Heat Stress in Construction

Construction workers exposed to hot environments or extreme heat can be at risk of heat-related illnesses (HRIs) and injuries. Heat stress is the combination of a worker’s exposure to heat from physical activity, environmental factors, and their clothing which results in an increase in the body’s heat storage, known as the net heat load [NIOSH 2016]. Heat strain is the physiological response to heat stress when the body tries to increase heat loss to the environment in order to maintain a stable body temperature [NIOSH 2016]. Core body temperature must be maintained within 1°C (1.8°F) of normal (about 37°C or 98.6°F) in order to continue to function normally [NIOSH 2016]. Factors that influence our ability to maintain a normal core body temperature include air temperature, humidity, skin temperature, the speed and temperature of air moving over the body, radiant temperature (e.g., working in direct sunlight), clothing type and amount, hydration, and other individual physical and medical characteristics [NIOSH 2016]. Heat stress can lead to unrelieved heat strain, which in turn can increase the risk for HRIs. HRIs include heat stroke, heat exhaustion, fainting, heat cramps, and heat rash. Heat stroke can occur in two forms, classic and exertional. While both can occur in hot environments, exertional heat stroke can happen in the absence of a hot environment, such as when working hard in the winter while wearing protective clothing that doesn’t allow the body’s heat to dissipate adequately. Sweating, while usually absent in classic heat stroke, is often present in exertional heat stroke [NIOSH 2016].

Construction work can be very labor intensive which can cause the body to generate excessive heat within the body. Construction workers often work outdoors during the hottest times of the year. Some construction work occurs in non-climate-controlled spaces, such as attics and crawlspaces, or in direct sunlight on roofs, roadways, and runways. Other construction jobs may expose workers to heat sources that are part of the work process, such as welding and cutting torches or hot asphalt contained in roofing kettles and paving machines. All of these factors can place construction workers at an increased risk for HRIs.

The following should be considered by employers and safety and health professionals when taking steps to protect construction workers from the adverse health effects of working in the heat:

• engineering and work practice controls,
• training and acclimatization (allowing the body to gradually adjust to the heat),
• measuring and assessing heat stress,
• medical monitor­ing and heat-protective clothing and personal protective equipment (PPE).

Underlying health conditions, age, lack of acclimatization to heat, pregnancy, restricted fluid intake, some over the counter and prescription medications, and previous HRIs are all factors that can put some construction workers at risk of HRIs [NIOSH 2016]. One study noted that a high proportion of heat-related fatalities occurred during the first days on the job, emphasizing the importance of acclimatization [Arbury et al. 2014]. Susceptibility to HRIs may also be higher among workers who live in homes that lack cooling or have high humidity [Quandt et al. 2013] because it reduces their ability to recover from the hot day at night. Engineering controls for heat such as shielding, barriers, insulating hot processes, tempered ventilation, fans, and eliminating steam leaks and other sources of humidity are not always feasible on construction sites.

Construction Industry Heat Stress Fatalities and Injuries

Construction workers are at risk of death, injuries, illness, and reduced productivity resulting from heat exposure on the job. Between 1992 and 2016, 285 construction workers died from heat-related causes, more than a third of all U.S. occupational deaths from heat exposure [Dong et al. 2019]. It is possible that heat-related deaths were undercounted due to misclassification. Heat-related deaths have had an upward trend that corresponds with an increase in average summer temperatures during the same time period. Approximately 75% of these fatalities occurred during the summer months of June, July, and August [Dong et al. 2019]. Non-fatal HRIs are more common and may not be captured in surveillance data, as they may often go unreported. For example, from 2008–2010, data from North Carolina showed that among patients aged 19–45 years, occupational HRIs resulted in more trips to the emergency room than any other work-related cause [Arbury et al. 2014]. An examination of Washington state workers’ compensation claim data from 2006 to 2017 found that the largest number of accepted HRI claims (but not the highest rate) was in the construction industry, and identified a strong correlation between a higher rate of HRI in the third quarter and hotter summers [Hesketh et al. 2020].

Injuries to construction workers on hot job sites can occur from impaired thinking; dizziness; sweaty, slippery hands; slowed response time; muscle fatigue and cramping; and clouded eyewear that blocks vision [Calkins et al. 2019, NIOSH 2016]. A study of heat-related injuries among outdoor construction workers in the state of Washington found a 0.5% increase in the chances of experiencing traumatic injuries per 1 °C increase in maximum daily humidex (a measure of how hot it feels that combines temperature and humidity) [Calkins et al. 2019]. As the planet warms and the construction workforce ages, it is likely that heat exposure and the risk of HRIs can be expected to grow [NIOSH 2016]. Except for 1998, 19 of the 20 warmest years all have occurred since 2001 [NASA 2020].

