By virtue of its position within the skull, the auditory system is generally well protected against injuries from external physical forces. There are, however, a number of physical workplace hazards that may affect it. They include:
Barotrauma. Sudden variation in barometric pressure (due to rapid underwater descent or ascent, or sudden aircraft descent) associated with malfunction of the Eustachian tube (failure to equalize pressure) may lead to rupture of the tympanic membrane with pain and haemorrhage into the middle and external ears. In less severe cases stretching of the membrane will cause mild to severe pain. There will be a temporary impairment of hearing (conductive loss), but generally the trauma has a benign course with complete functional recovery.
Vibration. Simultaneous exposure to vibration and noise (continuous or impact) does not increase the risk or severity of sensorineural hearing loss; however, the rate of onset appears to be increased in workers with hand-arm vibration syndrome (HAVS). The cochlear circulation is presumed to be affected by reflex sympathetic spasm, when such workers have bouts of vasospasm (Raynaud’s phenomenon) in their fingers or toes.
Infrasound and ultrasound. The acoustic energy from both of these sources is normally inaudible to humans. The common sources of ultrasound, for example, jet engines, high-speed dental drills, and ultrasonic cleaners and mixers all emit audible sound so the effects of ultrasound on exposed subjects are not easily discernible. It is presumed to be harmless below 120 dB and therefore unlikely to cause NIHL. Likewise, low-frequency noise is relatively safe, but with high intensity (119-144 dB), hearing loss may occur.
“Welder’s ear”. Hot sparks may penetrate the external auditory canal to the level of the tympanic membrane, burning it. This causes acute ear pain and sometimes facial nerve paralysis. With minor burns, the condition requires no treatment, while in more severe cases, surgical repair of the membrane may be necessary. The risk may be avoided by correct positioning of the welder’s helmet or by wearing ear plugs.