The current International Standard (ISO 2631, 1978) on whole‐body vibration defines a complex relation between discomfort and duration of vibration exposure. There are doubts concerning the accuracy of this relation and there are difficulties and dangers in its application to some non‐steady‐state conditions. This paper compares the ISO time‐dependency with those based on root mean square (rms) and root mean quad (rmq) averaging of levels over time. [The rmq averaging procedure is based on experimental studies reported by Griffin and Whitham (1980) J. Acoust. Soc. Am. 68, 1277–1284.] It is shown that although the rmq procedure was developed from studies involving only short duration vibration, it defines a time dependency which may be acceptable for much longer periods.