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The Six Basic Factors

The most commonly used indicator of thermal comfort is air temperature – it is easy to use and most people can relate to it. But although it is an important indicator to take into account, air temperature alone is neither a valid nor an accurate indicator of thermal comfort or thermal stress. Air temperature should always be considered in relation to other environmental and personal factors.

The six factors affecting thermal comfort are both environmental and personal. These factors may be independent of each other, but together contribute to a worker’s thermal comfort.

Environmental factors:

Personal factors:

Jigsaw style diagram showing the six different factors Jigsaw style diagram showing the six different factors

Environmental factors

Air temperature

This is the temperature of the air surrounding the body. It is usually given in degrees Celsius (°C) or degrees Fahrenheit (°F).

Radiant temperature

Thermal radiation is the heat that radiates from a warm object. Radiant heat may be present if there are heat sources in an environment.

Radiant temperature has a greater influence than air temperature on how we lose or gain heat to the environment. Our skin absorbs almost as much radiant energy as a matt black object, although this may be reduced by wearing reflective clothing.

Examples of radiant heat sources include: the sun; fire; electric fires; furnaces; steam rollers; ovens; walls in kilns; cookers; dryers; hot surfaces and machinery, molten metals etc.

Air velocity

This describes the speed of air moving across the worker and may help cool the worker if it is cooler than the environment.

Air velocity is an important factor in thermal comfort because people are sensitive to it.

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.

Small air movement in cool or cold environments may be perceived as draught. If the air temperature is less than skin temperature it will significantly increase convective heat loss.

Physical activity also increases air movement, so air velocity may be corrected to account for a person's level of physical activity.


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 some offices, humidity is usually kept between 40-70% because of computers. However, in workplaces which are not air conditioned, or where the climatic conditions outdoors may influence the indoor thermal environment, relative humidity may be higher than 70% on warm or hot humid days. 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 loss in humans.

When vapour-impermeable 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 microclimate of the garment may be high.

Personal factors

Clothing insulation

Clothing, by its very nature, interferes with our ability to lose heat to the environment. Thermal comfort is very much dependent on the insulating effect of clothing on the wearer.

Wearing too much clothing or personal protective equipment (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 frost bite 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 live and play. You may add layers of clothing if you feel cold, or remove layers of clothing if you feel warm. However, many companies remove this ability for their employees to make reasonable adaptations to their clothing.

It is important to identify how the clothing may contribute to thermal comfort or discomfort. It may also be necessary to evaluate the level of protection that any PPE is providing – can less or other PPE be used?

Work rate/metabolic heat

The work or metabolic rate, is essential for a thermal risk assessment. It describes the heat that we produce inside our bodies as we carry out physical activity.

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.

When considering these factors, it is also essential to consider a person's own physical characteristics.

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.

Next: Measuring thermal comfort