The six basic factors
Thermal comfort in the workplace is more complex than most people realise. While it is tempting to judge a working environment simply by whether it feels warm or cold, the reality involves a combination of six distinct factors, each capable of influencing how the human body experiences its surroundings. Understanding these factors is essential for employers and health and safety professionals who want to maintain safe, productive working conditions all year round.
The Health and Safety Executive recognises that no single measurement can capture thermal comfort accurately. Air temperature, for instance, is the most widely used indicator precisely because it is easy to measure and straightforward to communicate. However, relying on it alone can lead to poorly informed decisions, particularly in environments with high radiant heat, poor ventilation, or where workers are required to wear heavy personal protective equipment. A thorough assessment must consider all six factors together.
Environmental factors:
Personal factors:
Why Air Temperature Alone Does Not Determine Thermal Comfort at Work
Air temperature refers to the temperature of the air directly surrounding the body, typically expressed in degrees Celsius. It is the starting point for any thermal assessment, and in most indoor workplaces it remains the primary metric used to judge whether conditions are acceptable. That said, it tells only part of the story.
The reason air temperature is insufficient on its own is that the body does not exchange heat with the air alone. It also absorbs and emits radiant heat, is cooled or warmed by air movement, and is affected by how easily sweat can evaporate from the skin. A workplace at 22°C with high radiant heat sources, still air, and high humidity may feel significantly hotter and more physiologically demanding than a straightforward thermometer reading would suggest. This is why thermal comfort assessments in occupational health contexts must always combine air temperature with the other five factors before drawing any conclusions about worker welfare.
How Radiant Temperature Affects Worker Heat Stress in Industrial Environments
Radiant temperature is, in many practical situations, a more significant driver of thermal discomfort than air temperature. Thermal radiation is heat energy emitted by warm objects and surfaces; it travels through the air and is absorbed directly by the body without needing to heat the surrounding air first. This distinction matters enormously in workplaces where heat sources are present.
Common sources of radiant heat include furnaces, kilns, ovens, molten metals, hot pipework, and even direct sunlight through industrial glazing. A worker standing close to a kiln wall or operating near a furnace may experience significant heat gain through radiation even if the measured air temperature in the room appears moderate. The mean radiant temperature, which reflects the combined effect of all surrounding hot and cold surfaces, has a greater influence on how much heat the body gains from or loses to its environment than air temperature alone.
For employers carrying out thermal comfort risk assessments, understanding radiant temperature is particularly important in manufacturing, foundry, bakery, and outdoor construction settings. Shielding workers from radiant sources, using reflective barriers, and rotating tasks to limit exposure time are all practical controls that address radiant heat specifically rather than simply adjusting the thermostat.
Air velocity
This describes the speed of air moving across the employee and may help cool them if the air is cooler than the environment.
Air velocity is an important factor in thermal comfort for example:
- still or stagnant air in indoor environments that are artificially heated may cause people to feel stuffy. It may also lead to a build-up in odour
- moving air in warm or humid conditions can increase heat loss through convection without any change in air temperature
- physical activity also increases air movement, so air velocity may be corrected to account for a person’s level of physical activity
- small air movements in cool or cold environments may be perceived as a draught as people are particularly sensitive to these movements
Humidity
If water is heated and it evaporates to the surrounding environment, the resulting amount of water in the air will provide humidity.
Relative humidity is the ratio between the actual amount of water vapour in the air and the maximum amount of water vapour that the air can hold at that air temperature.
Relative humidity between 40% and 70% does not have a major impact on thermal comfort. In workplaces which are not air conditioned, or where the weather conditions outdoors may influence the indoor thermal environment, relative humidity may be higher than 70%. Humidity in indoor environments can vary greatly and may be dependent on whether there are drying processes (paper mills, laundry etc) where steam is given off.
High humidity environments have a lot of vapour in the air, which prevents the evaporation of sweat from the skin. In hot environments, humidity is important because less sweat evaporates when humidity is high (80%+). The evaporation of sweat is the main method of heat reduction.
When non-breathable vapour-impermeable personal protective equipment (PPE) is worn, the humidity inside the garment increases as the wearer sweats because the sweat cannot evaporate. If an employee is wearing this type of PPE (eg asbestos or chemical protection suits etc) the humidity within the PPE will be high.
Personal factors
Clothing insulation
Thermal comfort is very much dependent on the insulating effect of clothing on the wearer.
Wearing too much clothing or PPE may be a primary cause of heat stress even if the environment is not considered warm or hot.
If clothing does not provide enough insulation, the wearer may be at risk from cold injuries such as frostbite or hypothermia in cold conditions.
Clothing is both a potential cause of thermal discomfort as well as a control for it as we adapt to the climate in which we work. You may add layers of clothing if you feel cold, or remove layers of clothing if you feel warm. Many companies inhibit this ability for employees to make reasonable adaptations to their clothing as they require them to wear a specific uniform or PPE.
It is important to identify how the clothing contributes to thermal comfort or discomfort. By periodically evaluating the level of protection provided by existing PPE and evaluating newer types of PPE you may be able to improve the level of thermal comfort.
Work rate/metabolic heat
The more physical work we do, the more heat we produce. The more heat we produce, the more heat needs to be lost so we don’t overheat. The impact of metabolic rate on thermal comfort is critical.
A person’s physical characteristics should always be borne in mind when considering their thermal comfort, as factors such as their size and weight, age, fitness level and sex can all have an impact on how they feel, even if other factors such as air temperature, humidity and air velocity are all constant.
