At what atmosphere pressure gradient barotraumatic otitis media occurs?

Otitic barotrauma is a nonsuppurative condition resulting from the failure of the eustachian tube to maintain middle ear pressure at ambient atmospheric level. The usual cause is rapid descent during an air flight, underwater diving or compression in a pressure chamber. When atmospheric pressure is higher than that of the middle ear by a critical level of 90mm Hg, the eustachian tube gets "locked" i.e. soft tissues of the pharyngeal end of the tube are forced into its lumen. Reference: Diseases of Ear, Nose and Throat, PL Dhingra, Shruthi Dhingra,7th edition, pg no.71