Several hundred young men were given careful audiometry before beginning duty in noise of 105–110 dB SPL at one or more of the octaves 300–600, 600–1200, 1200–2400 cps. These men were then given the same audiometric examination intervals up to five years. Less than 15% of ears had permanent threshold shifts (PTS) of more than 20 dB at any frequency. Trend curves extrapolated over log time predict a median PTS of 8 dB at 4 kc/sec for 10 years' exposure. The PTS actually found was thus 20 dB less than predicted by the ASA Z‐24 Committee report; the 4‐kc/sec TTS2for an 8‐h exposure to this noise was over 60 dB; use of the 4‐kc/sec TTS2index would thus vastly overpredict both the actual PTS of 8 dB and the Z‐24 Committee prediction of 22 dB. Ear defenders as actually worn by this population reduce median PTS by no more than 5 dB. It is concluded that a damage‐risk criterion of 100 dB SPL at any of the relevant octaves would be conservative, protecting at least 850/0 of young healthy ears from PTS of over 20 dB at any frequency.