Regulations and Exposure Limits

While OSHA does not have a heat stress standard, employers can be cited under the general duty clause, related standards, and state specific standards in California, Washington, and Minnesota [see https://www.osha.gov/SLTC/heatstress/standards.html for more information]. In 2016, NIOSH published revised criteria for a recommended standard for occupational heat stress that includes recommendations employers should use to prevent HRIs [NIOSH 2016]. While not a mandate, it is appropriate guidance that can be successfully used to prevent occupational HRIs.

Role of Clothing

Clothing, which can protect against exposure to the sun can also limit or prevent sweat evaporation and convective cooling [Davis and Bishop 2013]. In general, the thicker or less permeable the clothing is, the more it hampers heat exchange [NIOSH 2016]. Cool, dry air moving freely over exposed skin effectively removes heat from the body and some types of clothing can interfere with that mechanism, even in cool ambient conditions [ACGIH 2017]. Clothing and personal protective equipment (PPE) such as impermeable coveralls, can also reduce sweat evaporation, and clothing and PPE can create microenvironments that trap heat close to the skin. Adjustments to exposure limits and work-rest schedules are required based on the type of clothing and PPE worn and the physical demands of the work to be performed. Employers must recognize this and take appropriate steps to address those circumstances on their worksites.

NIOSH has traditionally encouraged construction workers to wear clothing made of breathable fabrics like cotton and avoid wearing non-breathable synthetic fabrics [NIOSH 2010]. Synthetic sports apparel has become popular in recent years and is often advertised to keep athletes cooler, drier, and more comfortable. A review article of fourteen studies of clothing worn during exercise in hot environments found that while a few studies showed that sports textiles (synthetic fabrics and blends) were more comfortable when exercising, most studies found no difference in thermoregulation or comfort between synthetic and natural fabrics [Davis and Bishop 2013]. The authors recommended additional research on clothing construction, thermoregulation, and comfort [Davis and Bishop 2013]. We have received questions asking if microfiber materials are better than cotton for preventing heat-related illness. While athletic apparel technology helps the wearer feel more comfortable, research has not identified it as an effective control for heat stress.

Recommendations for Employers

See the NIOSH Criteria for a Recommended Standard: Occupational Exposure to Heat and Hot Environments for a complete list of recommendations for preventing HRIs. Key prevention measures are highlighted below.

Assess the risk –

• Use the Wet Bulb Globe Temperature Index (WBGT) and occupational exposure limits to protect workers. Small businesses that do not have access to WBGT measurement devices may choose to calculate the heat index of their outdoor work environment by using the OSHA-NIOSH Heat App or the NOAA heat index chart.
• Screen workers for heat intolerance:
  • Identify previous HRIs, low fitness and other factors that can reduce workers’ ability to tolerate physical activity in hot environments.
  • Educate workers to consult their healthcare provider about medications and underlying medical conditions that may affect their ability to tolerate heat and physical activity.

Limit exposure –

• Ensure and encourage workers to take appropriate rest breaks to cool down and hydrate.
• Encourage workers to wear light-colored, loose fitting, breathable clothing [NIOSH 2018]
• Schedule hot jobs for the cooler part of the day, and/or schedule them for alternate (not successive) days.[1]
• When possible, schedule hot routine maintenance work for cooler times of the year.
• Provide cool shaded or air-conditioned areas for rest and recovery.
• Provide a large container (a cooler, large bucket, or plastic tub) of cool (10‐20°C or 50-68°F) water for workers to immerse their hands and forearms, which reduces skin and core temperature [U.S. Army 2016, DeGroot et al. 2015, Barr et al. 2011]. Replace the water when its temperature exceeds 27°C or 80°F.
• Add extra workers to the crew to reduce heat exposure to each crew member and allow the job to continue while some crewmembers rest.
• Require a worker to stop working when they feel heat-related discomfort.
• Assign new and unacclimatized workers lighter work and longer, more frequent rest periods.
• Modify work/rest schedules to allow more rest time. Shorten work periods and increase rest periods:
  • As temperature, humidity, and sunshine increase.
  • When there is no air movement.
  • If protective clothing or equipment is worn.
  • For more physically demanding work.

Reduce metabolic heat load –

• Mechanize the physical parts of the work (e.g., use a backhoe instead of picks and shovels).
• Increase the size of the crew to reduce the workload on each worker.
• Reduce the time worked (shorter workdays, more rest time, restrict double shifts).
• Conduct prefabrication tasks in a temperature-controlled environment when possible.

