Vehicle drivers are a group of workers that are exposed to noise and whole-body vibration (WBV) several hours a day. Some drivers may also be exposed to high mental loads – monitoring and manipulating physical controls while engaging problem solving activities often with strong short-term memory and spatial manipulation components. Present standards and regulations that govern health risk assessment do not take into consideration the complexities of these multiple exposure environments. The effect of one factor (for example, noise or WBV) may be different than the effect of two factors presented together. This thesis investigates whether the combination of noise and WBV affects the performance of cognitive tasks more than when the exposures are presented separately.

A series of studies were designed to expose subjects to noise and WBV stimuli designed to simulate real life working conditions. Different combinations of subjective ratings, cognitive tests, and cortisol measurements were conducted both during and immediately after exposures, which ranged from 20 to 45 minutes.

The studies have shown that a combination of noise and WBV do not degrade cognitive performance more than a single stimulus. However, WBV can degrade attention performance after exposure is turned off when drivers have been working under high mental load during exposure. The combined stimuli are also experienced as more annoying and work is more difficult in such conditions. The exposure times and task difficulty levels used in this thesis did not produce biological stress as measured by cortisol. Nevertheless, subjective ratings are sometimes seen as early indicators of other symptoms and with increased task difficulty and/or longer exposure times there may appear other measurable outcomes of the combined stimuli.