The air temperature determines whether you feel hot or cold. relative humidity of the atmosphere. the presence of items that are either hot or cold in the area. the existence of air movement (breeze, ventilation). effort on the body. Clothing. These environmental elements are combined by different occupational heat exposure measurement techniques to provide a single value that represents the total heat load. Wet bulb globe temperature (WBGT) index is the measurement that is most frequently used in the workplace. Please Take Note Information on the prevention and treatment of illnesses brought on by the heat is provided in this OSH Answers document. For more on the physical impacts of working in hot situations, please visit Hot Environments - Health Effects and First Aid. The Temperature of the Wet Bulb Globe (WBGT) Using a formula that accounts for air temperature, air movement speed, radiant heat from hot objects, sunlight, and body cooling brought on by sweat evaporation, the wet bulb globe temperature is determined. A typical thermometer is used to measure the temperature of the air. A black globe thermometer is used to calculate the contribution from radiant heat. An ordinary thermometer is put into a hollow copper ball with a six-inch diameter and a flat black paint coating through a rubber stopper. At the center of the copper ball is the thermometer bulb. Usually, it takes the black globe thermometer at least 20 minutes to reach equilibrium. Using a natural wet bulb thermometer, the cooling effects of evaporation and air movement are taken into consideration. A standard thermometer with the bulb wrapped in an absorbent cotton wick is known as a natural wet bulb thermometer. The bottom end of the wick is submerged in distilled water, and it reaches 30 to 35 millimeters above the thermometer bulb. There is a wet wick exposed between the water and the thermometer's bulb for around 25 mm. Water for evaporation is continuously provided by the moist wick. The natural wet bulb thermometer takes at least 20 minutes to attain equilibrium, just like the black globe thermometer. WBGT is calculated in the workplace using two separate approaches: one for locations that receive direct sunshine and the other for locations that do not. Time-weighted WBGT is frequently employed when working conditions are highly variable. Examples of WBGT computations are provided in the question ""How do I compute the WBGT Index? "", which is below. There are additional WBGT direct reading meters that provide a direct WBGT reading without the need for computations. These meters are frequently referred to as heat stress analyzers.
What are the heat stress exposure limits?
For the majority of Canadian workplaces, exposure limits are regulated by provincial and territorial governments. For workplaces subject to federal law, they are set by Employment and Social Development Canada (ESDC). The American Conference of Governmental Industrial Hygienists' exposure (heat stress) rules are typically followed by these organizations (ACGIH). The net heat load to which a worker may be exposed as a result of the combined effects of metabolic heat, air temperature, air movement, humidity, radiant energy, and clothing is defined as heat stress by ACGIH. According to ACGIH, heat strain refers to the entire physiological reaction to heat stress. These restrictions are set by the ACGIH in WBGT (wet bulb globe temperature) degrees Celsius (°C) units. The WBGT unit takes into account environmental elements that affect how hot it feels to a person, such as air temperature, humidity, and air movement. Humidex values and WBGT values are not the same. When calculating the WBGT in specific workplace circumstances, solar load (heat from radiant sources) is also taken into account. Below you can find more information about WBGT. The most recent booklet or ACGIH publication, """"2022 TLVs® and BEIs®,"""" offers suggested screening standards for workers' exposure to heat stress (Table1). For further information on these screening criteria, classifications of work demands, suggestions for limiting heat stress, and heat stress management, check this paper and the """"Documentation of TLVs® and BEIs®"""". Table 1 ACGIH Screening Criteria for Heat Stress Exposure (WBGT values in °C for 8 hours of labor, five days a week, including traditional breaks) Work Distribution in a Work/Rest Cycle Limit of Acclimatized Action (Unacclimatized) Low Moderate High Heavy Very Light Medium Heavy Very Heavy 75-100% 31.0 28.0 -- -- 28.0 25.0 -- -- 50-75% 31.0 29.0 27.5 -- 28.5 26.0 24.0 -- 25-50% 32.0 30.0 29.0 28.0 29.5 27.0 25.5 24.5 0-25% 32.5 31.5 30.5 30.0 30.0 29.0 28.0 27.0 Notes: Assumes 8-hour workdays in a 5-day workweek with conventional breaks. TLVs assume that workers exposed to these conditions are adequately hydrated, are not taking medication, are wearing lightweight clothing (long-sleeve shirts and pants) and are in generally good health. Examples of workloads: Rest - sitting (quietly or with moderate arm movements) Light work - sitting or standing to control machines; performing light hand or arm work (e.g. using a table saw); occasional walking; driving Moderate work - walking about with moderate lifting and pushing or pulling; walking at moderate pace; e.g. scrubbing in a standing position Heavy work - pick and shovel work, digging, carrying, pushing/pulling heavy loads; walking at fast pace; e.g. carpenter sawing by hand Very Heavy - very intense activity at fast to maximum pace; e.g. shovelling wet sand Adapted from: 2022 TLVs® and BEIs® - Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices. Cincinnati: American Conference of Governmental Industrial Hygienists (ACGIH), 2022 , p. 233 .
