Microclimatic conditions and heat stress
Measurement of microclimatic conditions describes the environment at the workplace. It mainly monitors air temperature, radiant heat, air velocity and relative humidity.
Heat stress expresses the effect of the work environment and the work performed on the employee. The assessment therefore also takes into account energy expenditure during work, the duration of individual work operations, work clothing and the use of personal protective equipment.
The same temperature may be acceptable for seated office work but unacceptable for physically demanding work in production. A uniformly heated hall is also assessed differently from a workplace with intense radiation from a furnace, dryer, molten metal or hot product.
The conditions of health protection are laid down by Government Decree No. 361/2007 Coll..
When measurement is advisable or necessary
Heat stress measurement is usually addressed in connection with risk assessment and job categorisation, when technology or working conditions change, at the request of the regional public health authority (KHS) or in the event of repeated complaints from employees.
The basis for measurement is typically:
- the operation of a furnace, dryer, paint line, foundry equipment, bakery technology or another significant heat source,
- high temperatures in the hall during the summer period, work at an outdoor workplace or the introduction of longer shifts,
- a change in ventilation, work clothing, work intensity, production capacity or the organisation of work operations.
Measurement is also necessary if its result is to be used for a proposal to classify work into the second, third or fourth category, or to change the existing classification. Under Section 38 of Act No. 258/2000 Coll., on the Protection of Public Health, such measurement must be carried out by the holder of the relevant accreditation or authorisation.
The work class fundamentally affects the result
Work is classified into classes I to V according to the total average energy expenditure. The work class is not determined by the name of the job position, but by the activities actually performed and their share of the shift time.
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| Work class | Energy expenditure M | Examples of activities | Maximum permissible temperature |
|---|---|---|---|
| I | up to 80 W/m² | Office work, computer work, inspection activities, light laboratory work | 27 °C |
| IIa | 81–105 W/m² | Light manual work with the hands, assembly of small parts, mechanic's work with low physical load | 26 °C |
| IIb | 106–130 W/m² | Work mostly performed standing, welding, painting, machine operation, handling loads up to 10 kg | 30 °C |
| IIIa | 131–160 W/m² | Assembly lines, bakers, machine maintenance, warehouse workers, meat processing | 30 °C |
| IIIb | 161–200 W/m² | Construction work, operation of rubber presses, grinding of smaller castings | 26 °C |
| IVa | 201–250 W/m² | Work in foundries, work with a jackhammer, grinding of large castings, carrying loads of around 25 kg | 24 °C |
| IVb | 251–300 W/m² | Heavy manual work in quarries, agriculture or machine forging | 20 °C |
| V | 301 W/m² and more | Very heavy work, excavation work, manual forging of large pieces, transport of heavy loads | 20 °C |
The maximum temperatures given apply to year-round permissible microclimatic conditions at an indoor workplace with uncontrolled temperature that is naturally, combined or mechanically ventilated. The values relate to the operative temperature t<sub>o</sub> or the globe temperature t<sub>g</sub> and to work clothing with a thermal resistance of 0.5 clo.
The table also shows why a reading from an ordinary wall thermometer is not sufficient. The permissible value changes with energy expenditure and the type of workplace.
How the work class is correctly determined
For a simple and repetitive activity, the work class can usually be determined from the examples given in Annex 1 to the Government Decree. For combined job positions, it is necessary to describe the individual operations and their duration.
A worker may, for example, operate a machine for part of the shift, carry products by hand for part of the shift and perform seated inspection for part of the shift. The assessment takes into account the time share of these operations and their energy demand.
An incorrectly determined work class can lead to an incorrect assessment of the measured temperature, of protective drinks and of the permissible working time.
Which quantities are monitored during measurement
According to the Government Decree, heat stress is determined by the metabolic heat generated during work and by work environment factors. The basic measured or calculated quantities include:
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| Quantity | Symbol | Significance for the assessment |
|---|---|---|
| Air temperature | t<sub>a</sub> | Temperature of the surrounding workplace air |
| Globe temperature | t<sub>g</sub> | Takes into account air temperature and radiant heat |
| Operative temperature | t<sub>o</sub> | Resulting quantity taking into account air temperature, radiation and air movement |
| Air velocity | v<sub>a</sub> | Affects body cooling and sweat evaporation |
| Relative humidity | R<sub>h</sub> | At high humidity, the possibility of cooling by evaporation is reduced |
| Energy expenditure | M | Determines the amount of heat generated by the body during work |
| Thermal resistance of clothing | clo | Affects heat dissipation from the body surface |
If the air velocity does not exceed 0.2 m/s, the operative temperature can, under the specified conditions, be replaced by the globe temperature.
At an outdoor workplace, heat stress is assessed according to the globe temperature. Under the Government Decree, air movement and relative humidity are not taken into account separately in this assessment.
Measurement must capture representative operation
The result must correspond to the employee's actual exposure. Measurement points are selected in the area where the worker performs the individual operations, and the measurement must capture their time share.
At a workplace with a technological heat source, it is necessary to measure at normal or least favourable permitted output of the technology. At a workplace affected by outdoor temperature, the date is chosen so that it corresponds to the period to be assessed.
Measurement during a shutdown, at reduced production, with open gates or outside the hot period may not be usable for assessing summer heat stress.
If the worker moves between several locations during the shift, the average operative temperature is determined as a time-weighted average over the effective working time.
When the permissible working time is determined
For work of classes IIb to V, where the permissible heat stress values are exceeded, a regime of alternating work and safety breaks must be determined. At the same time, the long-term permissible and short-term permissible working time is determined.
