[frobozz]

If you suffer from sinus inflammation, either from an temporary infection or a chronic condition, they do not open.

If you have problems in your ear, such as a perforated eardrum or an inflammation; you may still be able to equalise pressure, but it can be painful to do so.

Other genetic or congenital conditions exist that can prevent or hinder pressure equalisation.

[StavrosK]

Ah, I see... Hmm, I imagine air travel would be quite painful in that case. Why doesn't the cabin pressure perfectly equal ground pressure? Doesn't it make sense that an airtight cabin will keep the pressure it's sealed at?

[w1ntermute]

It's not that easy. The pressure difference between the inside and outside puts a great amount of strain on the fuselage. I think what they do is just reduce the rate at which the pressure changes.

[StavrosK]

I see, thank you.

[assemble]

The pressure difference between the interior and exterior of the aircraft causes considerable stress on the aluminum. One of the advantages with composite fuselages is their greater durability. This will allow for pressurization to an equivalent 6000ft altitude, as opposed to the current standard of 8000ft. This will help with numerous things, including but not limited to: air quality, ear popping, humidity, etc.

[StavrosK]

Ah, I had no idea it wasn't pressurized at sea/airport level, thank you.

[frobozz]

Given a perfectly rigid container, you would be correct.

Planes can't even get close to being perfectly rigid, because they'd be too heavy to get off the ground.