In brief
At melting furnaces, casting lines, heat treatment furnaces and boilers, the heat load does not depend on the season, because it comes mainly from radiation from hot surfaces rather than from the air. The indicator that expresses this load as a single number is WBGT, assessed under ISO 7243 over a representative period of about one hour, together with workload and clothing. If the reference value is exceeded, radiation is first cut off at source; a work-rest cycle including rest in a cool area and regular hydration are then planned.
Working at furnaces: why is heat stress independent of the season?
The heat that hits your face when the furnace door opens does not come from the air; it comes from radiation from the glowing metal and the furnace wall. That is why the heat load at the furnace exists in January as well as in July; summer only adds to it. The foundry temperature shown by the thermometer on the wall does not reflect this load; the way to measure heat stress is WBGT measurement. For employees in foundries supplying parts to Bursa’s automotive and machinery industry, in forging and heat treatment shops, at glass and ceramic kilns and in boiler rooms, the problem is not thermal comfort but heat stress: the body’s inability to get rid of the heat it produces and receives from outside.
The legislation makes this distinction. Article 19 of Annex 1 of the Regulation on Health and Safety Measures in Workplace Buildings and Annexes requires the ambient temperature to be appropriate to the working method and the physical effort expended, and states that the TS EN 27243 standard can be used in measurement and assessment. Article 20 requires measures to protect employees from excessive heat where, due to the nature of the work, working continuously in a very hot environment is unavoidable. TS EN 27243 is based on the 1989 version of ISO 7243, the standard that assesses heat stress using the WBGT indicator; the current version of the standard is ISO 7243:2017. For temperature complaints in offices and shops, see our article on thermal comfort measurement; this article focuses on work at hot processes.
What is WBGT measurement, and how is the heat stress index calculated?
WBGT (wet bulb globe temperature) is a heat stress index that combines four environmental factors, namely air temperature, humidity, air movement and radiation, into a single temperature value. It is measured with three sensors:
| Sensor | What does it measure? | What does it capture at the furnace? |
|---|---|---|
| Natural wet bulb thermometer (Tnw) | A thermometer wrapped in a wet wick and exposed to natural air movement; shows the combined effect of humidity and air movement | Whether sweat can evaporate |
| Black globe thermometer (Tg) | The temperature inside a matt black globe; shows the radiant load | Radiation from the furnace, ladle and hot parts |
| Dry bulb thermometer (Ta) | Air temperature | Enters the formula only under sunlight |
ISO 7243:2017 gives two formulas:
- Environments without solar load (indoor workshop): WBGT = 0.7 × Tnw + 0.3 × Tg
- Environments with solar load: WBGT = 0.7 × Tnw + 0.2 × Tg + 0.1 × Ta
The fact that the natural wet bulb thermometer carries the greatest weight in the formula shows the importance of humidity and sweat evaporation in the heat balance. In a foundry, however, the globe thermometer reads far higher than the air temperature at the furnace; this difference is a sign that the problem will be solved by cutting off radiation, not by ventilation.
How is WBGT measurement planned: point, duration and workload
For a WBGT measurement to be useful, the questions “when, where and for whom” must be answered in advance. The measurement rules of ISO 7243:2017 are:
- Time: Measurement is made during the period of highest heat stress. At the furnace, these are the melting and pouring hours; in summer, the outdoor environment in the afternoon also has an effect.
- Duration: The assessment is made over a representative period of about 1 hour. If the environment varies, the values are time-weighted and averaged.
- Point: The sensors are normally placed at the employee’s abdomen height. If the environment is not homogeneous, the measurement is made at the position of highest heat stress.
- Types of work: If light work is done in the morning and heavy work in the afternoon, each is measured and assessed separately.
- Clothing: The reference values are based on cotton work clothing. For different clothing such as aluminised aprons or vapour-impermeable coveralls, a clothing adjustment value (CAV) is added to the measured value; if the adjustment is not known, a detailed analysis is made.
In addition to the measured environment, the workload, that is the metabolic rate, is also determined; in the same environment, an employee carrying a ladle produces far more heat than an employee waiting at a control panel. The standard uses five classes. The table below gives examples from foundry work; the exact class is determined by the measurement report:
| Metabolic rate class | General description | Example from the foundry |
|---|---|---|
| 0 – Resting | Resting while seated | Break in a cool rest room |
| 1 – Low | Light hand and arm work, standing inspection | Control panel, reading samples |
| 2 – Moderate | Sustained hand, arm and trunk work | Placing cores, light grinding |
| 3 – High | Intense arm and trunk work, heavy loads | Carrying a ladle by hand, removing slag, shovelling sand |
| 4 – Very high | Heavy work at the highest pace | Fast shovelling in an emergency, carrying while climbing stairs |
If there is doubt about which class applies, the standard requires the higher class to be used. We plan WBGT measurements with laboratories holding a competence certificate from the Directorate General of OHS (ISGGM) and incorporate the results into the risk assessment. We have listed which measurements are mandatory in our article on workplaces where environmental measurements are mandatory.
Interpreting the result: what if the reference value is exceeded?
ISO 7243 compares the measured WBGT with the reference values in Annex A. The reference values are set for exposure that can be sustained for up to 8 hours; they fall as metabolic rate rises and are lower for people who are not used to heat (not acclimatised). If the measured value is above the reference, the risk of heat-related illness increases, and the standard offers two routes: reducing heat stress directly in the workplace, or carrying out a detailed analysis under ISO 7933.
Consider the measures in the order of the hierarchy of controls:
- At source: Shorten the time the furnace door stays open, place radiation screens (reflective panels, water curtains, chain curtains) at the furnace mouth and casting line, and separate the cooling area for hot parts from the work position.