The long-term permissible time protects the employee against excessive fluid loss during the shift. The short-term permissible time limits the accumulation of heat in the body during a continuous work period.
The calculation is based mainly on the employee's heat balance, the work class, the measured conditions, clothing and exposure duration. The result may be, for example, a regime in which a certain period of work at the heat source alternates with a safety break in a cooler area.
The permissible time is therefore not a universal figure determined solely by air temperature. The same 32 °C can lead to a different regime for light machine operation and for physically demanding work in a foundry.
Acclimatised and non-acclimatised employees
An employee is considered acclimatised if they have performed work at the assessed workplace for at least three weeks.
If a non-acclimatised employee performs work of classes IIb to V under conditions where the permissible heat stress values are exceeded, their long-term permissible working time is reduced by 30% during the first three weeks.
This requirement is important when new employees start, after a longer absence, when agency workers are used or when an employee is temporarily reassigned to a hot workplace.
Fluid loss and protective drinks
The hygiene limit for fluid loss under heat stress is 1.25 litres per eight-hour shift. If this value is exceeded, the employer provides replacement of fluids and minerals.
The protective drink is provided in an amount corresponding to at least 70% of the fluid loss through sweat and breathing over an eight-hour shift.
For work of classes I to IIIa, the protective drink used is mainly lightly mineralised mineral water, spring water or water with similar properties. For work of classes IIIb to V, half of the required replacement is provided as moderately mineralised water and the other half as water suitable for the lighter work classes.
The maximum permissible fluid loss corresponding to the long-term permissible stress is 3.9 litres per eight-hour shift for a standard person.
The protective drink must be genuinely available to employees during work. Topping up the entire calculated amount in one go at the end of the shift is not a suitable way of protecting health.
An exceptionally hot day for light work
For the purposes of the Government Decree, an exceptionally hot day is a day on which the highest outdoor temperature exceeds 30 °C.
For work of class I and IIa at an indoor workplace with uncontrolled temperature, compliance with the maximum temperatures from the table of year-round permissible conditions is not required on such a day. However, the employer must ensure replacement of fluid loss to the extent specified in the Government Decree.
This rule applies primarily to naturally or mechanically ventilated offices and light workplaces without permanently controlled temperature. Air-conditioned workplaces have their own requirements for setting microclimatic conditions.
How heat stress is reflected in job categorisation
The criteria for categorisation are laid down by Decree No. 432/2003 Coll., on the conditions for classifying work into categories.
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| Category of the heat stress factor | Basic characteristics |
|---|---|
| Category 2 | The maximum permissible temperature is exceeded, but a regime of alternating work and safety breaks is not yet required |
| Category 3 | The exceedance already requires a regime of alternating work and safety breaks |
| Category 4 | A high risk of damage to health cannot be ruled out even when protective equipment is used and the work regime is adjusted |
Work classified in the third and fourth categories is risk work. Its classification is decided by the competent regional public health authority on the basis of the employer's proposal and the submitted measurement results.
Work is classified in the second category by the employer, who notifies the competent public health protection authority of the classification. The specific procedure also depends on the other risk factors of the given job position.
Technical and organisational measures
The result of the measurement should serve as a basis for specific protective measures. Preference is given to modifications that reduce heat stress directly at the source or along the path of heat propagation.
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| Type of measure | Examples |
|---|---|
| Source reduction | Thermal insulation of furnaces and piping, covering of hot surfaces, automation of handling |
| Reduction of radiant heat | Reflective screens, shielding of the workstation, increasing the distance from the source |
| Environment adjustment | Local cooling, air conditioning, fresh air supply, improved ventilation efficiency |
| Organisation of work | Rotation of employees, safety breaks, moving demanding operations to a cooler part of the day |
| Employee facilities | Air-conditioned rest room, available protective drinks, possibility of changing clothes |
| Personal protection | Suitable work clothing, reflective clothing or special cooled clothing |
Simply opening the gates or installing an ordinary fan may not improve the situation. In some operations, a fan may bring in warm air, worsen the movement of polluted air or disrupt the function of technological extraction.
It is advisable to verify the proposed measure subsequently by control measurement under comparable operating and climatic conditions.
Which documents to prepare before measurement
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| Required information | Why it is important |
|---|---|
| List of job positions | Determines which work is to be assessed |
| Description of individual operations | Used to determine the work class |
| Duration of operations | Enables calculation of time-weighted exposure |
| Length and organisation of the shift | Affects the permissible working time and break regime |
| Work and protective clothing | Changes the possibility of heat dissipation from the body |
| Heat sources | Determine suitable measurement points and operating state |
| Ventilation and cooling | Affect temperature and air movement |
| Number of employees and shift pattern | Required for job categorisation |
| Previous protocols and KHS decisions | Allow comparison of changes in conditions |
Before measurement, it is also advisable to agree on the required purpose of the output. An indicative check of conditions, accredited measurement for job categorisation or control measurement after technical measures have been implemented may each have a different scope.
What you can send us and what we will verify
For a measurement proposal, you can send us:
- a description of job positions and individual operations, including their duration, shift pattern and breaks,
- photographs of the workplace, heat sources, ventilation and the work clothing used,
- previous protocols, job categorisation, the KHS opinion or information on employees' complaints.
We will propose measurement points and a suitable measurement date so that representative conditions are captured. We will determine the work class and evaluate heat stress, fluid loss and any need for safety breaks or limitation of working time.
You can find details on the page Heat and cold stress measurement.
Related legal regulations
Government Decree No. 361/2007 Coll., laying down conditions for occupational health protection
Decree No. 432/2003 Coll., on the conditions for classifying work into categories