Humidex Rating and Work

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What is humidex?

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Humidex is a measure of how hot we feel. It is a parameter intended for the general public to express how the combined effects of warm temperatures and humidity are perceived. It provides a number that describes how hot people feel, much in the same way the equivalent chill temperature, or "wind chill factor," describes how cold people feel.

Environment Canada uses humidex ratings to inform the general public when conditions of heat and humidity are potentially uncomfortable.

Table 1
Degree of Comfort
20-29Little discomfort
30-39Some discomfort
40-45Great discomfort; avoid exertion
46 and overDangerous; heat stroke possible

Source: Warm season weather hazards. Government of Canada

What is the importance of humidity?

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The body attempts to maintain a constant internal temperature of 37°C at all times. In hot weather, the body produces sweat, which cools the body as it evaporates. As the humidity or moisture content in the air increases, sweat does not evaporate as readily. Sweat evaporation stops entirely when the relative humidity reaches about 90 percent. Under these circumstances, the body temperature rises and may cause illness.

What are some of the hazards of working in hot environments?

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There are several common heat-related illnesses. Some are more severe than others.

Heat rash, or prickly heat, occurs when blocked sweat glands become inflamed. This painful rash reduces the body's ability to sweat and tolerate heat.

Heat cramps are painful spasms of the muscles. The muscles used in doing the work are most susceptible. The spasms are caused by the failure of the body to replace its lost body salts and usually occur after heavy sweating.

Heat exhaustion results when the body loses large amounts of fluid by sweating during work in hot environments. The skin becomes cool and clammy. Symptoms include profuse sweating, weakness, dizziness, nausea, and headaches.

Heat stroke is the most serious condition and requires immediate medical attention. The body temperature becomes very high (even exceeding 41°C). Complete or partial loss of consciousness is possible. Sweating is not a good symptom of heat stress as there are two types of heat stroke – "classical," where there is little or no sweating (usually occurs in children, persons who are chronically ill, and the elderly), and "exertional" where the body temperature rises because of strenuous exercise or work and sweating is usually present.

What index should workplaces use to monitor conditions that may result in heat-related illness?

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Occupational (Industrial) hygienists recommend using the Wet Bulb Globe Temperature (WBGT) index to measure workplace conditions. This method closely relates to the human body's response to heat.

The WBGT measurement takes into account air temperature, air movement, radiant heat and humidity. There are direct-reading WBGT meters. These are also called "heat-stress indicators," commercially available. The WBGT measurements can then be related to the physical demands of the job. Only qualified professionals, whether they be in-house staff, consultants, or from the local occupational health and safety regulatory agency, should perform the measurement.

Direct comparison between WBGT and humidex is not possible--there are no conversion tables or mathematical formulas to do such conversions. However, one can estimate WBGT and humidex for a given ambient air temperature and humidity when radiant heat sources (hot and cold surfaces) are absent, and air movement is less than 0.5 m/sec (100 feet per minute). Under these conditions, the globe temperature equals room temperature. The natural wet bulb temperature (on the WBGT apparatus) is approximately 2°F (1.1°C) higher than the wet bulb temperature measured using a psychrometer.

Standard charts are available to determine wet bulb temperature from given air temperature and relative humidity values. For indoor or outdoor conditions with no direct sunlight, WBGT is calculated by using the following formula:

WBGT = 0.3 x globe temperature + 0.7 x natural wet bulb temperature

Can workplaces use humidex to monitor conditions that may result in heat-related illness?

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Humidex, as reported by weather forecasters, is intended for the general public to express the combined effects of warm temperatures and humidity.

Heat-related illnesses depend on many workplace factors, in addition to air temperature and humidity. Wind speed or air movement, workload, radiant heat sources, and a person's physical condition are also important.

Under certain workplace conditions, the humidex may serve as an indicator of discomfort resulting from occupational exposure to heat.

For example, when humidity is high, but when workload, wind speed, and radiant heat sources do not significantly contribute to the heat burden, humidex may be useful. Offices are typical of workplaces where humidex could be used. It is important to use the values of the temperature and relative humidity obtained by actual measurements taken in the workplace. Conditions inside the workplace may significantly differ from those given by the Weather Service.

How do I know what the humidex is?

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If you know the temperature and relative humidity, the humidex table from the Government of Canada can be used to determine the humidex rating. For example, if the temperature is 30°C and the relative humidity is 70%, the humidex rating is 41. This level is considered a level of "great discomfort" and exertion should be avoided.

As noted by the Government of Canada:

"An extremely high Humidex reading is any reading over 40. In such conditions, you should reduce all unnecessary physical activity. If the reading is in the mid to high 30s, then you should tone down or modify certain types of outdoor exercise, depending on the individual age and health, physical shape, the type of clothes worn and other weather conditions.

If working outdoors is an absolute necessity, drink plenty of liquids and take frequent rest breaks. In hot, humid conditions, there is a considerable risk of heat stroke and sunstroke."

How is humidex interpreted?

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The relationship between humidex and comfort is subjective. It varies widely between individuals.

Workplaces must use caution when applying the humidex rating. A high humidex rating can serve as a cue to assess workplace conditions more precisely.

For more information, please see the OSH Answers fact sheets on:

The Occupational Health Clinics for Ontario Workers Inc. (OHCOW) created a humidex-based response plan that translates the WBGT values into humidex values and provides recommended responses for each humidex range. This plan was developed as a tool to help workplaces that may not need to use WBGT values because work load, wind speed, and radiant heat sources do not significantly contribute to the heat burden.

While technically there is no way to directly compare WBGT and humidex values, this humidex response plan provides an additional guideline that uses information that is easily available to most employers. OHCOW notes, "in the translation process some simplifications and assumptions have been made; therefore, the plan may not be applicable in all circumstances and/or workplaces (follow steps 1 through 5 to ensure the humidex plan is appropriate for your workplace)." This plan is available on the OHCOW website:  Humidex-Based Heat Stress Calculator and Plan

See Table 2 for details.

Notes: These humidex levels are for unacclimatized workers performing moderate physical activity. The American Conference of Governmental Industrial Hygienists (ACGIH) specifies an action limit and a Threshold Limit Value (TLV®) to prevent workers' body temperature from exceeding 38°C (38.5°C for acclimatized workers). Below the action limit (Humidex 1), most workers will not experience heat stress. Most healthy, well-hydrated, acclimatized workers not on medications will be able to tolerate heat stress up to the TLV® (Humidex 2). Between Humidex 1 and Humidex 2, general heat stress controls are needed, and above Humidex 2, job-specific controls are needed.

Table 2 
Recommended Actions Based on the Humidex Reading
Table 3 - Recommended Actions Based on the Humidex Reading

IMPORTANT: Consult with the OHCOW humidex-based response plan for how to interpret and use this chart. ALWAYS follow steps 1 to 5 as listed in this plan. Also, see the Humidex-based Heat Stress Calculator.

  • Fact sheet last revised: 2023-07-11