At takeoff, the air in a commercial passenger aircraft is the same as the air at the airport. As the plane gains altitude, it enters thinner air, so it reduces its indoor air pressure to keep from bursting. But to keep passengers from having to wear oxygen masks, the plane maintains pressure equivalent to being 8,000 or 9,000 feet about the ground.
Low Pressure Effect On the Lungs
Oxygen enters the blood through the lungs following simple pressure dynamics. Higher oxygen pressure in the inhaled air passes into blood that has a lower oxygen pressure. When the inhaled air has less oxygen pressure, say the oxygen pressure equivalent to being 8,000 feet above sea level, less oxygen enters the blood. This is okay for most people who are just sitting around in an aircraft. But for people with lung disease, who already have impaired oxygen flow into their blood, it can cause unhealthy low blood oxygen, leading to uncomfortable shortness of breath. It can even threaten the heart and brain, organs that need absolute levels of blood oxygen to survive.
Added to this low pressure effect is the fact that aircraft cabin air is dry. This is because fresh air, always taken in during flight, can be extremely cold and needs to be heated. Humidity is added, but in general, the air is dry, which can make lung membranes dry, adding to discomfort.
Passengers with chronic lung disease such as COPD, emphysema, or chronic bronchitis should check with their clinicians about whether air travel is safe for them, and what they might do. People with recent and active acute lung disease, like acute bronchitis, should postpone their trip if it involves significant high altitude flying.
Passengers Can’t Bring Their Own Oxygen
Passengers who need supplemental oxygen must notify the airline at least 72 hours before gate departure time. Not all flights offer in-flight supplemental oxygen.
Most airlines will allow certain portable oxygen concentrators onboard. These devices take cabin air and separate out the oxygen, providing an oxygen rich stream.
Passengers who need oxygen at ground level, i.e., at the airport, must arrange for the equipment themselves. A brochure is helpful.
Barotrauma – Ears Feel As If Bursting
As the aircraft climbs, cabin pressure goes down. Air in the sinuses and middle ear is at ground level pressure. At lower cabin pressure, it expands (simple laws of physics—gases expand at lower pressure). That can cause pain and inflammation. The adverse effects of ambient pressure change is called barotrauma.
People with sinus infection or severe congestion may wish to postpone their trip, according to the (US) Centers for Disease Control.
Travelers who are especially affected by barotrauma may wish to talk with their doctor about the benefits and risks of taking pseudoephedrine 30 minutes before the flight, or taking a nonsteroidal anti-inflammatory (NSAID) drug.
Air, and, unfortunately, gases, in the stomach and bowel also expand with ascent. They also must leave the body, with predictable effects. Technically, these effects are called eructation and flatulence.
Cabin Air Quality is Good
Except for dryness, cabin air quality remains good in modern commercial aircraft. Inside air is mixed with outside air and filtered 20-30 times an hour. Newer aircraft even use HEPA filters, which remove 99.9% of bacteria, fungi and other particles. Airflow is in horizontal layers, not turbulent or up and down.
In-flight Insect Spraying (Disinsection)
Mosquitoes can bring serious diseases from one country to another. To reduce this threat, some countries require spraying the cabin before landing. Currently, incoming US flights are not sprayed, but if the threat is increased, spraying could be done.
Generally the spray, usually phenothrin, 2%, is felt to be safe, and any discomfort to passengers lasts only seconds.
Blood clots and emboli to the lungs are also a risk of air travel.