The indoor climate is an essential environmental factor in the design of work systems. An indoor climate that is felt to be comfortable by employees will have positive effects on their well-being, health, and productivity. If the indoor climate is perceived to be uncomfortable, however, the exact opposite is found - employees’ well-being and performance deteriorate. At the same time it is certainly possible for a room temperature of 12°C to be ideal if employees are wearing appropriate clothing and performing appropriate activities, although this would not be acceptable for office spaces. It is the complex interaction between numerous parameters that characterises climate conditions and allows them to be assessed objectively. The following parameters play a role in such assessments.

Physical quantities

The four most important quantities for the analysis of the climate conditions in workspaces are air temperature, humidity, air velocity, and thermal radiation. In many cases, a single climate variable can be determined using a variety of measurement methods/principles. However, the application of different measurement methods may also deliver inconsistent results.

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Air temperature

The air temperature is the temperature of the air surrounding an object not subject to thermal radiation. Air temperature is easily felt and judged by the senses.
If the air temperature is lower than the temperature of the skin and air velocity is increased, it is felt to be cooler. At a raised level of relative air humidity, the air temperature is felt more intensely on the skin than the same temperature at a low relative humidity.
The room temperature is an aggregate temperature variable calculated from the local air temperature and the radiant temperatures of the individual surrounding surfaces (e.g. walls and ceilings).
What is referred to as the operative room temperature is found from the air temperature and the mean radiant temperature. This may be determined using a special globe thermometer.

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Humidity

When analysing the moisture content of air, a distinction is drawn between absolute and relative humidity. The absolute humidity is the ratio of the mass of water vapour to the mass of dry air (g/kg) and therefore indicates how much water vapour there is in the air. Relative air humidity represents how saturated the air is with water vapour.
Relative air humidity cannot be felt by the senses, but the air temperature is perceived more intensely at raised levels of relative air humidity.
"Close", "sultry", or "oppressive" conditions are experienced if the absolute humidity is higher than approx. 11.5 g/kg (the "upper humidity limit"), and so outside the limits of the comfort zone. The process of heat dissipation from the body may be restricted at these concentrations. This may in turn result in a rise in body temperature, thus placing strain on the circulatory system.
When walls are at low temperatures, an excessively high level of air humidity may result in the condensation of water vapour on their cool surfaces because they are below the dew point, which can also cause structural damage (e.g. as a result of the formation of mould).
In turn, according to a study of "dry air" conducted by the Federal Institute for Occupational Safety and Health (Bundesanstalt für Arbeitsschutz und Arbeitsmedizin, BAuA) in collaboration with the Institute for Occupational Safety and Health of the German Social Accident Insurance (Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung, IFA), increases in skin roughness and electrostatic charge (e.g. on hair) are associated with low air humidity.

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Air velocity

There is considerable variation in perceptions of moving air. Draughts represent one undesirable form of air movement that is perceived as a noticeable current. According to Technical Rule for Workplaces (Arbeitsstättenregel) ASR A3.6, Ventilation (Lüftung), a draught is an uncomfortable flow of air that leads to local cooling, particularly on body surfaces not covered by clothing. Draughts can be caused both by natural ventilation and by air-conditioning (AC) systems.
Air velocity is perceived differently depending on the air temperature, air humidity, and air turbulence. It also plays a role what kind of clothing is being worn and what activities are being performed.
Higher air velocities promote the evaporation of sweat on the skin and consequently heat transfer, which is why an increased air velocity is felt to be less uncomfortable by people who are performing physical work. Indeed, higher air velocities when performing physical work may be required to maintain the body’s thermal balance.

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Thermal radiation

It is only possible to distinguish to a limited extent between the perception of hot ambient air and the effects of mean thermal radiation. The aggregate heat flow consequent upon irradiation from different directions is decisive for the effects of thermal radiation on an employee in the workplace.
Heat can be absorbed by or dissipated from the surrounding environment in the form of thermal radiation. The flow of heat produced by thermal radiation is captured with the quantitative variable "effective irradiance" (E_eff in W/m²).
Thermal radiation can be determined by combining the globe temperature and the radiant temperature.
The globe temperature (t_G in °C) is the temperature inside a black hollow sphere made of a thin material with good thermal conductivity (copper or aluminium).
The mean radiant temperature (t_r in °C) is an artificial quantity, defined as the uniform temperature of a black enclosure in which a human being would exchange the same amount of heat by radiation as in the existing environment.

