Chapter 6 showed that oxygen at high pressure can harm the brain within minutes. Oxygen can also harm the lungs. This happens at lower pressures, but only after many hours of breathing it.
The PO2 (partial pressure of oxygen, also written ppO2) is the share of the total gas pressure that comes from oxygen. It is measured in ATA (atmospheres absolute, about the same as bar); 1 ATA is the air pressure at sea level. Air at the surface has a PO2 of 0.21 ATA. Pure oxygen at the surface has a PO2 of 1 ATA.
Lung damage from oxygen is called pulmonary oxygen toxicity, or whole-body oxygen toxicity. It is also called the Lorrain Smith effect, after James Lorrain Smith, who described it in animals in 1899.
The brain effect and the lung effect behave differently. Brain toxicity is a risk at high PO2, even for short times. Lung toxicity builds up slowly from the total dose over hours and days. So divers track them separately.
What it feels like
It usually starts as irritation in the chest: a tickle or a burning feeling when you breathe in, then a cough. With a larger dose, a deep breath becomes painful and you feel short of breath.
Tests show a fall in vital capacity. This is the largest volume of air you can breathe out after your fullest breath in.
At the doses divers normally get, these changes usually go away over days to weeks once the exposure stops. Any chest symptoms after long oxygen exposure are a reason to stop further exposure and see a doctor.
Counting the dose in OTU
Divers count the lung dose in oxygen tolerance units (OTU). One OTU is 1 minute of breathing pure oxygen at 1 ATA. Below a PO2 of 0.5 ATA no OTU build up.
The unit came from Bardin and Lambertsen at the University of Pennsylvania in 1970. They measured how much vital capacity volunteers lost after breathing oxygen at different pressures. They called it the unit pulmonary toxic dose (UPTD). OTU is the same unit under the name divers use.
The OTU you gain each minute depend on how far the PO2 is above 0.5 ATA. The rate is 1 OTU a minute at 1.0 ATA and rises gently above that. DiveLogic uses this standard formula for the rate:
For a whole dive, DiveLogic works out the dose for each part and adds them up.
The dose rises with PO2, but much more slowly than the CNS clock, the running total for brain toxicity from chapter 6. Think of it like sun exposure: the total time outside matters more than a slightly stronger sun.
- OTU per minute
Planned with DiveLogicOpen the CNS and OTU calculator
OTU per minute
The bars show the OTU you gain in 1 minute at each PO2. At 1.0 ATA the rate is 1 OTU a minute. At 1.6 ATA it is about 1.93, still less than 2.
So time matters more than a small change in PO2. 3 hours at 1.0 ATA gives about the same dose as 90 minutes at 1.6 ATA: 180 × 1 = 180 OTU against 90 × 1.93 = about 174 OTU.
Compared with the CNS clock
Here each rate is shown as a multiple of its value at 1.0 ATA. From 1.0 to 1.6 ATA the CNS clock runs almost 7 times faster, because the NOAA limits fall from 300 to 45 minutes. The OTU rate less than doubles.
A short dive on an oxygen-rich gas mainly adds to the CNS clock. Long exposures at a moderate PO2 mainly add OTU. Examples are hours on a rebreather or several days of nitrox dives.
Recompression treatment
Divers with decompression sickness are treated in a recompression chamber, a steel room pressurised with air. They breathe oxygen at up to 18 m (2.8 ATA), a PO2 of 2.8. That is beyond any diving limit and gives about 3.6 OTU a minute.
So a treatment can give a large lung dose in a few hours. Doctors supervise it and give scheduled breaks on air to lower the dose.
Daily limits
DiveLogic warns when your OTU for a day pass 200. It treats 300 OTU as the limit for one day. These are standard values in technical diving.
OTU do not go down while you rest at the surface between dives. So the planner adds them up across every dive of the day.
There are also limits for several days in a row. The best known is the REPEX method, published by R. W. Hamilton and colleagues for NOAA in the 1980s. It allows a larger dose on a single day and a smaller daily dose as the number of days in a row grows.
- OTU
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One technical dive
The figure shows OTU through a dive to 45 m for 30 minutes. The bottom gas is trimix 21/35: 21 % oxygen, 35 % helium and the rest nitrogen. At 21 m the diver makes a gas switch to EAN50, a nitrox with 50 % oxygen, as deco gas for the ascent.
On the bottom the PO2 is about 1.2 ATA and OTU rise steadily. At the switch the PO2 jumps to about 1.6 ATA, then falls as the diver rises through the decompression stops, the pauses at set depths on the way up. On this dive a little under half of the dose comes after the switch.
A day of 3 dives
The same dive is made 3 times, with 2 hours at the surface between dives. OTU stay level at the surface and carry on from the same value on the next dive.
Each dive adds a little more than the one before. The later dives start with nitrogen left over from the earlier ones, so they need longer stops. The day passes the 200 OTU warning level on the third dive.
OTU and the CNS clock over a day
Here both are shown as a percentage of their limits. The CNS clock falls by half every 90 minutes at the surface, so it starts each dive lower than it ended the one before.
OTU do not fall at the surface, so they reach a larger share of their limit by the end of the day. On a day like this, OTU can limit your diving before the CNS clock does.
Try it
Set a PO2 and a time. The gauge shows the OTU for that one exposure. It marks the values where the DiveLogic planner gives advice and where it warns. Compare 2 hours at 1.3 ATA, a typical rebreather dive, with 30 minutes at 1.6 ATA.
Planned with DiveLogicOpen the CNS and OTU calculator
What to remember
- Pulmonary oxygen toxicity, the Lorrain Smith effect, is lung irritation from a long oxygen dose. It builds up over hours and days.
- The dose is counted in OTU: 1 minute of pure oxygen at 1 ATA is 1 OTU. Nothing builds up below 0.5 ATA.
- The OTU rate rises slowly with PO2, so time is the main factor.
- OTU do not fall between dives. DiveLogic adds them across the day, warns above 200 and treats 300 as the daily limit.
- It matters most for long oxygen decompression, rebreathers, several days of technical diving and recompression treatment.
Sources
- Smith J. L. (1899). The pathological effects due to increase of oxygen tension in the air breathed. Journal of Physiology 24(1): 19-35.
- Bardin H., Lambertsen C. J. (1970). A quantitative method for calculating pulmonary oxygen toxicity: use of the unit pulmonary toxicity dose (UPTD). Institute for Environmental Medicine, University of Pennsylvania, Philadelphia.
- Clark J. M., Lambertsen C. J. (1971). Pulmonary oxygen toxicity: a review. Pharmacological Reviews 23(2): 37-133.
- Hamilton R. W. (1989). Tolerating exposure to high oxygen levels: REPEX and other methods. Marine Technology Society Journal 23(4): 19-25.
- NOAA (2001). NOAA Diving Manual: Diving for Science and Technology, 4th edition. Best Publishing, Flagstaff, AZ. (Oxygen exposure limits.)