Would the airzound bike horn cause hearing damage even if used well under the duration limit? Would the airzound bike horn cause hearing damage even if used well under the duration limit?

The Airzound is 115 dB which means according to the NIOSH standard, the duration limit is 28 seconds using a 3 dB exchange rate!
https://mecart.com/blog/2016/06/03/stop-noise-from-harming-your-employees-health-and-productivity/

A safety conscious college instructor taught his class that when loud sounds caused ringing, you've already permanently damaged your hearing. 

Customers on Amazon mentined that the Airzound caused ringing in their ears. I experienced ringing in my ears when exposed for less than a second! That means according to the instructor, I've lost some hair cells. Is that really true?

An example here is a cyclist using it for 10 seconds in total in a day in the worst case during a round trip commute. It's possible to have a few right hooks, close passes, etc in one day. If 10 seconds in a day is enough to destroy hair cells and it's repeated once a week, it may become noticeable years later. Light duty hearing protection may be enough to prevent hearing loss. It needs to be light enough for hearing the traffic.
A little differently: the 115 dB exposure is culmulative with other noise that day. So the tables tell us that if you work in an environment of 85 dB(A) for eight hours and you use the horn on your way to work, then you will damage your hearing. 

Ringing in your ears is a great warning sign from the body that you are risking damage to your hearing. 

Ear protection has its uses; you want it to bring your noise exposure down to 85dB(A) or below, which is quite difficult especially from 115 dB. It would be much better not to use the horn at all, instead of using ear plugs, because that’s a better risk management strategy. 

After all, noise-induced hearing loss is preventable.