The effects of exposure of humans to underwater impulsive sound (such as from blasting or piling) depend on the level of exposure, and may be divided into three categories:
Primary, or life threatening physical injury, including death and severe physical injury.
Secondary, or non-life threatening physical injury, and in particular auditory damage (see Subacoustech’s work on underwater hearing).
Tertiary, injury due to behavioural effects, for instance if a diver is startled and spits out the diving valve or surfaces missing decompression stop.
In the vicinity of the source, high pressures may be superimposed on divers in the water, and primary, secondary and tertiary injuries of the diver will result when he is too close. The effects of the shock wave reduce with distance, and at sufficient range secondary injuries alone will occur. At greater ranges, no physical injuries will be directly caused by the sound, but nevertheless behavioural effects may still cause injury or fatality
As a result, around an impulsive source there are at least three concentric zones of reducing effect, and an outer zone where no effect occurs. It is important to be able to specify the radius of these zones, in order that appropriate types and levels of resources may be provided to ensure the safety of divers and swimmers.
Two measures of the strength of shock waves are usually used to correlate injury with the shock waves’ strength:
- The peak pressure of a shock wave is the maximum level of overpressure (that is, pressure above the local ambient pressure) caused by the shock wave. Conventionally, this is usually considered to be the initial peak of the waveform. This quantity is related to the transient deformation of tissues in the diver and hence there is physical reason to believe that it may be related to effects such as stunning.
- The impulse is defined as the integral over all time of the pressure, i.e., it might be considered to be the average level of the shock wave pressure multiplied by its duration. It has been demonstrated to correlate well with severe physical effects such as pulmonary injury.
In shallow water a diver receives sound not only from the direct propagation of the shock wave from the charge, but also from the reflected waves which may bounce from the surface and bottom. Nedwell  points out that since the surface reflected wave is inverted a significant reduction of the level of impulse at the diver may occur due to the water surface reflection introducing an opposing contribution to the integral from that of the main pressure peak. In addition, however, this wave is distorted by bulk cavitation such that it is both reduced in level and extended.