Air temperature
This is the temperature of the air surrounding the body. It is usually given in degrees Celsius (°C).
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.
Examples of radiant heat sources include: the sun; fire; electric fires; ovens; kiln walls; cookers; dryers; hot surfaces and machinery, molten metals etc.
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.