For those people who think breathing in oxygen for a few hours in a day will help you even though your saturation levels are above 90-92 % .
1. All organisms breathe in air which has 21 percent oxygen and the rest mainly Nitrogen and other gases.
2. This air fills up d Airways

3. Across the wall of the airways is blood which gives away CO2 and takes in O2 & moves to the left side of the heart which pumps this blood everywhere.
4. The small airway is still open because of the residual nitrogen which keeps it patent.

5. When you breathe pure oxygen this replaces the nitrogen in the airways. Oxygen is very soluble in blood unlike nitrogen. So this airway, filled with oxygen, is emptied of the oxygen when the O2 moves into the blood
6. The small airway thus collapses a bit.

7. This goes on with inhalation of more and more oxygen and is called absorption atelectasis in medical parlance.
8. It takes tremendous external pressure to open up a closed airway. Just regular breathing and even the deepest breath cannot accomplish that.

9. So you have effectively lost that part of the lung which is the site of exchange of O2 and CO2.
10. Doctors do not give patients on ventilators high percentage of oxygen unless it is needed.
11. The pulse oximeter measures the oxygen in the blood and gives a number.

12. Between the range of 90 to 100 this number does not reflect a major change in the amount of oxygen in the blood. This is because the oxygen dissociation curve is sigmoid in nature. Between these values the curve is flat.

13. It would be tedious to explain the oxygen dissociation curve. Please Google it.
14. So taking oxygen when you don't need to is harmful to you. Best practice deep breathing to increase the volume and patency of your small airways.
Hope this helps.

@docdeepika @Dwari1008 @PriyankaSAP @udairaina