- Engineering: Provide local cooling (spot air) at fixed work positions, move the control point into an enclosed, air-conditioned cabin, and replace manual carrying with mechanical ladle transport.
- Organisation: Work-rest cycle, task rotation, moving heavy work to the cooler hours of the day.
- PPE: Aluminised aprons and face shields block radiation, but they also prevent sweat from evaporating; this is why the clothing adjustment must be taken into account in measurement.
How is work-rest time in hot environments planned?
ISO 7243 requires a time-weighted average where the environment and workload change within the hour. This makes the work-rest cycle calculable: rest lowers both the average WBGT and the average metabolic rate. But this only works if the rest is taken in a cool place.
An example calculation (the values are illustrative; use the values in your own measurement report):
| Scenario | At the furnace (WBGT 31 °C) | Rest location | Hourly weighted WBGT |
|---|---|---|---|
| A | 40 minutes | Air-conditioned rest room, WBGT 22 °C, 20 minutes | (31 × 40 + 22 × 20) / 60 = 28.0 °C |
| B | 40 minutes | Bench next to the furnace, WBGT 29 °C, 20 minutes | (31 × 40 + 29 × 20) / 60 = 30.3 °C |
| C | 30 minutes | Air-conditioned rest room, WBGT 22 °C, 30 minutes | (31 × 30 + 22 × 30) / 60 = 26.5 °C |
The only difference between A and B is where the rest is taken. Sitting next to the furnace may count as a break, but it hardly lowers the heat load at all. The hourly average obtained is compared with the reference value for the average metabolic rate class calculated for the same hour; if the exceedance continues, a longer rest as in C or a cooler rest location is needed.
When setting up the cycle, put these points in writing:
- The rest room should be close to the furnace area, shaded or air-conditioned, with seating and cold drinking water.
- Heavy tasks (carrying ladles, removing slag, furnace maintenance) should not fall to the same person back to back; prepare a rotation schedule.
- On days with high heat load, give the shift supervisor authority to reduce the pace of work.
The time limit must not be forgotten either. The Regulation on Work That Must Be Performed for a Maximum of Seven and a Half Hours or Less per Day for Health Reasons limits daily work in casting operations, from the preparation of moulding sand to metal melting and pouring, to 7.5 hours. Under Art. 41 of Labour Law No. 4857, overtime cannot be required in such work whose duration has been shortened for health reasons. For other risks in foundries, see our article on occupational safety in foundries.
Hydration and health surveillance
Under heat stress, the body cools itself by sweating; if sweat loss is not replaced, blood volume falls, the load on the heart increases and attention wanders. NIOSH’s recommendations are a practical starting point on the floor:
- In moderately hot work, drink about 1 cup (240 ml) of water every 15–20 minutes; do not wait until you are thirsty.
- Fluid intake should generally not exceed 6 cups (about 1.4 litres) per hour.
- Water should be close to the work area, easy to reach and below 15 °C.
- For intense sweating lasting hours, drinks containing electrolytes are needed; alcohol and high-caffeine and sugary drinks are limited.
In practice, do not leave this to the employee: a cold water station near the furnace, a personal bottle with the employee’s name on it, the shift supervisor reminding people to drink water during breaks, and explaining in training that darkening urine is a sign of dehydration all work.
On the health surveillance side, the workplace physician assesses conditions that may reduce heat tolerance and medications used at pre-employment and periodic examinations, and imposes restrictions on the employee if necessary. Dizziness, nausea, muscle cramps and excessive fatigue are early signs of heat-related illness; confusion is an emergency. For symptoms and first response, see our article on heat stroke symptoms and response.
Common gaps found in inspections
- Mistaking air temperature for a heat stress measurement. A wall thermometer does not show radiation; an assessment at the furnace without a globe thermometer is incomplete.
- Measuring on a cool morning. Measurement must be made at the hour of highest heat load and with the actual workload.
- Taking breaks next to the furnace. As the example above shows, this leaves the cycle on paper only.
- Not incorporating the result into the risk assessment. Art. 10 of Law No. 6331 requires the measurements and investigations needed for the risk assessment to be carried out; the measurement result and the measure taken must be traceable in the same file.
You can plan measurement and inspection dates with the OHS calendar, and for sector-specific topics see our metal and machinery page.
Frequently Asked Questions
In which workplaces is WBGT measurement needed?
The legislation does not give a specific list of workplaces; it requires protective measures wherever work is carried out continuously in a very hot environment and refers to TS EN 27243 for the assessment. Foundries, forges, heat treatment, glass and ceramic kilns, boiler rooms and areas where heavy work is done outdoors in summer are typical examples; the risk assessment determines whether it is needed.
What is the difference between WBGT and air temperature?
Air temperature shows only the temperature of the air. WBGT also takes into account humidity, air movement and radiation from hot surfaces. At the furnace, air temperature may look reasonable while WBGT is very high because of radiation.
Is work-rest time set in the legislation?
No, the legislation does not give a fixed ratio. The ratio is calculated from the measured WBGT, the workload and the temperature of the rest location. In casting work, there is also a daily limit of 7.5 hours and a ban on overtime.
Does a rest taken at the furnace count?
It may count as a break, but it does not significantly lower the heat load. To lower the hourly average, the rest must be taken in a cool, shaded or air-conditioned area.
A risk assessment that identifies and scores hazards on site, links each measure to an owner and a date, and has a clear renewal schedule.
Bursa SİSTEMER OSGB Technical Team — written from the field experience of our OHS specialists and workplace physicians. Legal information is valid as of the publication/update date; contact us for an assessment specific to your business.