Temporary Worker Initiative

Exposure to Outdoor and Indoor Heat‐Related Hazards

This is part of a series of guidance documents developed under the 

Occupational Safety and Health Administration’s (OSHA’s) Temporary Worker 

Initiative (TWI). This Initiative focuses on compliance with safety and health 

requirements when temporary workers are employed under the joint 

employment of a staffing agency and a host employer.

Introduction

Temporary workers are entitled to the same 

protections under the Occupational Safety and 

Health Act of 1970 (the OSH Act) as all other 

covered workers. When a staffing agency supplies 

temporary workers to a business, typically, the 

staffing agency and the staffing firm client, 

commonly referred to as the host employer, are 

joint employers of those workers. Both employers 

are responsible for determining the conditions of 

employment and complying with the law. In these 

joint employment situations, there are common 

questions regarding how each employer can fulfill 

its duty to comply with the OSH Act and other 

applicable standards. This bulletin addresses what 

both the staffing agency and the host employer can 

do to ensure that temporary workers in high-hazard 

industries are protected from heat-related hazards. 

Dangers of Exposure to Heat Hazards

Exposure to heat hazards both outdoors and 

indoors could lead to serious illness, injury, 

or death. Heat-related illnesses and injuries 

can happen at varying ambient temperatures, 

especially in cases where workers are not 

acclimated, perform moderate or higher physical 

activity, or wear heavy or bulky clothing or 

equipment, including personal protective 

equipment. Heat-related illnesses and injuries also 

generally occur when body heat generated by 

physical work is performed in conditions of high 

ambient heat, especially when combined with 

humidity and inadequate cooling. See Appendix A

for a list of serious heat-related illnesses, along 

with common signs and symptoms of exposure.1

Heat-related fatalities have occurred with a heat 

index below 80°F, particularly when aggravating 

factors are present, and are usually the result of 

exertional heat stroke, where physical activity in 

hot environments causes the body temperature to 

reach 104°F or higher (normal body temperature is 

around 98.6°F).

Heat Index

The National Weather Service (NWS) uses a heat 

index (HI) to classify environmental heat into four 

categories: 

y Caution (80°F – 90°F HI); 

y Extreme Caution (91°F – 103°F HI); 

y Danger (103°F – 124°F HI); and, 

y Extreme Danger (126°F or higher HI). 

The HI, sometimes referred to as the apparent 

temperature, is given in degrees Fahrenheit 

and is a measure of how hot it really feels when 

relative humidity is factored in with the actual air 

1. Appendix F in the Heat NEP.

TWI BULLETIN NO. 12
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temperature.2 When the HI is 80°F or higher, serious 

occupational heat-related illnesses and injuries 

become more frequent, especially in workplaces 

where unacclimatized workers are performing 

strenuous work (e.g., intense arm and back/lifting 

work, carrying, shoveling, manual sawing, pushing 

and pulling heavy loads, and walking at a fast 

pace), without easy access to cool water, or cool/

shaded areas, when working in direct sunlight or 

areas where other radiant heat sources are present. 

The OSHA-NIOSH Heat Safety Tool App is a 

resource for finding the forecasted and current heat 

index near your location. The Heat App indicates 

the hazard levels using the HI as: 

y Caution less than 80°F HI; 

y Warning 80°F to 94°F HI; and,

y Danger at 95°F HI or higher. 

Currently OSHA does not have a specific standard 

addressing heat-related hazards. However, the 

agency recently issued an Exposure to Outdoor 

and Indoor Heat-Related Hazards National 

Emphasis Program (NEP) to protect workers from 

such hazards. The Heat NEP outlines policies 

and procedures to identify and eliminate or 

reduce worker exposures to occupational heatrelated illnesses and injuries in general industry, 

construction, maritime, and agriculture. It targets 

specific high-hazard industries expected to have 

the highest exposures to heat-related hazards and 

resulting illnesses and deaths. On any day that 

the NWS announces a heat warning or advisory 

for the local area, compliance safety and health 

Officers (CSHOs) will conduct on-site inspections 

at these targeted high-hazard industries. Likewise, 

on a heat priority day, a day when the heat index 

for the day is expected to be 80°F or more, CSHOs 

will provide compliance assistance to employers 

when conducting on-site inspections, even if the 

establishment is not on the high-hazard industries 

list. On heat priority days, CSHOs are encouraged 

to be aware of heat hazards they may observe in 

their job-related travel and conduct compliance 

assistance or self-referrals for inspections.

