Mindey, 
Life-Saving Oximeter Handbands
Just like smoke alarms, but for human bodies.
Fire alarms work sensing alarming levels of fumes, and oximeters work by measuring how red light and infra-red light is absorbed by the blood, which can be done by really small light-powered diodes and photo sensors, connected via bluetooth to phone.
The advantage of oximeter, is in lower false positives rates. When measuring just pulse (heart-rate), the reasons of absence of pulse could be numerous, such as removal of the handband. However, the absence of measurement of oxygen would not raise the alarm. Only the measurement of oxygen level that is dangerously low, would, and automatically summon help.
(via "Reverse engineering a bluetooth low energy oximeter)
Bluetooth scanner, github/apk. In my case, for YWK-J1 oximeter, github.com/PolyU-IC/oxiMeter code was useful, though, I bumped into this bleak.exc.BleakError issue. There is though interesting Android Flutter project apparently successfully connecting to it, that I didn't try yet.
There are other BLE oximeters, and another approach described in this video alongside this code in Adafruit.
测量血氧水平，Nature上有具体化这个想法的实验 https://www.nature.com/articles/d41586-019-03483-7
[Transiency@0D38482D], [+] :)
“Wearable graphene sensors use ambient light to monitor health” use ambient light———— Exactly same idea, mabe you can cooprerate with them
Most of deaths are because of lack of oxygen, so, an indicator in the case of very large spectrum of issues.
It looks like the Nimb rings are very close from usability standpoint, and Beurer PO60 looks very close from functionality point of view. Would be interesting to see, what could be achieved collectively.