A series of three experiments with 15 normal-hearing listeners measured temporary threshold shifts (TTS) as a function of various parameters of impact noise exposure. A criterion of 15 dB TTS at 4, 6, and 8 kHz, measured 3 min after exposure was adopted. Results indicated that; (1) the growth of TTS is a power function of the peak level of impact noises, (2) recovery from TTS appears to proceed linearly with the logarithm of the time in quiet, (3) the amounts of TTS3 are exponentially related to the logarithm of the decay time (or e duration: te) of the impact sound pressure envelope, (4) with constant total amounts of energy and a constant repetition rate, increasing the number from 60 to 1000 increases the final amount of TTS; the derived trading relationship involves a 12.7-dB level change for a tenfold change in the number of impacts, (5) repetition rates ranging between 0.5 and 1 pps (pulse per second) are the most harmful to hearing and, (6) daily exposure to 100 impacts (te: 200 ms; repetition rate: 0.5 pps) should be limited to a 130-dB peak level, in order to protect 90% of the exposed people. Results are discussed in terms of the possible use of the equivalent continuous noise level as an index in setting safe limits of occupational impact noise exposure.