Relationship between mental workload and salivary cortisol levels: A field study

BACKGROUND: Mental workload (MW) assessment is a key point in research and development of human-machine interfaces in the workplace. OBJECTIVE: This study aimed to examine the relationship between mental workload and the amount of salivary cortisol amongst technicians of an Iranian combined cycle power plant. METHODS: A total of 18 technicians participated in this pilot study. First, task analysis was carried out to determine operation steps that forced us to separate them in to two groups. Next, the Visual, Auditory, Cognitive, and Psychomotor (VACP) technique and NASA Task Load Index (NASA-TLX) questionnaire were used to examine their mental workload. Then, the operator's salivary samples were collected at four specific times of both off and working days. Finally, the relationship between mental workload and salivary cortisol was examined with the analysis of Pearson correlation. RESULTS: The results of mental workload assessment with subjective methods showed a high level of mental workload amongst all technicians. No significant association was observed between the NASA-TLX questionnaire and VACP technique with concentration of salivary cortisol (p = 0.081). CONCLUSION: Based on the results, it can be concluded that salivary cortisol might not be an appropriate physiological method to assess mental workload, since cortisol levels are highly dependent on individuals' circadian rhythm.