WBGT VS Heat Index: The WBGT (Wet Bulb Globe Temperature and Heat Index are both thermal stress indices used to assess the effects of temperature, humidity and radiation on the human body. Both measure how hot it feels outside, but they are calculated differently and take different factors into account. Both methods of measuring the potential for heat-related stress in the body are good to use. However, there is a big difference. The heat index is calculated for shaded areas, and the wet bulb globe temperature is calculated for areas exposed to direct sunlight. (HSE guidelines)
You can see that both are useful depending on where you plan to spend your time outdoors. If it is mostly in the shade, use the heat index. If you plan to be mostly in the sun, then use the wet bulb globe temperature. Here in this blog, we will cover WBGT and heat index chart, heat index calculator.
The heat index is a term most often heard when talking about the weather in general. It was first developed for the US military in the 1950s. The index gives us an idea of the apparent temperature it feels like outside. When the relative humidity is higher, less evaporative cooling is allowed to occur, making the temperature “feel” hotter.
The wet bulb globe temperature (WBGT) is an indicator of heat-related stress on the human body when working or performing work in direct sunlight. Military agencies, the Occupational Safety and Health Administration (OSHA), event associations and organizations use this index to manage workload or activity level.
However, they differ in terms of the parameters they consider and the conditions in which applicable following:
WBGT vs Heat Index: Parameters and applicability
- Parameters considered:
WBGT: The WBGT takes into account three factors: It is measured using a specialized thermometer that has the natural wet bulb temperature (reflecting humidity), the globe temperature (reflecting radiation), and the dry bulb temperature (reflecting air temperature). It provides a measure of the integrated effects of heat, humidity, and radiation on the body.
Heat Index: The Heat Index considers only the air temperature and relative humidity, providing an estimation of how hot it feels to the body based on these two factors.
- Applicability:
WBGT: In general, WBGT is considered a more accurate measure of how hot it feels outside because it takes into account more factors than the heat index. The WBGT is primarily used in occupational settings and sports activities to assess the risk of heat-related illnesses, particularly when physical exertion is involved. It is considered more accurate in evaluating heat stress as it includes additional factors like radiation.
Heat Index: The Heat Index is commonly used in weather forecasting to provide information about how hot it feels outdoors to the general population. It helps in issuing heat advisories and warnings to protect people from dangerous heat conditions.
What is the heat index mean
The heat index, or wind chill, provides information about how hot it feels outside in the shade. It is a measure of the air temperature about the relative humidity for a particular day. The National Weather Service publishes the heat index daily in newspapers to alert people to the risk of heat illness. Direct sun exposure can increase the risk of heat-related illness on days when the heat index is high.
Babies, older adults, or anyone with a health condition may be more at risk of heat problems due to their age and general health. Preventive measures on days, when the heat index is high, will help reduce the risk of heat-related illness. When the outdoor humidity is higher than 75%, the loss of body heat due to sweating is not as effective, so other measures are necessary to keep cool.
Heat index calculator
The heat index incorporates information about ambient temperature and relative humidity to assess risks during heat waves and warn of conditions dangerous to human health. This is especially important during humid heat waves when temperatures alone are not sufficient to assess the thermal stress experienced by the human body.
Based on the NWS chart, we present 5 heat stress levels (“Safe”, “Caution”, “Extreme Caution”, “Danger”, and “Extreme Danger”) corresponding to different combinations of temperatures and relative humidity. There are several heat indices in operation, the one presented here is the US Expanded Heat Index and the corrected version of the National Weather Service (NWS) Heat Index.
Heat index calculator (relative humidity).
Humidity
Relative humidity, for its part, measures the amount of water in the water about the maximum amount of water vapour (humidity). The relative humidity is precisely the humidity that they talk about in the weather report. Humidity is a natural component of the atmosphere and comes from the amount of water vapour in the air.
Water vapour enters the atmosphere by evaporating from the large bodies of water that occupy the Earth’s surface, such as lakes, oceans, and seas. The heat index incorporates information about ambient temperature and dew point to assess risks during heat waves and warn of conditions dangerous to human health.
Heat index calculator (relative dew point)
What is the dew point?
The dew point is the temperature to which the air must be cooled for water vapour to condense into dew or frost. At any temperature, there is a maximum amount of water vapour that air can hold. This maximum amount is called the saturation pressure of water vapour. The addition of more water vapour produces condensation.
