involve a recompression chamber, which most diving incidents do. Medical treatments generally start with a PO2 of 2.8 and go up from there, so it is easy to see why hyperbaric doctors would want to know the oxygen levels in a diver prior to starting a treatment. Prior oxygen exposures aside, chamber operators will run whatever the appropriate schedule is, it is just best to provide as much information as possible to them. The best way to handle this is: send the diver’s computer along with them, send their logbook or send their decompression schedules, but – if time permits – get copies of all of this information. When items start changing hands, they have a tendency to get lost and as any diver knows, you need a backup plan. At minimum, the diver’s dive computer should be downloaded and their dive profiles can be quickly hand written or scanned.

The best time management technique to accomplish all necessary tasks is to assign a person for each task and collect all the information as soon as time permits. Fingers crossed you will never have to deal with a diving incident, but if you do find yourself in such a situation, be prepared and everything will go as smooth as possible.

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In addition to Central Nervous System (CNS) toxicity, technical divers must also be aware of pulmonary, sometimes called whole body, oxygen toxicity. Pulmonary toxicity is only of concern for very long exposures to oxygen (at least several hours). Most tech courses teach that if you look after the CNS toxicity, then pulmonary is not going to be a problem.

Pulmonary toxicity is tracked using Oxygen Toxicity Units, known as OTUs for short. One OTU is earned by breathing 100% oxygen at one bar for one minute. The most conservative limit sets a maximum of 300 OTUs per day for multi day diving trips. Thirty minutes on oxygen at 6m would give 48 OTUs, well short of the 300 OTU limit, whereas it would give a CNS percentage of 67% which is approaching the limit of 80% of total CNS exposure recommended for technical divers.

As we can see from the example above, the advice that if you look after the CNS, then the pulmonary toxicity will look after itself seems sound. It is really only with very long decompression dives or long cave dives that there is any risk of pulmonary toxicity. The other time that it can be a problem is during multi day, repetitive rebreather dives. Although open circuit divers will use a partial pressure of 1.4 on the bottom and up to 1.6 during deco, they are not exposed to this maximum partial pressure for very long.

The 1.4 on the bottom only occurs at maximum depth, and even if the diver switches onto a deco gas at 1.6, as soon as they begin to ascend this partial pressure starts to drop. On the other hand, a rebreather diver will run their unit at a lower partial pressure, typically 1.3. However, they will be experiencing this partial pressure for the whole duration of the dive. At every point the rebreather, or the diver in the case of a manual CCR, will be maintaining a partial pressure of 1.3. During the whole of the bottom phase of the dive, during the ascent and during every decompression stop, the diver will be at a partial pressure of oxygen of 1.3 bar. As a result, their total exposure might be considerably higher than an open circuit diver.

Combined with easier gas logistics, lower gas costs, reduced decompression obligation and warmer breathing gas, the rebreather diver is also likely to do more diving than an open circuit diver on a similar expedition. As a result, rebreather divers can often rack up significant pulmonary oxygen toxicity levels. Three hours diving a day on a set point of 1.3 will result in 234 OTUs, which is approaching the recommended limit of 300 per day.

In a number of cases, rebreather divers carrying out multi day, repetitive diving expeditions will complain of chest irritation similar to a chest infection after very long dives or after several days of carrying out long dives. This is with no symptoms of CNS toxicity. Still,iIt is certainly an area rebreather divers in particular need to pay close attention too.