Woot, found this on Google: “ A healthy person should be able to achieve normal blood oxygen saturation levels (SpO2) of 94% to 99% consistently. For patients with mild respiratory diseases, the SpO2 should be 90% or above. Supplementary oxygen should be used if SpO2 levels falls below 90%, which is unacceptable for prolonged periods of time.”

This is also consonant with remarks my daughter-in-law has made; she’s a home health care nurse.