Short Answer

The chances are extremely negligible.

Long Answer

There are all the reasons for an anxious flyer to think about a scenario where an airplane door or emergency exit swings open mid air. After all, it’s just a door and that’s what doors do – they swing open.

However, the chances of the same happening on a flight are next to none. And yes, this includes both accidental as well as deliberate opening of the door.

And the reason behind the same is – cabin pressurisation.

Modern aircraft fly at a cruising altitude of up to 45,000 feet. At that altitude, the air pressure outside is about 24 kilo-Pascal as compared to about 100 kilo-Pascal at the earth’s surface. In simpler words, the air outside an aircraft is very thin. If you had to breathe at that pressure, you would be unconscious in less than a minute.

For the same reason, an aircraft cabin is pressurised. Fresh air is pumped into the cabin when in flight, pressurised to the levels in which humans feel comfortable, and ‘old’ air pushed out.

What does all of this have to do with an airplane door? Well, because the cabin has air at a much higher pressure than outside, there is a huge positive force acting on the airplane door against it opening. According to an estimate, the pressure on an airplane door is 8 pounds per square inch, way more than a human can displace.

However, at lower altitudes, the pressure differential between the inside of an aircraft and its outside is lesser. For this reason, it is more likely for the door to be opened at lower altitudes. However, at these altitudes as well, it is highly unlikely for a lone human to be able to open the door.

While cabin pressurisation ensures that the doors cannot be swung open easily, design failures can lead to the same happening in the event of an accident. One can cite a good number of commercial airline accidents where rapid loss of cabin pressure (or depressurisation) took place due to the fuselage or cabin/cargo hold door failure. This rapid depressurisation can be fatal for the passengers as the human lungs are not able to withstand the rapid air pressure change and collapse. Again, the chances of the same happening due to aircraft-related factors like metal fatigue or door failure are very less. Majority of the depressurisation accidents have happened due to human factors like explosions or mid-air collisions.

In closing, mid-air opening of airplane doors are highly unlikely. But such an event has all the likelihood of being a fatal affair.

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Cover credits: Jean-Philippe Boulet, https://commons.wikimedia.org/w/index.php?curid=4442268