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.