Heat exposure at work can be rapidly fatal and a complex hazard to protect against, but basic knowledge and key work practices – Rest, Shade and Water – can easily save lives if employers and workers collaborate smartly. Workers who are at highest risk of heat exhaustion, stroke and death are typically new to the job and not acclimatized to heat stress at work. Acclimatization to hot work takes about at least 1-2 weeks of gradual daily exposure. It helps the body to sweat more efficiently while losing fewer electrolytes (sodium and potassium salts).

To remain healthy, humans must maintain an internal (core body) temperature between 36.0-38.0^oC. Heat stress at work leads to heat-related illnesses (HRI), which begin to occur at core body 38.0^oC and above, leading rapidly to heat symptoms of dehydration, skin rash, muscle cramps, fainting (syncope) and exhaustion, the latter with profuse sweating, nausea, headache and weakness.

Too hot to work

While some workers are more vulnerable, even young healthy workers can be at great risk of acute and chronic heat injury. All stakeholders are challenged by and vulnerable to heat: workers and their families, employers, community leaders, local ecological and food systems, and local health service capabilities (clinics and hospitals).

Early HRI symptoms can often be managed at the work site, but heat exhaustion can quickly evolve to life-threatening heat stroke with core body temperature at and above 39.0^oC, typically with a change in mental status, rapid heart rate, loss of sweating, and collapse. Rest, rapid cooling and fluids is critical first aid for early symptoms leading to heat exhaustion, but heat stroke is an immediate medical emergency requiring external cooling with cold packs and emergency hospitalization. Workers and managers need to know how to recognize HRI symptoms and workers must be empowered to protect themselves and co-workers. Each worksite should implement a heat protection program to prevent HRI and maintain productivity safely.

The physiology of heat tolerance is a balance between the worker’s ability to maintain normal core body temperature in settings of heat stress. Heat stress is a combination of internal body (metabolic) heat generated by level of activity (or workload) coupled with external heat exposure (to air temperature, humidity, radiant heat and air movement), wherein ability to maintain normal core body temperature can easily become overwhelmed. In other words, overload of internal heat generation and external heat combine to surpass a worker’s ability to eliminate internal heat through the normal ways of sweating and skin vessel dilation to dissipate heat through evaporation, convection and (infrared) radiation. If fluid intake is inadequate, sweating ability is diminished.

Muscle breakdown also occurs with elevated core body temperature (hyperthermia), leading to increased blood protein that, together with dehydration, can overpower kidney function resulting in acute or chronic kidney insufficiency and possible failure. Muscle cramps are symptoms of muscle breakdown in hot work, and an early warning to take a rest and fluid break. Salt loss from excessive sweating poses threats to critical organ functions. Exposure to radiant heat (sunlight or hot equipment) greatly lowers the ability to tolerate heat for any given temperature, thus the importance of shade and cooler air.

Workers more prone to risk include any who lack heat protection programs especially when doing moderate to heavy workload jobs. Also at greater risk of HRI and injury are workers who are young or older, unacclimatized to heat, pregnant, or who have certain pre-existing medical conditions (asthma, lung or heart conditions, diabetes, malnourished or significantly overweight, sleep-deprived, seizure disorder, cancer, chronic pain, psychiatric conditions, using certain medications, or dependence on alcohol or illicit drugs). Women are slightly more prone to heat stress than men in general due to subtle differences in physiology, but women workers are often more at risk of heat exposure due to the types of jobs they do and because many often work overtime and second jobs to survive and support their children if single.

Strategizing heat safety

The best approach to protection of worker health and workplace productivity is to have an established comprehensive framework program, heat safety guidelines, and a workplace organizational effort involving all stakeholders. A collaborative worker-manager health and safety committee specifically tasked with ensuring worker heat protections can help a lot. Effectiveness depends most importantly on worker, employer and community knowledge of the basics about heat related illnesses, the financial impact of work in hot environments, and key strategies to mitigate and adapt to heat at work, both outdoors and indoors.

