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. However, air temperature alone is not a valid or accurate indicator of thermal comfort or thermal stress. It 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 an employee's thermal comfort.
This is the temperature of the air surrounding the body. It is usually given in degrees Celsius (°C).
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.
Examples of radiant heat sources include: the sun; fire; electric fires; ovens; kiln walls; cookers; dryers; hot surfaces and machinery, molten metals etc.
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
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.
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.