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The room climate and its parameters
Information on physical parameters, personal factors, and air quality
The room climate is an essential environmental factor regarding the design of work systems. A room climate perceived as comfortable by the employees has positive effects on the well-being, the health, and the productivity. However, if the room climate is perceived as uncomfortable by the employees, the exact opposite occurs – well-being and performance deteriorate. Yet, a room temperature of 12°C may absolutely be ideal if clothes and work are adapted to this temperature, while this temperature is unacceptable for office premises. Only the complex interaction of many parameters characterises a climate and the condition can be estimated objectively only when being familiar with these parameters, respectively. The following parameters play a role in this regard:
Physical variables
The four most important climate variables in working spaces include the air temperature, the humidity, the air velocity, and the thermal radiation. In many cases, a climate variable can be determined using different measurement methods/principles which may lead to different results.
Air temperature
The air temperature is the temperature of the air surrounding us, without the effects of thermal radiation. We can readily perceive and assess the air temperature by instinct.
We perceive air temperatures below the skin temperature as cooler if the air velocity is increased. In the event of an increased relative humidity, we perceive the air temperature on the skin more intensively than the same temperature at a lower relative humidity.
The room temperature is a summarised temperature variable consisting of the local air temperature and the radiation temperatures of the individual surrounding surfaces, i.e. walls and ceilings, for example.
The operative room temperature is a temperature resulting from the air temperature and the average radiation temperature. This temperature may be determined using a special thermometer, the globe thermometer.
Humidity
In order to describe the moisture content of the air, a differentiation is made between absolute and relative humidity. The absolute humidity refers to the water vapour content of the air. It is the mass-ratio between water vapour and dry air (g/kg). The relative humidity specifies the degree of saturation of the air with water vapour.
We are not capable of assessing the relative humidity specifically, but perceive the air temperature more intensively if the relative humidity is higher.
If the limits of the comfort zone are exceeded, this is called closeness feeling. At these concentrations, the process of heat dissipation of the body may be limited. In turn, this may result in an increased body temperature and thereby in a circulation strain.
Furthermore, a high relative humidity in combination with low wall temperatures may result in water vapour condensation and thereby possibly even in structural damages (formation of mould) when the saturation temperature is fallen below.
An older literature study of the Industrial Health and Safety Institute of the German Social Accident Insurance (IFA) shows the huge range of the issue "dry air". It has been proven beyond doubt that an increase of the skin roughness (complaining about dry and itchy hands) and an increased number of electrostatic charges ("floating" hair) are directly connected to low humidities.
Air velocity
We perceive air movements very differently. A desired form of air velocity is the so-called draught that is perceived as a draught phenomenon. Pursuant to the workplace rule ASR A3.6 "Lüftung" (ventilation), draught is a disturbing puff of air resulting in a local temperature reduction, particularly on body surfaces not covered by clothes. Draught can be caused both by natural ventilation and by air-conditioning systems (AC systems).
The air velocity is perceived differently depending on the air temperature, the air humidity, and the degree of turbulence of the air. The clothes we wear and the things we are doing also play a role.
For example, we perceive an increased air velocity as less disturbing when we are working manually, it may even be required in order to compensate the heat balance. Higher air velocities promote the evaporation of sweat from the skin and thereby the heat dissipation.
Thermal radiation
We are able to differentiate between the perception of hot ambient air and the effect of the average thermal radiation to a very limited extent only. The totalized heat flow caused by the different directions of irradiation is decisive for the effect of thermal radiation on an employee at the workstation.
We are able to accept or dissipate heat from our environment via thermal radiation. This heat flow caused by the thermal radiation is captured by the measurement parameter "effective irradiance" (Eeff in W/m²).
The globe temperature and the radiation temperature may be used to determine the thermal radiation.
The globe temperature (tG in °C) is the temperature inside a blackened hollow sphere made of a thin material with a good thermoconductivity (copper or aluminium).
The average radiation temperature (t in °C) is an artificial variable, defined as uniform temperature of a black environment, inducing the same radiation exchange between human being and environment as the actual environment.
