What is meant by thermal comfort?
Thermal comfort refers to a person’s satisfaction with the thermal environment. It is influenced by physical conditions and by the characteristics of the activity and clothing. Different perceptions can coexist in the same space, so a single set temperature does not fully describe the well-being of all occupants.
This analysis is part of the ergonomic design of the workstation. It allows for the optimization of facilities and conditions of use to reduce discomfort. It should be distinguished from the control of extreme exposures: an uncomfortable environment is not necessarily assessed with the same tools as a situation capable of producing heat overload or cold injury.
What do PMV and PPD mean?
PMV, Predicted Mean Vote, estimates the average perceived temperature vote of a group using a heat balance model. PPD, Predicted Percentage of Dissatisfied, estimates the percentage of people who would be dissatisfied under those conditions. These are population predictions and not measurements of how each individual occupant necessarily feels.
The relationship between the two helps interpret deviations from thermal equilibrium. An average value close to neutral does not imply universal satisfaction. Nor does it allow us to dismiss an individual complaint. The analysis should be used to understand conditions and guide improvements, not as an argument to claim that a person cannot feel cold or heat because the calculation is favorable.
Necessary variables
The model considers air temperature, radiant temperature, humidity, air velocity, metabolic activity, and clothing insulation. These variables must accurately represent the situation being studied. A thermostat reading may differ from the conditions near a window, an air vent, or a surface emitting thermal radiation.
Activities and attire can also vary within the same venue. It is advisable to describe relevant groups and tasks rather than assigning a single value for convenience. Data should be collected using appropriate methods and record units, location, and time so that calculations are reproducible and can be correlated with usage observations.
Local thermal discomfort
Overall indices do not fully capture localized discomfort. Drafts, vertical temperature differences, hot or cold floors, and radiant asymmetries can all cause discomfort even if the overall estimate seems acceptable. The specific areas of the workstation where the person spends time and how exposure changes during the task should be examined.
This is particularly relevant in workstations with screens, reception areas, and spaces near entrances. A draft in the neck or legs can be related to air distribution and cannot be resolved simply by changing the building’s temperature. The solution must address the root cause and be tested in the specific affected areas.
Scope of the standard and the tools
ISO 7730:2025 is the international edition published in 2025 for this field and replaces ISO 7730:2005. Previous technical notes can help in understanding concepts, but their references and criteria should be reviewed against the edition used. The report should identify the standard, tool, and assumptions, without automatically presenting an older guideline as the complete and current standard.
This approach applies to moderate indoor environments within its conditions of validity. When heat stress is a concern, methods such as WBGT and other specific analyses may be necessary. Meeting a comfort target does not replace verifying legal obligations or risks beyond the scope of PMV and PPD.
Practical example
In an office, complaints about the cold are received even though the thermostat reads a temperature that the organization considers adequate. Observation identifies workstations near a diffuser and others near a sunny facade. Using a single measurement point conflates different situations and makes it difficult to explain why people disagree.
The assessment characterizes occupancy zones and conditions, including air velocity and radiation. The measures adjusted for distribution and control are compared with measurements and subsequent consultation. The example shows that varying the overall setpoint may not solve the problem and could even worsen other positions if local differences are not identified.
Design, participation and monitoring
Solutions may include zoning, radiation protection, maintenance, and relocation of workstations or diffusers. They must be compatible with air quality and other building needs. Reducing ventilation to avoid discomfort without assessing the consequences can simply shift the problem to another aspect of the indoor environment.
Participation allows us to understand schedules, seasonal changes, and situations that a brief visit might miss. It’s advisable to verify results under representative conditions and review modifications to occupancy, equipment, or activity. Ergonomics combines technical data with user experience, preventing the evaluation from being limited to a single figure disconnected from daily work.
Common mistakes
Common mistakes include relying solely on the thermostat, ignoring clothing and activity levels, evaluating a single point for the entire building, or confusing comfort with safety in extreme heat. It is also a mistake to dismiss discomfort simply because the average is favorable, or to compare results calculated using different assumptions without explaining those differences.
A useful assessment acknowledges the variability and limitations of the model. It must specify which conditions it describes, which areas require intervention, and how the improvement is verified. PMV and PPD provide criteria for designing and evaluating environments, provided they are integrated with local conditions, applicable regulations, and information from the people who use the space.
