How does prolonged exposure to high frequency noise affect hearing?
The ear uses a series of hairs to parse out sound frequency in the cochlea.  When the cochlear hairs die, we lose some frequency response.  

As we get older, even without exposure to loud noises, the higher frequency responses tend to lose their response first.  But loud noises have been linked to hearing loss and high tones are often lost first. 

Hot off the presses: there's been some success in regenerating the cells which respond to sound in our ears. 

I want to add @Armatus has a good point.  Ultrasonic sounds - above hearing range - seem to not adversely affect hearing at all.  Will humans and animals be harmed by frequencies outside their hearing range?