https://www.siliconvalley.com/2021/12/23/how-to-monitor-oxygen-levels-while-awake-or-sleeping/

If you’ve had a checkup, a medical professional may have clipped a device to your finger to measure your oxygen saturation (SpO2) and pulse. Until last year, that was my only experience with such devices, but during the pandemic, I and many others purchased our own pulse oximeters to check at home for possible signs of COVID 19 or other

respiratory issues. They can be found online for as little as $15, but, as the FDA warns, “although pulse oximetry is useful for estimating blood oxygen levels, pulse oximeters have limitations and a risk of inaccuracy under certain circumstances that should be considered.”  The agency points out several factors that can affect the accuracy of a pulse oximeter reading, “such as poor circulation, skin pigmentation, skin thickness, skin temperature, current tobacco use, and use of fingernail polish.”  A study of six inexpensive pulse oximeters, as reported in the journal  Anesthesia & Analgesia found that “Many low-cost pulse oximeters sold to consumers demonstrate highly inaccurate readings.”