Heat hazards, aside from heat exposure itself, include mental fatigue and stress which impairs ability to make safe judgments about work tasks. Physical hazards include rapid and severe skin burns from contact with hot surfaces, especially if a worker collapses to the ground, as well as kidney failure due to dehydration and lack of sufficient clean cool water and food, falls from heights, power tool and moving vehicle accidents, and possibly aggressive behavior due to mood changes from heat fatigue and work demands.

Inhibitors of safe and decent work include piece-rate and pay schemes based upon work quotas, working alone, informal work, confined and semi-confined space work, and inability to take cooling rest breaks and to drink clear fluids. Indoor work can be just as hot if not hotter depending upon air circulation and methods of cooling such as use of fans. Informal work can be especially risky and include: street vendors, transport and delivery workers, waste pickers, domestic workers, migrant farm workers, and very small industry sweatshops. Urban settings are notorious for “heat islands” which are local areas that trap and concentrate temperature, humidity and radiant heat, and block sources of cooler air flow.

Local governments, employers and worker organizations are most effective when they can collaborate at the work site, and when they can rely upon regulatory or policy-driven Occupational Safety and Health (OSH) heat standards in combination with strategies that employ basic occupational health services (BOHS) at these 3 key levels of OSH prevention:

• Primary – workplace controls to eliminate or reduce heat hazards for each worker at risk;
• Secondary – worker health surveillance for early onset of heat illness symptoms; and,
• Tertiary – rapid medical care for heat illness and injuries with local emergency services.

At these prevention levels, we employ value-based knowledge, protections and services which rely on worker empowerment. It is critical to provide safety training and to give workers voice, ability to be heard, and to be trusted. Workers often have very good ideas about when protections are needed and what might help adjust to heat and to enhance productivity safely.

Key primary and secondary preventive measures include:

• Baseline exposure risk assessment and action for controls specific to the work setting, job types and workload and of each job, including other hazard exposures such as chemicals, radiant heat, clothing and any need for personal protective equipment
• Workload classification for specific jobs (there are ways to measure workload, with categories ranging from sedentary, to mild, moderate, heavy and very heavy)
• Ongoing environmental monitoring for heat level exposure, ideally measured as the WBGT (wet bulb globe temperature which accounts for air temperature, humidity, air movement and radiant heat from the sun or local heat sources), or at least the heat index (air temperature and relative humidity), or the most basic (ambient air temperature alone); and then compare the heat level to standards for work rest cycles or work cessation criteria; this typically involves hourly heat measurements and weather predictions several days in advance
• Medical fitness clearance by a doctor or nurse work clearance for heavy workloads
• Training of all workers at time job placement and periodically, as well as training of emergency first responders among the work team and site managers who are able to act quickly
• Acclimatization for moderate, heavy and very heavy workload jobs (one week of gradual exposure to the hot work tasks)
• Prompt reporting of new symptoms by workers and co-workers monitoring for early onset of hyperthermia (internal core body temperature elevation above 38.0^oC or 100.4^oF) that presents with symptoms such as profuse sweating, fatigue, stomach upset, headache, lightheadedness, and eventually mental status changes in behavior, thinking and level of consciousness; hot work should not be done alone, but rather with a buddy system on the lookout for early heat symptoms among co-workers
• Work-Rest cycles, removal from work, and stop work parameters based both on heat exposure risk and also worker symptom onset; this is best accomplished by hourly air temperature measurements and ability of managers and workers to see thermometers or heat index indicators in real-time at work
• Water (clean cool), shade and cooler rest areas away from radiant heat at the job site are key protections
• Partnership with local medical emergency and fire services for rapid response when needed 
• Root-cause investigation of heat illness and injury events
• Precautionary rules that consider if one worker begins to get heat exhaustion symptoms, other workers may soon follow, and that increased work protection or work stoppage should occur, and zero tolerance for pushing the limits, delay tactics, threats and reprimands for heat symptom complaints, harassment, or aggression
• Recordkeeping and reporting of heat-related illnesses and injuries

Heat emergencies can occur within minutes to hours, or may be of gradual onset with repeat exposure. The range of heat related illnesses by diagnosis include: heat cramps, heat fainting (syncope), heat exhaustion, and heat stroke (which is rapidly fatal and requires immediate hospital care).

