A few years ago I conducted an evaluation of various ambulances to determine if hearing loss could be a risk for paramedics. It's a very real possibility, not from the siren, but simply the ambient noise during travel. A brand new ambulance with zero equipment in it was just below the threshold for long term exposure. However, an equipped ambulance that had over 60,000km on it would be in the low 90 dbs. With the siren on it didn't change much unless the siren was a roof mount like back in the 1980s.
Looking at this chart, exposure to 90db for 2 hours can cause hearing loss. When I travel at highway speeds with my ambulance, we are well over that threshold for 4 - 5 hours. I wanted to conduct actual evaluations over a 5 year period with rural medics, urban medics, and the standard norms but I couldn't find an audiologist to assist me.
Here's a little more info, This may be frivolous, however, there is a definite risk.
http://www.dangerousdecibels.org/education/information-center/noise-induced-hearing-loss/
http://www.nidcd.nih.gov/health/hearing/pages/noise.aspx