[Ammers]

This is a bizarre one.  On open circuit I virtually never had a problem clearing my ears and never had any problems after dives.  Now my right ear hurts now and again on the surface...here's why!

Typically, in mixed gas closed circuit rebreathers, the percentage of oxygen in the breathing mix or oxygen fraction is very high at the beginning of the dive, typically 70 percent or higher. The middle ear gets filled with a gas containing a high concentration of oxygen.

During the dive, this presents no problem, you simply equalise whenever a pressure imbalance is felt.  However, upon reaching the surface, it's no longer considered that equalisation is necessary as a stable pressure has been reached.  

But the middle ear may still be filled with an oxygen rich gas.  Slowly, over time, the oxygen in the middle ear may be metabolised by the tissues surrounding the area. The volume of gas slowly decreases, resulting in a net pressure imbalance between the outer and middle ear. This can result in a squeeze, identical in effect to that experienced by descending without equalising.  

This is middle ear oxygen absorption syndrome, or O2 ear.. Military aviators occasionally experience it as well because they use oxygen during high altitude operations and experience the squeeze some time after landing.  

Because onset is so gradual, an individual may not be aware of the pressure imbalance until noticeable damage has occurred.  The is expecially true if one goes to sleep soon after the causative event.  

You are in the risk group for this problem if you use an oxygen rebreather, a semi closed rebreather with a high oxygen content nitrox mix, a mixed gas closed circuit rebreather, or any high oxygen nitrox during open circuit diving.  The risk is generally less for open circuit divers because descent and related pressure equalisation usually occur on air or whatever bottom mix is being utilised.  

Not everyone will experience this problem but manual equualisation after reaching the surface at periodic intervals should prevent this type of squeze, every 15 to 30 minutes for 2 to 4 hours.