Personal factors
In addition to the physical parameters, the personal factors must also be taken into consideration when assessing the room climate situation.
Personal factors include:
- activity level while performing the work
- clothing situation
- acclimatisation
- additional personal factors (e.g. age, sex, health condition, dehydration).
More detailed information on the energy turnover of different physical activities and on the estimation of an insulation value of clothing combinations can be found in DIN EN ISO 7730.
Air quality
Pursuant to ASR A3.6 "Lüftung" (ventilation), enclosed working spaces must contain sufficient amounts of breathing air beneficial to the health. Normally, this corresponds to the external air quality. The room air quality in working spaces may be affected adversely by substances, moisture, or heat.
If activities involving hazardous substances or biological substances are performed at the workstation, the provisions pursuant to the Ordinance on Hazardous Substances or the Ordinance on Biological Agents, including the technical guidelines, are applicable regarding the hazards caused by substances. These hazards are not taken into consideration within the framework of a climate assessment and must be taken into account separately within the framework of a risk assessment, considering the corresponding guidelines.
In office and office-like working spaces, the room air quality is mainly determined by the substances dissipated by the human beings themselves (e.g. carbon dioxide). In order to determine the room air situation, carbon dioxide must be measured or the air change must be determined. In this, the carbon dioxide concentration is a recognised measure for subjectively evaluating the air quality. As experience teaches, increased carbon dioxide concentrations have adverse effects on the concentration.
If employees complain about the air quality despite using the working space as intended, it must be checked whether and possibly which additional measures must be performed. For example, the following come into question:
- temporarily increased ventilation,
- change of the use of the room,
- relocation of the employees to other rooms,
- installation or adaptation of an AC system.
CO2 measurement
"If there is a comprehensible suspicion regarding the CO2 concentrations being too high, measurements should be performed under usual conditions of use and with the normal number of persons present, e.g. covering the period of use on a working day. The current value is assessed.
Prior to performing the measurement, the room must be ventilated as usual for the work. For rooms with a surface area of 50 m² at most, normally one measurement site in a height of about 1.50 m and a distance of 1 to 2 m from the walls is sufficient. In larger rooms, several measurement sites may have to be set up. The measurement site should be located in the area the persons are located in - but with sufficient distance to the persons, in order to avoid a direct influence of the measured result through the breathing air of persons." (Source: ASR A3.6 Lüftung)
Ventilation measurement
One reason for a poor quality of the room air may be that, particularly in the event of natural ventilation (e.g. using the windows), too little or no ventilation at all takes place and thereby the necessary ventilation rates are not complied with.
One option of determining this is to determine the ventilation rate. For measurement purposes, a non-toxic tracer gas (e.g. SF6) is introduced into the rooms to be measured and the concentration drop is determined over a period of approximately one hour at a defined mode of use. In so doing, certain boundary conditions (particularly operating condition of inlet air/extraction systems, openings of the windows, room use) should be varied in a targeted manner. This is used to calculate the ventilation rate. This measurement is based on DIN EN ISO 12569.
Based on the measured results, additional measures may be defined, e.g. changing the ventilation behaviour or installation/conversion of an air-conditioning system (AC system).
Natural ventilation
Natural ventilation of rooms may occur as inrush ventilation or continuous ventilation, for example. Accordingly, sufficiently dimensioned ventilation cross-sections are required.
Inrush ventilation must be performed at regular intervals. In so doing, the time between two ventilation intervals and the duration of the ventilation phases are decisive for the room air quality achieved.
It is recommendable to ventilate office premises after 60 minutes and meeting rooms (i.e. rooms with a larger number of persons) after 20 minutes. The minimum duration of inrush ventilation depends on the temperature difference between inside and outside and the wind.
In order to not to exceed a carbon dioxide concentration of 1000 ppm, the following orientation values can be assumed for the respective ventilation time:
Summer: 10 minutes,
Spring/autumn: 5 minutes
Winter: 3 minutes
(Source: ASR A3.6 Lüftung).