low saturation during airline flight

I recently experienced low sats during a flight.  I was using a Portable Oxygen Concentrator (POC) continuous setting of 3 LPH.  Most of the 4 hour flight my sats were in the 91-93% range. When I had to make a battery change my sats dropped to 78-83% and I couldn't get them back to the 90%+ reading.  I remained @ the lower levels for about 30-45 mins before I got them back to 90%+.  
Questions: 1) What is the potential harm?
                 2) How long before some real harm?
                 3) What could have caused the low sat reading after the battery change?
Your responses are appreciated.