In every workplace, the structure and practices of work must be thoughtful for each worker and for the environment, the latter by employer practical steps to reduce greenhouse gas (GHG) emissions that contribute to climate change, and to eliminate local chemical pollution.

There are standards for measuring heat exposure, and while these may vary by country, some countries have not yet employed standards specific for heat exposure controls. As mentioned, the WBGT or heat index are, respectively, the best and second-best measures that each workplace must adopt.

Heat recordings should be kept daily in hot environments to help guide work safety, and job workloads and work practices must be aligned both to (1) heat measurements and controls and (2) early symptom monitoring.

The best controls are often low cost, as mentioned in the list above: cool water, shade, work-rest cycles, training, and early intervention when symptoms arise.

Government role in heat safety

In the future, we can expect better regulations and tools for employers and workers to better know the immediate risk and to refine protective controls. Worksite mobile applications for real-time and predictive heat index and WBGT measures, as well as wearable devices to measure a worker’s core body temperature are in evolution.

Lest there be any doubt, since 2022 all UN International Labor Organization member governments and their employers are obliged to provide worker health protections in conformance with the OSH ILO Conventions 155 and 187, regardless of whether the member country has adopted and ratified them. It is mandatory, as a fundamental principle and right at work, to protect workers against dangerous heat stress and related illnesses and injuries. Trade unions can file complaints directly with the ILO in Geneva.

Protecting workers from heat stress

Research has found a strong association between thermal stress due to global warming and occupational heat stress, so heat stress at work is a major challenge for work safety in future

Research has found a strong association between thermal stress due to global warming and occupational heat stress, so heat stress at work is a major challenge for work safety in future

• Personal, including physiological, health status, hydration, acclimatization
• Environmental – high level heat (WBGT 37.5-49^oC)
• Occupational barriers at work

3 protective factors combine to prevent HRI and improve resilience:

• Personal
• Managerial
• Engineering

Some key protections:

• Worker-Manager OSH committee dedicated to hot work protection
• Volunteer first responders with first aid and BLS training
• Written heat stress protection OSH plan
• Job and process hazard analysis, including heat stress risk level
• Portable shade, rest areas, fluids
• Planning a Work-Rest Cycle for the work shift based upon heat exposure and workload
• Ventilation, cooling gear, fans, HVAC, good clothing choices 
• Medical clearance and monitoring, buddy system, acclimatization
• Emergency medical partnership with local clinic, emergency services, hospital
• Hourly monitoring of air temperature and weather forecasts
• Stop work if daily maximum temp is 40^oC+
• Taking immediate action on sentinel events
• Protecting workers from falls, skin burns, UV and direct sun and hot equipment
• Eye and skin protection
• Sharing and rotating work tasks
• Substitution of mechanical methods for manual labor
• Self-pacing that allows workers to act for self-care, rest, rehydration
• Weather recording of air temperature, heat index, WBGT, and work pacing
• Recordkeeping and program evaluation and improvement

The workload intensity classification table shows the metabolic rate for different work intensities, with associated physical task and occupation examples.

The ACGIH publication “2024 TLVs® and BEIs®” (the most current booklet) provides recommended screening criteria for heat stress exposure for workers (Table 1). The table provides guidance on Work-Rest Cycles (% work) for given workload at WBGT levels for an 8-hour shift, 5 days per week (heat-related work rest breaks in addition to conventional work breaks). Note the difference between acclimatized and unacclimatized workers.

The recommended actions based on the humidex reading according to the Occupational Health Clinics for Ontario Workers (OHCOW) are found in the table below:

How would you provide adequate hydration for work in hot environments?

• Provide cool potable water (10-15°C) easily available at the work area, and encourage 1 cup intake every 15-20 minutes.
• Encourage to salt food, but do not give salt tablets
• Do not provide salted drinking water, unless workers are not acclimatized.
• If provided, salted water should be 0.1% conc. (1g/1L, or 1 level tablespoon per 15 quarts) completely dissolved.
• Rehydration drinks can include electrolytes, typically sold as sports drinks.

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The author, Dr. Thomas H. Gassert is an Instructor in Occupational and Environmental Medicine at Harvard T.H. Chan School of Public Health, focusing on international training and health initiatives related to occupational health and safety, climate change, and global health crises. He holds a master’s in Occupational Hygiene and an MD, and resides in Portsmouth, New Hampshire.