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Personal factors

Apart from physical indicators, personal factors also have to be taken into consideration when assessing indoor climate conditions in workspaces.
Personal factors include:

• the work activity
• the clothing situation
• acclimatisation
• other personal factors (e.g. age, sex, health status, dehydration)

More detailed information on the metabolic rates of different physical activities and the estimation of the thermal insulation of clothing ensembles can be found in DIN EN ISO 7730.

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Air quality

According to ASR A3.6, Ventilation, enclosed work rooms have to contain sufficient quantities of healthy, breathable air. This usually means air of the same quality as the outdoor air. The quality of indoor air in work rooms may be affected adversely by contaminant, moisture, or thermal loads.
If activities involving hazardous substances or biological agents are performed in the workplace, the provisions set out in the Ordinance on Hazardous Substances (Gefahrstoffverordnung, GefStoffV) or the Ordinance on Biological Agents (Biostoffverordnung, BioStoffV), including the relevant technical rules, apply with regard to the hazards that arise from such substances there. These hazards are not given consideration in a climate assessment and are to be analysed separately as part of a risk assessment that takes account of the relevant rules and regulations.
In offices and office-like workspaces, the indoor air quality is mainly affected by the substances human beings themselves emit (e.g. carbon dioxide (CO₂)). In order to analyse the indoor air situation, it is necessary to measure CO₂ or determine the air exchange rate. In this respect, the CO₂ concentration is a recognised measure for the subjective evaluation of air quality. Experience shows increased CO₂ concentrations have adverse effects on employees’ attention.
If employees complain about air quality despite their work room being used as intended, it is to be examined whether and, if so, what additional action should be taken. For example, the following measures may come into question:

• temporarily increased ventilation
• changes to the use of the room
• relocation of employees to other spaces
• installation or adaption of an AC system

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CO₂ measurement

Where there is a justifiable suspicion that CO₂ concentrations are too high, measurements should be taken under the conditions that usually prevail when the space is in use and with the usual number of persons present, for example during the hours when it is occupied on working days. It is the instantaneous value that is evaluated.
The room must be ventilated as usual during working hours before the measurement is taken. For rooms with a floor area of up to 50 m², one measurement point at a height of about 1.50 m and a distance of 1 to 2 m from the walls is usually sufficient. Several measurement points may have to be set up in larger spaces. In order to prevent the measurement results being directly influenced by exhaled air from anyone in the room, the measurement point should be set up away from the zone where employees are present or at a sufficient distance from them. (See ASR A3.6, Ventilation, for further information.)

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Air exchange measurement

One possible reason for poor indoor air quality may be that rooms are not being ventilated sufficiently, if at all, especially where there is reliance on natural ventilation (e.g. via windows), and the necessary ventilation rates are therefore not being reached.
The calculation of the airflow rate in accordance with DIN EN ISO 12569 means determining relevant parameters, in particular the operational status of air inlet and extraction systems, the opening of windows, and how rooms are used. Current ventilation conditions are measured using tracer gas dilution methods (e.g. with SF6) in order to calculate air exchange rates.
Based on the results of the measurements, additional steps may be stipulated, for example changes to the ventilation strategy or the installation/reconfiguration of an air-conditioning system.

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Natural ventilation

Rooms can be ventilated naturally by, for example, either purge or continuous ventilation. Free vent areas of sufficient dimensions are consequently required.
Purge ventilation is to be performed at regular intervals. When this is done, the amount of time that elapses between two ventilation periods and the duration of the ventilation periods are decisive for the level of indoor air quality that is achieved.
It is recommended that office rooms be ventilated after sixty minutes and meeting rooms (i.e. more densely occupied spaces) after twenty minutes. The minimum duration of purge ventilation depends on the difference between the indoor and outdoor temperatures and the wind speed.
The following guide times for ventilation periods during different seasons may be complied with in order to ensure the carbon dioxide concentration does not exceed 1000 ppm:
summer: ten minutes,
spring/autumn: five minutes
winter: three minutes
(Source: ASR A3.6, Ventilation). 

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