Blood oxygen on a plane

How the estimate is worked out

US certification rules limit cabin pressure altitude to 8,000 ft in routine operation. Aircraft with composite fuselages, the Boeing 787 and Airbus A350, hold about 6,000 ft. For other types the tool uses a figure towards the upper end of what has been measured, and “Not sure” uses the 8,000 ft limit.

The cabin climbs gradually after take-off, and a short flight does not go high enough for it to reach the cruise setting, so the tool lowers the peak for flights under about half an hour. The expected SpO2 is the figure the altitude oxygen calculator gives for a healthy adult at the same height on the ground.

Airlines do not publish cabin altitude for individual flights, and it varies with cruising level. Read the result as a likely upper figure, not a measurement of your flight.

Taking a reading on board

• Cabins are cold and dry. A cold finger gives a weak signal and a low reading, so warm your hand first.
• Sit still with your hand resting on your lap or the armrest. Movement and turbulence both disturb the sensor.
• Wait until the number has been steady for a minute before noting it.
• Take a reading on the ground before boarding, so you have your own figure to compare with.

Finger oximeters are typically accurate to within a few points at best. The oximeter accuracy tool shows the range a single reading could really be, and how accurate pulse oximeters are explains why.

Sleeping on a long flight

Breathing slows in sleep, so readings taken while asleep in a seat commonly sit a few points under waking ones at the same cabin altitude. A spot check before and after a nap does not show this. A ring or wrist oximeter that records continuously does, and the free session viewer opens the file and shows the lowest SpO2 and the time spent under 90%.

If the flight ends somewhere high, the first night there usually reads lower than the flight did. Look up the expected SpO2 at your destination, or read blood oxygen at altitude.

Common questions

Why is my SpO2 lower on a plane?

The cabin is pressurised to the equivalent of about 6,000 to 8,000 ft, not to sea level. At that pressure each breath delivers roughly three quarters of the oxygen it does on the ground, so blood oxygen sits a few points lower for the whole cruise.

What SpO2 do healthy passengers read in flight?

In one study of 84 passengers, average SpO2 was 97% on the ground and 93% at cruise, and just over half read 94% or less in flight. Readings in the low 90s are a common finding at cruise.

Which aircraft have the lowest cabin altitude?

The Boeing 787 and Airbus A350 hold the cabin at about 6,000 ft. Most other airliners cruise with the cabin somewhere between 6,000 and 8,000 ft, depending on how high the aircraft is flying.

Does the cabin altitude change during the flight?

Yes. It rises gradually during the climb, holds steady at cruise and falls again during descent. It can also step up partway through a long flight when the aircraft climbs to a higher cruising level.

Does my SpO2 go back up after landing?

In healthy people it usually returns to the ground-level figure once the cabin is back at ground pressure. If you land somewhere high, the expected reading is the one for that elevation.

What the rule and the studies say

1. Pressurised cabins must provide a cabin pressure altitude of not more than 8,000 ft in ordinary operation.

   US Federal Aviation Administration, 14 CFR 25.841(a), Pressurized cabins

2. The Boeing 787 holds a cabin altitude of 6,000 ft and the Airbus A350 holds 6,000 ft or lower. A Boeing 767 is typically at about 7,000 ft when cruising at 37,000 ft.

   Wikipedia, Cabin pressurization (aircraft figures and their manufacturer references)

3. Across 207 flights on 17 aircraft types the average peak cabin altitude was 6,341 ft. It averaged 7,085 ft on flights over 750 miles and 5,160 ft on shorter ones, and about 10% of flights peaked above 8,000 ft.

   Hampson NB, et al. Altitude exposures during commercial flight: a reappraisal. Aviat Space Environ Med. 2013;84(1):27-31.

4. In 84 passengers aged 1 to 78, average SpO2 was 97% on the ground (range 93 to 100) and 93% at cruise (range 85 to 98). 54% read 94% or less in flight.

   Humphreys S, et al. The effect of high altitude commercial air travel on oxygen saturation. Anaesthesia. 2005;60(5):458-460.

5. In 42 airline crew monitored continuously on 22 scheduled flights, the average lowest SpO2 fell from 97.0% before the flight to 88.6% at cruise. Individual lowest readings ranged from 93% down to 80%.

   Cottrell JJ, et al. Inflight arterial saturation: continuous monitoring by pulse oximetry. Aviat Space Environ Med. 1995;66(2):126-130.

6. In a 20-hour simulated flight with 502 adult volunteers, average SpO2 fell as cabin altitude rose, by up to 4.4 percentage points at 8,000 ft compared with near sea level.

   Muhm JM, et al. Effect of aircraft-cabin altitude on passenger discomfort. N Engl J Med. 2007;357(1):18-27.

These are averages from groups of passengers, crew and volunteers. They describe what was measured, not what any one passenger will read.