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References:

Heat at work: Implications for safety and health. ILO, 25 July 2024.

https://www.ilo.org/publications/heat-work-implications-safety-and-health

HEAT-SHIELD Project provides guidance on developing a customized occupational heat stress Work Heat Action Plan (WHAP) which provides both short-term and longer-term heat warnings to protect worker health and productivity. 

https://www.heat-shield.eu

Climate CHIP (Climate Change Heat Impact and Prevention) is a non-profit website that provides free access to a range of information, tools and resources about heat stress and other health impacts of climate change anywhere in the world. It also features a documentary film, “Too hot to work” and a useful Workplace Heat Effects Assessment Tool “WorkHeat Tool” (Oct. 2023).

https://www.climatechip.org

https://www.climatechip.org/sites/default/files/231111_WorkHeat_guidance.pdf

https://www.climatechip.org/heat-stress-index-calculation

Hot Environments. Canadian Centre for Occupational Health and Safety

https://www.ccohs.ca/oshanswers/phys_agents/heat/max_temp.html

Heat Stress Toolkit (2024)

Heat Stress Toolkit

Heat Stress Awareness Tool – Pocket Card

Use workplace Thermometer & Hygrometer readings

https://www.ohcow.on.ca/edit/files/heatstressawareness/Heat%20Stress%20Awareness%20Tool.pdf

Heat Stress Toolkit Videos

Heat Stress Toolkit Videos

AIHA Heat Stress Mobile App (NEW – July 2024)

Real-time WBGT index and 5-day WBGT forecasts

https://synergist.aiha.org/20240607-heat-stress-mobile-app

Hong Kong: Guidance Notes on Prevention of Heat Stroke at Work. Hong Kong Labour Department, Occupational Safety & Health Branch, April 2024 (2^nd ed.); 52 pages

https://www.labour.gov.hk/common/public/oh/Heat_Stress_GN_en.pdf

Singapore: Revised framework to guide employers and protect outdoor workers against heat stress

Singapore Ministry of Manpower, 06 September 2024

https://www.mom.gov.sg/newsroom/press-releases/2024/0906-revised-framework-to-guide-employers-and-protect-outdoor-workers-against-heat-stress

Poster: HEAT STRESS MEASURES FOR OUTDOOR WORK

https://www.mom.gov.sg/-/media/mom/documents/heat-stress-measures/heat-stress-framework-outdoor-workers.pdf

US OSHA Heat Standard – Final Phase Rulemaking 2024

https://www.osha.gov/heat-exposure/rulemaking

Criteria for a Recommended Standard. Occupational Exposure to Heat and Hot Environments. US National Institute of Occupational Health and safety (NIOSH); 2016

https://www.cdc.gov/niosh/docs/2016-106/ pdfs/2016-106.pdf  

Diagnostic and exposure criteria for occupational diseases – Guidance notes for diagnosis and prevention of the diseases in the ILO List of Occupational Diseases (ILO 2022) [open source, free]; Diagnostic (ICD-10 and ICD-11) billing codes are listed for diseases caused by exposure to extreme hot temperatures, Pages 382-385

https://www.ilo.org/sites/default/files/wcmsp5/groups/public/@ed_dialogue/@lab_admin/documents/publication/wcms_836362.pdf

ACOEM Guidance Statement. Proposed Mitigation and Adaptation Strategies Related to Climate Change: Guidance for OEM Professionals (2021)

Nabeel I, Caraballo-Arias Y, Perkison WB, et al. ACOEM Work Group on Mitigation/Adaptation Strategies Related to Climate Change. J Occup Environ Med. 2021;63(9):e650-e656.

https://acoem.org/acoem/media/News-Library/Proposed_Mitigation_and_Adaptation_Strategies-25.pdf

Heat, Worker Health, Climate

Climate conditions, workplace heat and occupational health in South-East Asia in the context of climate change. WHO South-East Asia Journal of Public Health. September 2017:6(2)

https://apps.who.int/iris/bitstream/handle/10665/329616/seajph2017v6n2p15.pdf?sequence=1&isAllowed=y