To let it go below 95% is not recommended by Mike White, but many medical doctors will say lower is still in a healthy range and 88% should be considered the very lowest end.  Learn more about oxygen here.

Measures Blood Oxygen, Heart Rate and Pulse Strength  Simultaneously.

I use this pulse oximeter device to gauge blood oxygen while using my oxygen concentrator.
Assessing Blood Oxygen (as a percentage of arterial hemoglobin: 95-97% is most common.

Below 95% is not recommended by Mike White, but many medical doctors will say lower is still in a healthy range and 88% should be considered the very lowest end. They mostly will not prescribe oxygen to those with more than 88% on the fingertip pulse oximeter SpO2 monitor. But even 95% may be a false reading as anyone can breathe faster or deeper and make their blood oxygen rise for the moment, Especially if they are anxious about their possible condition. The low blood oxygen can go lower during sleep and the person never knows it until they start waking up gasping and not breathing. By that time I have observed they have lost a great deal of sleep and breathability while waking and during sleep.

Features:

• Large Bright Display of SpO2, Pulse Rate, and Pulse bar.
• Low power consumption, automatic power off
• Approximately 30 Hour normal operation.
• Low battery indicator.
• Supports 2 AAA alkaline or rechargeable batteries.
• Accommodates widest range of finger sizes from pediatric to adult.
• Small & lightweight design ( total 0.09 lbs without batteries ).
• 2 year limited warranty.
• Perfect for Sports and Aviation Use.