"Low oxygen symptoms of hypoxemia vary depending upon its severity. If you or a loved one experience any of the symptoms listed below, contact a health care provider as soon as possible. If you experience more than one of the following symptoms of low oxygen levels, seek medical attention immediately:
Confusion
A sense of euphoria
Restlessness
Headache
Shortness of breath
Rapid breathing
Dizziness, lightheadedness and/or fainting spells
Lack of coordination
Rapid heart rate
Elevated blood pressure
Visual disturbances
A bluish tint to the lips, earlobes, and/or nail beds (cyanosis)
Elevated red blood cell count or polycythemia (if a long-term problem)"
https://www.inogen.com/blog/signs-your-loved-one-may-not-be-getting-enough-oxygen/