oxygen levels

my smoking 2 pk a day friend was recently in hospital where blood work and oxygen levels were taken.  He's had total knee replacement and after the blood work was completed the 'nurse' told him that "his oxygen level was at 100%".  After he was told this he is now 'assuming' that it is ok for him to smoke "because the nurse (and I use the term lightly in this instance) told him that he was at 100% on his oxygen level".  to which I replied " I guess so, it was being pumped into your body through the nose hose and you were laying flat on your back recovering from surgery!  geez!!  my question is... and as I am not a nurse but know just enough of the medical field to get myself into trouble... I would like to know in the simplest of form of an answer, how the 'oxygen level' is arrived at... and how to tell my friend that his oxygen level is more in the range of 25% due to his age of 56, shallow breathing and life time smoking habit... LOL  thanks to any who care to answer!

[National Jewish Health]

The oxygen level of 100% is meaningless when a person is receiving supplemental oxygen.  It may, but almost certainly does not reflect his oxygen level when he is out of the hospital and in a steady state.  That is the oxygen level that counts.

Red blood cells carry oxygen.  When all the red cells in your body are carrying the maximum amount of oxygen they are capable of carrying – that is 100%.  Nobody, not taking oxygen, walks around at 100%.  Even if your friend's oxygen level were in the normal range, when not in the hospital, that would not be evidence of minimal or no lung damage.  By the time the oxygen level falls below normal, that person has usually lost 50 to 75% lung power.

It is crazy for him to conclude that this information means there is no damage.  He should quit smoking now.