Enhance heat tolerance –

• Develop a plan to acclimatize workers to heat.
• Gradually increase the time spent in hot environments over 7-14 days.
• New workers should be limited to 20% of the time in the heat on the first day and no more than a 20% increase of time in the heat on each subsequent day.
• Experienced workers should be limited to 50% heat exposure on the first day, 60% on day 2, 80% on day 3. Beginning on the fourth day, full-time work in the heat is permitted. Remember to use the WBGT, or other tool to establish work/rest schedules in excessive heat for even acclimatized workers.
• Closely supervise new employees for the first 14 days or until they are fully acclimatized.
• Recognize that acclimatization can be lost in a few days away from work.

Encourage hydration-

• Estimate how much water will be needed and decide who will obtain and check on water supplies.

• Workers in heat <2 hours and involved in moderate work activities should drink 1 cup (8 oz.) of water every 15–20 minutes, but during prolonged sweating lasting several hours, they should drink sports drinks containing balanced electrolytes.
• Individual, not communal, drinking cups should be provided.
• Provide adequate, convenient toilet facilities so workers do not avoid hydration to delay bathroom use.

Create a heat alert program –

• Form a Heat Alert Committee each year by mid-April that includes representatives of management, labor, and a qualified healthcare provider or health and safety professional. Small businesses may not have the staff or resources to form a committee but may still benefit from the steps below.
• Train everyone involved in the program on the steps to take when the National Weather Service issues an Excessive Heat Watch. Focus on injury and illness prevention, recognition of symptoms of HRIs, and first-aid procedures.
• Instruct managers and supervisors, in writing, to ensure that adequate supplies of cool liquids, first-aid supplies and equipment (such as ice packs and iced bedsheets, or a child’s wading pool that can be quickly filled with cool water), and cool rest areas are ready at each site.
• Have procedures in place to deal with a heat alert such as postponing non-urgent work, increasing crew size at each site; increasing rest times; reminding workers to hydrate; monitoring heat at each site, including work and rest areas; monitoring workers’ core temperatures; taking additional precautions on the first day of a shift change to account for loss of acclimatization; sending workers who show signs of HRIs for medical evaluation and requiring written permission to return to work; restricting overtime; and eliminating piecework incentives.

Know the symptoms of these HRIs and the steps to take when they occur [NIOSH 2010, U.S. Army 2018].

• Heat stroke is a Medical Emergency! It can be fatal or cause permanent disability. Signs and symptoms of heat stroke include high body temperature; confusion; loss of coordination; hot, dry skin or profuse sweating; throbbing headache; and seizures or coma. Dial 911. Move the worker to a cool, shaded area. Cool the worker quickly with a cold water or ice bath if possible. Remove their outer clothing and apply iced bedsheets or cooling packs to their chest, armpits, and groin. Continue cooling the worker until EMS arrives, unless the worker is shivering.
• Heat exhaustion is the body’s response to excessive dehydration and loss of electrolytes and can quickly progress to heat stroke. Signs and symptoms include a rapid heart rate; excessive sweating; extreme weakness or fatigue; dizziness; nausea, vomiting; irritability; rapid, shallow breathing; and a slightly elevated body temperature. Move the worker to rest in a cool area. Loosen their clothing. Encourage them to drink plenty of water or other cool beverages. If facilities are available, allow them to take a cool shower, bath, or sponge bath. Call 911 if the worker’s condition worsens or if there is no improvement within 15 minutes.
• Heat cramps affect workers who sweat a lot during strenuous activity. Symptoms of heat cramps include muscle cramps, pain, or spasms in the abdomen, arms or legs. Have the affected worker stop all activity and sit in a cool place. Encourage them to drink clear juice or a sports beverage, or drink water with food. Avoid salt tablets. Do not allow the worker to engage in strenuous work for a few hours after the cramps subside. Seek medical attention if the worker has heart problems, is on a low-sodium diet, or if their cramps do not subside within one hour.

Following the steps above can help ensure that workers remain safe while working in hot environments. Please share with us how your workplace has worked to prevent heat-related illnesses.

CAPT Alan Echt, DrPH, CIH, is a Senior Industrial Hygienist in the NIOSH Office of Construction Safety and Health.

Scott Earnest, PhD, PE, CSP, is the Director for the NIOSH Office of Construction Safety and Health, Coordinator for the Construction Sector.

CDR Elizabeth Garza, MPH, CPH, is Assistant Coordinator for the Construction Sector in the NIOSH Office of Construction Safety and Health.

Christina Socias-Morales, DrPH, is a Research Epidemiologist in the NIOSH Office of Construction Safety and Health

This work is a product of the NORA Construction Council. The authors wish to thank reviewers at NIOSH and CPWR – the Center for Construction Research and Training for their comments

[1] Adjusting shift start/stop times (i.e., work from 6am-2:30pm instead of 7:30am-4pm) can protect workers from heat stress, but this adjustment can affect sleep. Sleep deprivation is a risk factor associated with traumatic injuries and driver fatigue. Local noise ordinances and coordinating the work of multiple trades can also make this difficult.

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