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# What a Bag of Chips Has in Common with a Pneumothorax

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0109/2026

**Physics you know – and physiology you should.**  Frequent flyers may recognize this little physics experiment, served up without a lab or safety goggles: you take a half-empty plastic bottle, seal it during cruise, and land to find it looking oddly crumpled. Or you watch a bag of chips that looked completely unremarkable on the ground suddenly making quite a scene in the cabin. There it sits, puffed up tight, as if the manufacturer secretly swapped in a family-size pack.

Nobody pumped extra air into it. The explanation is much simpler: the air pressure around the packaging has changed.

And that's exactly the point where a puffed-up bag of chips turns, remarkably quickly, into aviation medicine.

**The Boyle-Mariotte Law – 350 years old and still highly relevant**

The physical principle behind this is centuries old. In the 17th century, the Irish-English natural philosopher Robert Boyle, and somewhat later the French physicist Edme Mariotte, studied how gases behave under different pressure conditions. Today we know their findings as the Boyle-Mariotte law. Simplified: as long as temperature stays constant, the pressure and volume of a gas are inversely proportional.

Or even more simply: **less pressure outside means more space needed inside.**

On the ground, we're under full atmospheric pressure. A commercial aircraft, by contrast, typically cruises at altitudes of around 30,000 to 40,000 feet. Fortunately, the cabin isn't pressurized to match outside conditions – otherwise tomato juice and spotty Wi-Fi would probably be the least of our problems. The cabin is pressurized during cruise to correspond to a much lower altitude, usually around 6,000 to 8,000 feet.

Even so, cabin pressure remains lower than at sea level. And trapped gas responds just as reliably to that as it did 350 years ago in Boyle and Mariotte's experiments: it expands.

**When gas is trapped inside the body**

For a bag of chips, that's largely irrelevant. For a critically ill patient, it may not be.

The human body, too, can contain air or gas in a variety of places. Much of this is entirely normal. As long as a gas pocket can communicate with its surroundings and equalize pressure, it usually doesn't cause a bigger problem. Things get interesting – and sometimes dangerous – where gas is trapped and can't escape easily.

The classic example is **pneumothorax**. When air is present between the lung and chest wall, its volume increases as ambient pressure drops during climb. A small pneumothorax that initially looked clinically unremarkable can turn into a significantly bigger problem during flight. Under unfavorable circumstances, this can lead to worsening respiratory compromise and, eventually, hemodynamic effects.

In short: **a bag of chips is allowed to puff up. A pneumothorax, not so much.**

The same principle shows up in other places, too:

- **After neurosurgical procedures**, air can remain trapped inside the skull. If its volume increases at altitude, it does so inside a space that is famously reluctant to change its overall volume: the bony skull.
- **In certain eye surgeries**, a gas bubble is deliberately introduced into the eye. For these patients, meaningful altitude exposure can be particularly problematic, since the gas bubble expands under reduced ambient pressure and intraocular pressure can rise to dangerous levels.
- **In the gastrointestinal tract**, where gas is extremely common. The occasional abdominal discomfort associated with flying is therefore by no means solely attributable to the food on board. In healthy people, gas expansion here usually remains a fairly harmless side effect. In certain conditions, after surgery, or in cases such as bowel obstruction, the same physics can suddenly become clinically significant.

**The second effect: less oxygen**

Altitude has a second effect that matters just as much for aviation medicine.

The oxygen content of cabin air remains at roughly 21 percent, just as on the ground. Even so, the body has less oxygen available at altitude. The reason, once again, is the lower ambient pressure: as total pressure drops, so does the partial pressure of oxygen.

For a healthy passenger, this is normally not a problem. Oxygen saturation may dip slightly during a flight without causing any meaningful impairment. For critically ill patients, the picture looks different. Someone who already has limited respiratory reserve on the ground can be meaningfully affected by this additional drop in oxygen partial pressure. What still appears well compensated at sea level may no longer be the case at a cabin altitude of several thousand feet.

**What this means for flight planning**

That leaves us with two of the most important physiological challenges of a flight, summed up remarkably simply:

**Trapped gas gets bigger – and oxygen gets scarcer.**

This is exactly why planning an air ambulance flight involves more than just asking whether a patient appears fit for transport in principle. Just as critical is anticipating what the changing environmental conditions during flight will do to that patient's physiology – and addressing potential problems before they arise at 35,000 feet.

So the next time you're on a commercial flight and the bag of chips in front of you slowly puffs up, feel free to watch with a certain scientific calm. You're witnessing the Boyle-Mariotte law in real time.

For the chips, that's usually not a problem.

For our patients, we prefer to take a closer look.

![Infographic](https://fai-api.gigabit.dev/fileadmin/user_upload/Infografik_Englisch_09.2026_GHS.jpg)

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![Dr. Sebastian Gautsch](https://fai-api.gigabit.dev/fileadmin/user_upload/Sebastian_Gautsch.png)

_Author _**Dr. Sebastian Gautsch**Medical Director

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