Do you suffer on long airline flights as I do? If so, you may be interested to know about the carbon dioxide (CO2) levels in airline cabins. A few years ago I was inspired to buy a CO2 meter and take it along on some flights. The levels I measured were high enough to cause discomfort among most people.

Fresh air outdoors has about 360 parts per million (ppm) of CO2. According to Wikipedia “Concentrations higher than 1,000 ppm will cause discomfort in more than 20% of occupants, and the discomfort will increase with increasing CO2 concentration. The discomfort will be caused by various gases coming from human respiration and perspiration, and not by CO2 alone. At 2,000 ppm the majority of occupants will feel a significant degree of discomfort, and many will develop nausea and headaches. The CO2 concentration between 300 and 2,500 ppm is used as an indicator of indoor air quality.”

Local ordinances typically require office buildings to have sufficient fresh air flow to keep CO2 levels below about 900 to 1100 parts per million. With 360 ppm air outdoors, it doesn’t take much ventilation to accomplish that.

My symptoms include mild headaches, fatigue, and the “close” feeling of stale air. Suspecting CO2, I bought a CO2 meter and carried it on airline flights. I gather the meters are sold mainly for the benefit of greenhouse operators and building inspectors. (Plants grow faster with more CO2, so greenhouse operators artificially boost levels.) Since airline cabins are only pressurized to about 8000 feet, I made sure to get one that worked at altitude. I discovered that CO2 levels were rarely below 1100 ppm, and typically rose to 1500 ppm after an hour or so of flying. On long flights I got readings as high as 2200 to 2500 ppm. Activating the air nozzle above the seat makes no difference. It is recirculating the stale air.

The airlines claim that “despite the use of recirculation fans, in-flight cabin air is of better quality than air in the average office building and is changed completely in a much shorter period of time (3 to 5 minutes).”

That doesn’t square with my measurements. Beyond the instrument measurements, it’s clear to me that my feelings of unpleasantness vary considerably from flight to flight. I took a long flight on Japan Airlines recently and had little discomfort from the atmosphere, but a five hour flight on Northwest was truly oppressive. You could suppose than on the Northwest flight I might have been feeling poorly for other reasons, but a had no problems on a previous segment, and my symptoms cleared within an hour of landing.

I sent my CO2 data to an official committee studying air quality in airlines, and to various others I thought might be interested. I never received an acknowledgment from anyone.

CO2 is not toxic to humans until about 25,000 parts per million. Even at levels approaching 20,000 ppm, people can adapt to the exposure over a period of time. Flight crews that are used to it will not suffer nearly as much as passengers, and probably not at all. Despite not being poisonous, the human species uses CO2 levels to signal bad air. Humans have major problems if they cannot get enough oxygen, however, we do not readily sense lack of oxygen until the shortage is severe.

Years ago, a British physician named Jonathan Miller demonstrated breathing an atmosphere in which the CO2 was removed. On television, he put a plastic bag over his head — not something to try at home. Without CO2 accumulating to trigger distress, he conversed pleasantly up to the point when he passed out. Exotic rebreathing apparatus is available for scuba divers; it just scrubs out exhaled CO2.

An airliner traveling above 30,000 feet must heat and compress the thin -55 C air outside to provide fresh air to passengers. The plane has units called air packs to do that. A large jetliner may have a dozen air packs installed. Running the air packs costs money, so rather than run all dozen they might run about three of them. CO2 levels rise considerably in the cabin.

The air packs on airliners suck power from the engines, which lowers fuel economy. Consequently, there is an economic incentive to not make the air any fresher than required. Passing stricter government regulations is not likely to have much effect since the air packs are easily adjustable and the flight crews are not suffering. My solution is to require a CO2 meter accessible to passengers in the cabin of each large jet airliner. The meter would be marked with the 1000 ppm limit for acceptable air. The if the airlines are correct and all of my symptoms and all of your symptoms are psychosomatic, there will be evidence to show they are correct. Otherwise, they are going to get flak.

Meantime, I wonder how much those scuba rebreathing things cost.