A research team in Beirut has determined that professional singers may have a risk of slight hearing impairments when compared with non-singers. A.L. Hamdan, et al, used transient-evokd otoacoustic emissions (TEOAEs) to find this result. They played clicks or pure tones for the singers and nonsingers, and measured the amplified and filtered responses to see if they were normal or not. If an evoked emission is not in the normal range, that suggests a damaged cochlea. While the singers' responses were in the normal range, they were "less robust" than those of the non-singers. This is a preliminary study, concerned more with establishing methedology than in finding universal results. So you vocalists should not be panicking yet. I SAID, YOU SHOULDN'T BE PANICKING YET!
AL Hamdan, KS Abouchacra, AG Al Hazzouri, and G Zaytoun. (2008). "Transient-Evoked Otoacoustic emissions in a group of professional singers who have normal pure-tone hearing thresholds," Ear and Hearing (preprint, March 29).