The dew point is closely related to relative humidity, which is the relationship between the amount of water vapour present in the air at a given moment and the maximum amount of vapour that the air would be able to contain at that temperature. On the other hand, the relative humidity is 100% when the dew point and the temperature are equal.
If the temperature drops rapidly, condensation will occur and liquid water will begin to form. Also, the dew point is considered the most accurate way to measure ambient humidity, since it is an absolute measurement as opposed to relative humidity.
Heat index chart
In summary, the WBGT is a more comprehensive heat stress index that considers additional factors like radiation, making it suitable for occupational and sports settings. On the other hand, the Heat Index is a simpler index based on air temperature and humidity, commonly used for general weather forecasting and public awareness.
A heat index chart is a tool used to determine the level of discomfort or danger that can be caused by high temperatures and humidity. It takes into account the combined effect of air temperature and relative humidity on the human body and provides a measure of how hot it feels.
It’s important to monitor the heat index chart during hot weather conditions, and take appropriate precautions to stay safe and healthy, such as staying hydrated, wearing lightweight and light-coloured clothing, avoiding outdoor activities during the hottest part of the day, and seeking shade or air-conditioned spaces when possible.
Table of heat index values
The heat index chart typically ranges from 80°F to 130°F, with varying levels of discomfort and danger associated with each range. For example, a heat index of 80°F to 90°F is considered a “caution” level, where individuals should take extra care to stay cool and hydrated. A heat index of 105°F to 130°F is considered “extreme caution” to “danger” level, where there is a high risk of heat exhaustion, heat stroke, and other heat-related illnesses.
WBGT meaning
What is thermal stress and why is it measured? Workers who operate in facilities and machines that produce high energy or that have a high-temperature emission may occasionally suffer thermal stress (occasionally due to the influence of body temperature). This phenomenon can also cause symptoms such as skin changes, thermal cramps, heat syncope, heat exhaustion, heat stroke and, additionally, increased probability of an accident at work which can even lead to death.
Therefore it is necessary to use an apparatus to determine the thermal stress. Heat stress meters detect temperature and classify it as humidity. The world standard designation for this is the WBGT (Wet Bulb Globe Temperature Index).
The WBGT index is based on the combination of environmental heat loads and metabolic heat loads. It is used, due to its simplicity, to quickly discriminate whether or not a heat stress risk situation is admissible. Although its calculation often allows taking decisions, regarding the possible preventive measures to be applied.
The higher the WBGT, the greater the risk of heat stress. When WBGT reaches a certain level, it can be dangerous to work or exercise in hot weather. The specific WBGT levels that pose a risk vary depending on the individual’s activity level, acclimatization to heat, and other factors.
There is a strong relationship between WBGT and heat stress.
Studies have shown that WBGT is a good predictor of physiological changes that occur in response to heat stress, such as an increase in heart rate and core body temperature.
The use of WBGT can help reduce the risk of heat-related illness by allowing people to assess the level of heat stress and take steps to protect themselves. For example, if the WBGT is high, people may choose to work or exercise in the shade, drink plenty of fluids, and wear light-coloured, loose-fitting clothing. Here is a table that shows the relationship between WBGT and the risk of heat stress:
- Normal thermometer (dry bulb)
- Wet-bulb thermometer – (humidity)
- Globe temperature – (radiant heat)
The wet bulb temperature (also called cooling limit temperature) is the lowest temperature that can be achieved with direct evaporative cooling alone. The cooling effect achieved by the evaporation of water can be measured in this way and simulated under these conditions for the sweating human body.
The radiation temperature, which is also taken into account, is measured inside a thin-walled, matt-black, hollow sphere (globe), which reacts to the infrared thermal radiation of the sun.
The thermal stress can produce in all jobs exposed to high temperatures (firefighters, workers in blast furnaces, manual excavation, and work at height) a problem of fatal danger for workers. By using the corresponding heat stress meters and measuring heat stress, you can detect hazards in time and optimally coordinate working conditions.
WGBT meter
Keep in mind that these values can vary slightly depending on different sources and regions. It’s always important to check the specific heat index chart for your location, as it may account for local weather conditions and factors. This will help you determine how to protect yourself from heat-related illnesses during hot and humid conditions.
A WBGT meter is a device used to measure the Wet Bulb Globe Temperature (WBGT), which is a measure of the heat stress in an environment, taking into account air temperature, humidity, and radiant heat. WBGT meters are commonly used in workplaces, sports events, and military operations to assess the risk of heat-related illnesses and take appropriate measures to prevent them.
