What is mental workload?
Every task requires cognitive effort. Driving a forklift, answering calls, monitoring a control room, programming a machine, or reviewing files all require perceiving signals, interpreting them, making decisions, and acting within a specific timeframe. Mental workload is the measure of this effort. When demands consistently exceed responsiveness, overload occurs, leading to fatigue, errors, and decreased performance; when they are significantly lower, underload results in monotony, inattention, and lapses in vigilance. Both situations pose risks to safety and health.
The INSST has addressed mental workload since NTP 179 (1986), which defines it based on the quantity and complexity of the information to be processed, the time available to respond, and the individual’s capacity, placing it within the fields of ergonomics and psychosocial factors. The UNE-EN ISO 10075 series on ergonomic principles related to mental workload provides the international terminological framework, distinguishing between mental pressure (external demands) and mental strain (the effect on the individual).
There is no specific regulation in Spanish law. The obligation derives from the general principles of Law 31/1995: to adapt work to the individual, particularly in the design of workstations, the choice of equipment and work methods, with a view to mitigating monotonous and repetitive work and reducing its effects on health (Article 15.1.d), and to assess all risks, including those arising from the organization of work.
Factors that determine mental workload
- Task requirements. Amount and complexity of information, number of sources to monitor, memory requirements, decisions under uncertainty, required accuracy.
- Time. Imposed pace, deadlines, simultaneous tasks, interruptions, and the possibility of breaks.
- Environmental conditions. Noise, lighting, temperature and air quality , which consume care resources.
- Information and control design. Readability of screens and signals, consistency of interfaces, alarms and warnings.
- Organization. Working hours, shifts and night work, job descriptions, available support.
- Individual factors. Training, experience, fatigue level, age, and personal characteristics modulate the effect but are not used to justify excessive demands.
Teleworking and digitalization have added their own factors: multiple communication channels, constant interruptions, expectation of immediate response and blurring of the boundaries of the workday.
How is it evaluated?
Mental workload is assessed as part of the ergonomic and psychosocial evaluation of the job, combining several approaches:
- Task analysis: description of requirements, pace, interruptions, interfaces and consequences of error, through observation and interviews.
- Validated subjective methods: scales in which the person rates the demands of the task, such as the NASA-TLX described in NTP 544 or the SWAT described in NTP 545.
- Performance indicators: errors, response times, incidents and complaints associated with specific tasks or times of the day.
- Psychosocial factors: general methods, such as FPSICO, include dimensions of workload and cognitive demands that allow the position to be placed within the organization as a whole.
- Joint analysis: comparison of the results with the people who perform the work and with the managers, and identification of the specific conditions that explain the overload or underload.
Physiological indicators (heart rate, brain activity, eye movements) are used in research, but rarely in ordinary preventive practice, and in no case do they replace the analysis of working conditions.
Organizational application: how to reduce mental workload
The measures act on the task, the information, the time, and the organization, in that order of preference:
- Task design. Simplify processes, eliminate unnecessary steps, reduce simultaneity, define clear priorities and provide aids (lists, procedures, decision aids).
- Information design. Consistent screens and interfaces, hierarchical signals and alarms, accessible documentation.
- Working time. Sufficient breaks in sustained attention tasks, limits on the duration of surveillance tasks, rotation with tasks of a different nature, control of interruptions.
- Organization. Adequate staffing, balanced distribution of workload among people and periods, procedures for peak work periods, specific training.
- Environment. Noise reduction, adequate lighting and thermal comfort in positions with high cognitive demands.
INSST’s NTP 659 provides task design criteria to prevent undue mental workload. These measures are the responsibility of management, those who design processes and interfaces, and supervisors, with advice from the occupational health and safety service.
Limits and common mistakes
- Treat mental workload as an individual problem of capacity or endurance, rather than as a feature of the task that needs to be designed.
- Evaluate only the overload and forget the underload and monotony, which are common in surveillance tasks and highly automated jobs.
- Using subjective scales as the sole source, without task analysis or comparison with performance and incident data.
- Confusing mental workload with work-related stress: workload is a requirement of the task; stress is a person’s response to a perceived imbalance between demands and resources.
- Responding to overload with time management training alone, without changing the task or the organization.
- Ignoring the cumulative effect of interruptions and continuous digital communication.
Mental workload assessment is not a clinical instrument nor does it allow for diagnosis; health effects, when they exist, are addressed through health surveillance and procedures provided for work-related injuries.
Practical example
Situation: A pharmaceutical logistics company detects repeated errors in order preparation during the afternoon shift.
- Analysis. Job observation and interviews: each preparer simultaneously attends to a radio frequency terminal, an incident screen and supervisor calls; interruptions are concentrated in the last two hours of the shift; the system displays similar product codes with the same font.
- Evaluation. Application of the NASA-TLX to the order pickers on both shifts, with high scores in mental demand and time pressure in the afternoon shift; the errors coincide with the peaks of interruptions.
- Measures. Channeling incidents to a shift supervisor to avoid interruptions, visual differentiation of codes on the terminal, an additional ten-minute break in the second half of the shift, and redistribution of urgent orders.
- Follow-up. Reduction of errors in three months and re-evaluation with the same instrument; incorporation of mental workload analysis into the procedure for designing new positions.
Regulatory framework and technical references
- Law 31/1995, articles 15.1.dy 16. Adaptation of work to the person, mitigation of monotonous and repetitive work and evaluation of all risks.
- Royal Decree 488/1997, article 3 and annex . Risk assessment of workstations with display screens, including mental workload issues, and interface design principles.
- NTP 179: The mental workload: definition and evaluation (INSST) . Definition, factors and criteria for evaluating mental workload.
- NTP 544: Estimation of mental workload: the NASA-TLX (INSST) method . Multidimensional subjective method for estimating mental workload.
The UNE-EN ISO 10075 series of standards on ergonomic principles relating to mental workload establishes the terminology, design principles and requirements for measurement methods; it is a voluntary technical reference, not a legal standard.
