My Mother Is 90 Y/o Whose Oxygen Levels Keep Dropping.

My mother is 90 y/o whose oxygen levels keep dropping. about the highest is around 88-90 with oxygen on. it has dropped to as low as 73. She prefers not to use oxygen in order to not become dependent. The levels seems to be fairly good when she is sitting in her chair. What is a person's normal/average oxygen level? What could be causing hers to drop?