The WBGT meter typically consists of a thermometer, a hygrometer, and a globe thermometer, which measures radiant heat. The device calculates the WBGT value based on the readings from these sensors and provides a numerical value that indicates the level of heat stress in the environment.
WBGT Chart
The WBGT value is commonly used to determine if it is safe to work or exercise in a particular environment. For example, a WBGT value of 30 °C is considered safe for light work or exercise, while a WBGT value of 31°C is considered unsafe for even moderate work or exercise, and may require additional safety measures such as frequent breaks, hydration, and rest in a cool environment.
The threshold limit values (TLVs) of the ACGIH (American Conference of Governmental Industrial Hygienists), based on ISO 7243, to evaluate the thermal environment, into account the WBGT Index (Wet Bulb Globe Temperature), and calculated from the humid temperature, globe temperature and dry temperature.
The following chart was developed by SABIC (Saudi Arabia).
What is thermal stress?
The phenomenon of thermal stress is not exclusively linked to the sphere of the human being, but also to the animal field in stables (large feedlots) and the plant field. Sometimes small changes in environmental conditions produce both thermal stress and thermal comfort.
The World Health Organization (WHO) states that “health not only implies the absence of disease, on the contrary, it corresponds to a state of physical, mental, social and spiritual well-being” in all environments in which people are found.
Heat is one of the environmental and physical factors that can most affect the world of work, especially at certain times of the year. The most common is that workers who are exposed to heat cause discomfort, but sometimes if the conditions are extreme, the discomfort becomes immediately dangerous to life and health.
Many jobs require working in hot environments, both indoors and outdoors. Working in the heat and strenuous physical exertion can affect the body’s cooling system. If it cannot cool itself, the worker may experience heat stress. If it is not recognized and treated early on, more serious and even fatal conditions can develop quite quickly.
The effects of working in direct sunlight at various temperatures and the precautions to be taken have been determined, and developed by SABIC (Saudi Arabia) as follows:
Above mention work/rest period is minimal. You may plan for more rest but not less. Fluid recommendations are just general guidelines of SABIC. Workers can take more fluid but not less.
Excessive heat warning
Human beings are warm-blooded, homoeothermic, that is, we maintain our core temperature around 37ºC, in such a way that the body is capable of cooling itself when its temperature rises. The heat load that workers receive and accumulate in their bodies is the result of the interaction between environmental conditions, physical and metabolic activity, and the clothing they wear.
To try to eliminate excess heat, the body’s thermoregulation mechanisms are put into operation, whose centre is in the brain. This physiological thermoregulation is characterized by the fact that workers begin to sweat (as sweat evaporates from the skin, it cools) and, in addition, blood flow to the skin increases (peripheral vasodilation) to carry heat from inside the body to the outside.
Its surface and from here the heat is expelled to the outside. This is accompanied by an increase in circulating blood volume and heart rate. If despite the implementation of the physiological mechanisms of adaptation to heat, workers continue to work in hot conditions and accumulate it, the core body temperature can exceed 38ºC, and the worker suffers heat stress.
Health problems derived from heat stress are known as heat disorders, the severity of which will be related to the amount of heat accumulated in the body. When you work in hot conditions, for a long time and without taking breaks, there comes a time when you are so hot.
You experience a loss of motivation towards the activity, decreased quality of work, concentration, exhaustion, dehydration, syncope and heat stroke among others, which in addition to affecting the health of workers, increase the occurrence of work accidents and therefore absenteeism in companies.
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Heat stress index interpretation (according to SABIC)
|
Category
1 (Normal) |
|||
|
DUTY |
LIGHT |
MODERATE |
HEAVY |
|
30 WBGT |
26.7 WBGT |
25 WBGT |
|
|
Heat stress illness |
Fatigue and light
sweating |
||
|
Recommended action
recommended |
·
Monitor yourself and co-workers for signs of heat stress and
allow workers to set their own pace of work ·
If possible block direct sunlight and insulate heat sources
|
||
|
water intake |
1 Cup (250 ml) every 30
minutes |
||
|
Category
2 (CAUTION) |
|||
|
DUTY |
LIGHT |
MODERATE |
HEAVY |
|
36 WBGT |
28 WBGT |
25.9 WBGT |
|
|
Heat stress illness |
Heat cramps, Headaches, Vomiting,
Thirst |
||
|
Recommended action
recommended |
·
Wear lightweight, light-coloured and loose-fitting clothes.
