Application
Respiration from PPG requires caution
Respiration can appear in PPG through amplitude modulation, intensity variation, frequency variation, and autonomic coupling. But wrist motion and short windows can easily corrupt the estimate.
Practical method
A robust system should compare several respiration candidates and reject outputs when the spectrum is ambiguous, motion is high, or window length is insufficient.
Wellness use
Respiration trend is valuable for strain, recovery, sleep, and heat-stress context. The correct public claim is trend support, not clinical respiratory diagnosis.