A pulse oximeter shines two wavelengths of light through a fingertip and infers oxygen saturation from how much of each is absorbed. It is measuring light, not oxygen, and everything that changes the light path changes the result.
Under good conditions a consumer device is typically accurate to within about two to three percentage points of an arterial blood measurement.
What the numbers mean
95–100% is the usual normal range for a healthy adult at sea level.
91–94% is below typical and worth discussing with a clinician, particularly if it is a change from your own baseline.
90% or below generally warrants prompt medical attention.
People with chronic lung disease often have a lower personal baseline, and their clinician may have given them a different set of thresholds. Where that is the case, those thresholds apply rather than the general ones.
What degrades a reading
Cold hands and poor circulation. The most common cause of a falsely low or unstable reading, and the easiest to fix — warm the hand and try again.
Motion. Shivering, tremor, or simply not holding still. Most devices display a pulse waveform or a signal bar; if it is erratic, so is the number.
Nail polish and artificial nails, particularly dark colours. Try a different finger or remove the polish.
Poor perfusion from low blood pressure or vasoconstriction.
Skin pigmentation. The FDA has noted that pulse oximeters can be less accurate in people with darker skin, with a tendency to overestimate oxygen saturation — meaning a reading may look acceptable when the true value is lower. This is a recognised limitation of the technology rather than a device fault, and it argues for weighing symptoms more heavily and for treating a borderline reading with more caution.
Carbon monoxide poisoning. A standard oximeter cannot distinguish carboxyhaemoglobin from oxyhaemoglobin and will read falsely high — sometimes near 100% in someone who is seriously poisoned. This is the most dangerous limitation of the device, and it is why a normal oximeter reading means nothing in a suspected CO exposure.
Severe anaemia, where saturation can be normal while the total oxygen carried is not.
How to take a reading that means something
Warm hands. Sit still and rest for a few minutes first. Remove nail polish from the finger used. Rest the hand at heart level. Let the device settle for 30 to 60 seconds rather than reading the first number that appears. Check the waveform or signal indicator.
Then repeat on a different finger. Two readings that agree are worth more than one.
Trends beat single readings
The single most useful thing a home oximeter does is establish your own baseline when you are well, so that a change is recognisable later.
A reading of 94% means one thing in someone who normally reads 98% and something quite different in someone who normally reads 94%. Without the baseline, a single number is hard to act on in either direction.
Record the number, the time, and what you were doing. A drop on exertion that recovers at rest is a different observation from a low resting number.
What an oximeter does not tell you
It does not measure breathing effort, carbon dioxide, or how hard someone is working to maintain that saturation. A person breathing rapidly and using accessory muscles to hold 95% is not reassured by the 95%.
Nor does it detect carbon monoxide, as above.
Symptoms outrank the device. Significant breathlessness, chest pain, confusion or blue lips warrant medical attention regardless of what the display says, and a normal reading is not a reason to delay.
This page describes commonly cited reference figures and known limitations of the technology. It does not replace assessment by a clinician.