How do I use the heat stress exposure table (Table 1)?
As an example, the wet bulb globe air temperature was measured with a WBGT direct reading meter at 27.0ºC for an 8-hour work period. The worker is not used to hot conditions (i.e., unacclimatized) and is performing moderate cleaning duties (e.g., scrubbing floors and walls). These ACGIH guidelines suggest that an unacclimatized worker can do this work for approximately 25 to 50% of an 8-hour workday. A """"rest break"""" includes other job duties. Duties involving high activity or exertion levels may not allow a person’s body to cool effectively and should be avoided during the rest period. When it is very hot, the breaks should be distributed appropriately (e.g., shorter breaks every hour) rather than working for a longer time and taking longer breaks. See below for more information about controls and acclimatization. The ACGIH exposure limits are intended to protect most workers from heat-related illnesses. The limits are higher than they would have been if they had been developed to prevent discomfort. If you are wearing heavier clothing then the exposure limit should be lowered. ACGIH recommendations for such situations are suggested in Table 2. Table 2 Correction of TLV for Clothing (Values cannot be added when wearing multiple layers) Clothing Type WBGT Correction (°C) Work clothes (long sleeve shirt and pants) 0 Cloth (woven material) coveralls 0 SMS (Spunbonded - Meltdown - Spunbonded) polypropylene coveralls + 0.5 Polyolefin coveralls + 1 Double-layer woven clothing + 3 Limited-use vapour-barrier coveralls + 11 Note: These values are not to be used for completely encapsulating suits. Coveralls assume only modest clothing is underneath, not a second layer of clothing. Adopted from: 2022 TLVs® and BEIs®: Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices. Cincinnati, Ohio: American Conference of Governmental Industrial Hygienists, 2022 p. 232 For example, an acclimatized worker wearing double-layer woven clothing doing moderate work at a WBGT of 28°C would have a corrected exposure level of 28.0 + 3 = 31°C, which would lower their allowable exposure to 0-25% work (from 75 -100% work). If the measured WBGT is 30°C, then the adjusted WBGT value would be 33°C and would be over the screening value for moderate work at a work/rest cycle of 0 – 25%. If work is done in these conditions, the employer should monitor the workers for heat strain and implement controls as needed.
What control measures can be used to reduce the effects of heat?
The risk of heat-related illnesses can be reduced by:Engineering controls to provide a cooler workplace.
Safe work practices to reduce worker exposure.
Training employees to recognize and prevent heat illnesses.
Table 3 (below) provides a summary of these controls.
Engineering controls are the most effective means of reducing excessive heat exposure. The examples which follow illustrate some engineering approaches to reducing heat exposure. Reducing Metabolic Heat Production (heat produced by the body): Automation and mechanization of tasks minimize the need for heavy physical work and the resulting buildup of body heat. Reducing the Radiant Heat Emission from Hot Surfaces: Covering hot surfaces with sheets of low emissivity material such as aluminum or paint that reduces the amount of heat radiated from this hot surface into the workplace. Insulating Hot Surfaces: Insulation reduces the heat exchange between the source of heat and the work environment. Shielding: Shields stop radiated heat from reaching workstations. Two types of shields can be used. Stainless steel, aluminum or other bright metal surfaces reflect heat back towards the source. Absorbent shields, such as water-cooled jackets made of black-surfaced aluminum, can effectively absorb and carry away heat. Ventilation and Air Conditioning: Ventilation, localized air conditioning, and cooled observation booths are commonly used to provide cool workstations. Cooled observation booths allow workers to cool down after brief periods of intense heat exposure while still allowing them to monitor equipment. Reducing the Humidity: Air conditioning, dehumidification, and elimination of open hot water baths, drains, and leaky steam valves help reduce humidity. Personal Protection Ordinary clothing provides some protection from heat radiated by surrounding hot surfaces. Specially designed heat-protective clothing is available for working in extremely hot conditions. In hot and humid workplaces, light clothing allows maximum skin exposure and efficient body cooling by sweat evaporation. Workers who move back and forth between very hot, dry indoor environments and cold winter outdoor environments find that long underwear moderates the extremes in temperatures. Eye protection which absorbs radiation is needed when the work involves very hot objects, such as molten metals and hot ovens. Work that requires the wearing of impermeable clothing presents an added heat burden as the clothing reduces the body's ability to dissipate heat. Under such circumstances, it is often necessary to reduce the exposure limit values of WBGT to levels below those appropriate for workers wearing light clothing. Table 3 Summary of Control Measures Methods of Control Actions Engineering controls Reduce body heat production Mechanize tasks. Stop exposure to radiated heat from hot objects Insulate hot surfaces. Use reflective shields, aprons, remote controls. Reduce convective heat gain Lower air temperature. Increase air speed if air temperature below 35°C. Increase ventilation. Provide cool observation booths. Increase sweat evaporation Reduce humidity. Use a fan to increase air speed (movement). Clothing Wear loose clothing that permits sweat evaporation but stops radiant heat. Use cooled protective clothing for extreme conditions. Administrative controls AcclimatizationAllow sufficient acclimatization period before full workload. Duration of workShorten exposure time and use frequent rest breaks. Rest area Provide cool (air-conditioned) rest-areas. WaterProvide cool drinking water. Pace of WorkIf practical, allow workers to set their own pace of work. First aid and medical care Develop and implement emergency procedures. Assign one person trained in first aid to each work shift. Train workers in how to recognize symptoms of heat exposure in themselves and others.
