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.