Sorry for a Q. from a non-lens expert. Been to lens factories before to to quality assessment, and i noticed lenses are usually made in extremely clean environments. Now here is my question,

.. ( drum roll).... i fly quite often, and air pressure in cabins reduce...... this means air will exit the lens, and when you decend, the cabin will re-pressurize ( air goes in the lens )....... over alonger period of time, i assume it will suck in dust and 'contaminated" air........ which will affect the inside lenses which cannot be cleaned ( easliy).

Any veiws would be appreciated

Yes. Probably a few rare diseases inside em too!

Yes, but they can be cleaned by a trained professional. Some lenses are sealed better than others, i.e. less prone to dust build up.