Can we become acclimatized to hot environments?
The body adapts to a new thermal environment by a process called acclimatization. Complete heat acclimatization generally takes six to seven days, but some individuals may need longer. Loss of acclimatization occurs gradually when a person is moved permanently away from a hot environment. However, a decrease in heat tolerance occurs even after a long weekend. As a result of reduced heat tolerance, it is often not advisable for anyone to work under very hot conditions on the first day of their return to work. New employees should acclimatize before assuming a full workload. It is advisable to assign about half of the normal workload to a new employee on the first day of work and gradually increased over subsequent days. Although well-trained, physically fit workers tolerate heat better than people in poor physical condition, fitness and training do not substitute for acclimatization. Some medications interfere with acclimatization. For example, hypotensives (drugs causing low blood pressure), diuretics, antispasmodics, sedatives, tranquilizers, antidepressants and amphetamines decrease the body's ability to cope with heat. Workers should seek a doctor's advice on the suitability of a medication for them if they work in hot environments. Consumption of alcohol also interferes with acclimatization.
How can I prevent heat related illnesses?
If practical, workers in hot environments should be encouraged to set their own work and rest schedules. Infrequent or irregular tasks such as emergency repairs of hot process equipment often result in heat exposure. Experienced workers can often judge heat strain and limit their exposure accordingly. Inexperienced workers may need special attention as they may continue to work beyond the point at which signs of heat strain appear. People are generally unable to notice their own heat stress-related symptoms. Their survival depends on their coworker's ability to recognize these symptoms and seek timely first aid and medical help. Salt and Fluid Supplements: A person working in a very hot environment loses water and salt through sweat. This loss should be compensated by water and salt intake. Fluid intake should equal fluid loss. On average, about one litre of water each hour may be required to replace the fluid loss. Plenty of cool (10-15°C) drinking water should be available on the job site and workers should be encouraged to drink water every 15 to 20 minutes even if they do not feel thirsty. Alcoholic drinks should NEVER be taken as alcohol dehydrates the body. An acclimatized worker loses relatively little salt in their sweat and, therefore, the salt in the normal diet is usually sufficient to maintain the electrolyte balance in the body fluids. For unacclimatized workers who may sweat continuously and repeatedly, additional salt in the food may be used. Salt tablets are not recommended because the salt does not enter the body system as fast as water or other fluids. Too much salt can cause higher body temperatures, increased thirst and nausea. Workers on salt-restricted diets should discuss the need for supplementary salt with their doctor. Sport drinks, fruit juice, etc: Drinks specially designed to replace body fluids and electrolytes may be taken but for most people, they should be used in moderation. They may be of benefit for workers who have very physically active occupations but keep in mind they may add unnecessary sugar or salt to your diet. Fruit juice or sport and electrolyte drinks, diluted to half the strength with water, is an option. Drinks with alcohol or caffeine should never be taken, as they dehydrate the body. For most people, water is the most efficient fluid for re-hydration. Emergency Action Plan: In extreme environments, an emergency plan is needed. The plan should include procedures for providing affected workers with first aid and medical care. More information is available in our OSH Answers document Extreme Hot or Cold Temperature Conditions.
How do I calculate the WBGT Index?
The wet bulb globe temperature (WBGT) is calculated by using the following equations. For outdoors with direct sun exposure: WBGT = 0.7 x Tempwet bulb + 0.2 x Tempglobe + 0.1 x Tempair For indoors or outdoors without direct sun exposure: WBGT = 0.7 x Tempwet bulb + 0.3 x Tempglobe where: Tempwet bulb = natural wet bulb temperature measured by using a thermometer whose bulb is covered with wet cotton cloth and is cooled by the natural air movement Tempglobe = temperature measured using a black globe thermometer Tempair = temperature measured using a conventional thermometer All temperatures are to be expressed in °C. Example Workers employed in an outdoor workplace with direct exposure to the sun. Measurement of workplace conditions produced the following results. Tempwet bulb = 24°C Tempglobe = 42°C Tempair = 40°C WBGT = 0.7 x 24 + 0.2 x 42 + 0.1 x 40 = 29.2°C Time-Weighted Average (TWA) When thermal conditions of the workplace fluctuate widely, time-weighted average (TWA) WBGT is used to assess heat exposure. WBGT1,WBGT2, etc. = the wet bulb globe temperatures measured or calculatedt1, t2, etc. = the elapsed time spent in the corresponding conditions described by WBGT1, WBGT2, etc., respectively. Example Measurement and/or calculation of WBGT during a two-hour job produced the following results. Exposure duration (hours) WBGT (°C) 0.5 25 1.0 27 0.5 28 These data would yield the following time-weighted average."""