·
Allow
sufficient acclimatization period before full workload for
new employees. · Rest in a shaded and cool area, away from direct sunlight. Avoid caffeinated
drinks or heavy meals. ·
|
||
|
water intake |
1 cup (250 ml) every 20
minutes |
||
|
Category 3 (DANGER) |
|||
|
DUTY |
LIGHT |
MODERATE |
HEAVY |
|
31.4 WBGT |
29.4 WBGT |
27.9 WBGT |
|
|
Heat stress illness |
Heat stroke, exhaustion,
heavy sweating, dizziness, fainting |
||
|
Recommended action
recommended |
·
Wear lightweight, light-coloured and loose-fitting clothes.
·
Allow
sufficient acclimatization period before full workload for
new employees. · Rest in a shaded and cool area, away from direct sunlight. ·
Avoid
caffeinated drinks or heavy meals. |
||
|
water intake |
1 cup (250 ml) every 15
minutes |
||
|
Category 4 (EXTREME DANGER) |
|||
|
DUTY |
LIGHT |
MODERATE |
HEAVY |
|
32.2 WBGT |
29.4 WBGT |
27.9 WBGT |
|
|
Heat stress illness |
Heat stroke, no sweating,
mental confusion, unconsciousness |
||
|
Recommended action
recommended |
Restrict moderate and heavy outdoor activities |
||
|
water intake |
1 cup (250 ml) every 10
minutes |
||
Work-rest regimen and workload
Based on the results the average WBGT was measured when carrying out corresponding activities in the production area. TLV – WBGT (28°C) established by the ACGIH, for continuous moderate work and acclimatized person, it is necessary to determine a work time and a rest time for each hour of the workers.
Additional precaution under direct sunlight
Any outside work under direct sunlight must apply categories 4 interpretation and adhere to the following minimum controls:
- If possible reschedule work activities for early morning or after sunset.
- Provide cool drinking water and isotonic drinks at the workplace.
- Implementation job rotation every 15 minutes.
- Provide shaded areas with a source of air movement.
- Continuous monitor all personal heat stress symptoms
Heat stress workplace inspection checklist
| At ‘hot’ spots | YES | N/A | COMMENT |
|---|---|---|---|
| Is potable water available? | ☐ | ☐ | |
| Are supervisors trained to detect heat stress symptoms? | ☐ | ☐ | |
| Is ventilation in place and operating? | ☐ | ☐ | |
| Is air conditioning in place and in operation? | ☐ | ☐ | |
| Mist fan in place and operating? | ☐ | ☐ | |
| Liquid replacement plan? | ☐ | ☐ | |
| Work/rest schedule? | ☐ | ☐ | |
| Heat stress shelter? | ☐ | ☐ | |
| Heat stress training program? | |||
| Name of evaluator | Date | Time | Location |
| ---------------------- | -------------------- | --------------------- | ----------------------- |
CONCLUSION
Finally, the threshold limit values (TLVs) of the ACGIH (American Conference of Governmental Industrial Hygienists), will be taken into account to evaluate the thermal environment, with the WBGT index.
So, if the cases are estimated only with the WBGT index, it can be seen that the results are outside the acceptable ranges of exposure, which indicates that for the evaluated work environment, the activity can be carried out by taking breaks.
Regarding the above, it should be noted that an evaluation of the conditions of workers exposed to high temperatures allows obtaining information related to occupational exposure to such hot environments and the potentially harmful effects on health.
Recommendations
Normally heat sources transfer energy by conduction, convection or radiation. However, convection and radiation are the phenomena that create the greatest risk situations for workers. Administrative controls and work practices. Training is the key to improvement, a good training program for thermal risk is recommended, which includes:
- Make known the risks of exposure to heat.
- Recognize the predisposing factors, signs and symptoms of heat illnesses.
- Specific training in first aid to attend heat emergencies.
- Responsibility for unnecessary exposure.
- The danger of using drugs, including some therapeutics, and/or alcohol in hot environments
- Importance of using personal protective equipment.
- Rescue program and its importance.
- Workers should not be alone in conditions that can cause heat stress.
- An emergency plan will not only help us to evacuate a place at any given time but also to know how to act in adverse circumstances such as accidents caused by heat stroke.
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