2. See www.weather.gov/bgm/heat.

Employers Responsibilities

All employers covered under the OSH Act must 

ensure that workers are protected from heat-related 

hazards. To protect their workers, employers 

should implement the following: 

y Develop a program that outlines the steps the 

employer will take to protect their workers from 

heat hazards. 

y Provide regular access to cool water and to cool 

and/or shaded areas for rest.

y Allow new and returning workers to acclimatize 

to working in the heat. 

Acclimatization is a process by which a 

person gradually increases their exposure 

time to hot environmental conditions, causing 

beneficial physiological changes that improve 

body temperature regulation and minimize 

risk of HRIs by properly regulating body 

temperature that minimizes heat-related 

illnesses. It is therefore essential for employers 

to train new or returning workers on heatrelated hazards including heat stress, gradually 

increase their workloads, ensure more frequent 

breaks as they acclimatize to ambient conditions, 

and monitor them for signs of heat illness. 

y Implement a “buddy” system.

y Buddy system entails pairing the workers so 

that they can recognize when symptoms are 

observed in each other and take the necessary 

steps before heat illness develops. 

y Implement administrative controls (e.g., earlier 

start times, and employee/job rotation) to limit 

heat exposures.

y Train workers on heat illness signs, how to 

report signs and symptoms, first aid, how to 

contact emergency personnel, prevention 

methods, and the importance of hydration.

y Train supervisors on the Heat Illness Prevention 

Plan to include monitoring workers for signs 

and symptoms of HRIs, to monitor temperatures 

of outdoor sites, to remind workers to stay 

hydrated, and to establish and enforce rest and 

cool-down breaks.

Training must be done in a language the workers 

will understand.
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Host Employer Responsibilities

The host employer has the primary responsibility 

of developing a Heat Illness and Injury Program for 

the worksite, providing cool water and cool, shaded 

areas for rest, and training and implementing 

acclimatization procedures for new workers, 

including temporary workers, and returning 

workers. In addition, the host employer should 

become familiar with available tools (i.e., OSHANIOSH Heat Safety Tool App and the NWS website) 

that permit regular evaluation of workplace heat 

hazards at their worksite for determining local 

forecasts and current heat index.

The host employer should maintain the appropriate 

surveillance of workplace conditions and the degree 

of worker exposure(s). Therefore, upon identifying 

heat priority days the host employer is in a position 

to implement the program, adjust the work times 

(i.e., schedule work during the cooler times of day) 

and take steps to reduce the risk of health hazards 

associated with heat exposures. That is because the 

host employer is most familiar with and has control 

over the processes, workload, and equipment 

that may contribute to high heat hazards that the 

temporary workers will encounter. Therefore, 

the host employer is responsible for site-specific 

policies and procedures including training and 

should afford temporary workers the same training 

and protections as permanent workers.

Staffing Agency Responsibilities

The staffing agency shares responsibility for its 

workers’ safety and health and has an obligation to 

become familiar with the various tools mentioned 

above and to review the host employer’s worksite 

policies and procedures to control heat hazards prior 

to assigning workers. As a recommended practice, 

the staffing agency should take reasonable steps to:

y Ensure that the host employer provides cool 

shade, acclimatization procedures, and has 

existing procedures for new or returning 

workers, access to adequate cool water, use of 

administrative controls, and site-specific training 

for temporary workers in the same manner in 

which the host’s employees are protected; 

y Inform temporary workers of the heat-related 

hazards they may encounter; 

y Instruct temporary workers that they must follow 

all safety and health rules (i.e., taking required 

breaks/hydrating) required by the host employer; 

y Train the workers on how to identify signs and 

symptoms of exposure to heat and use of a 

buddy system; and

y Review and confirm that the host employer has 

a Heat Illness and Injury Program in place. 

The staffing agency is also responsible for 

maintaining communication with its workers 

and the host employer regarding acclimatization 

procedures for new and/or returning workers, 

heat exposures, and any signs or symptoms the 

temporary workers are exhibiting. Such ongoing 

communication alerts the staffing agency 

and temporary workers to additional or newly 

discovered heat-related hazards that may need to 

be addressed and ensures that responsibilities of 

the host employer’s Heat Illness and Injury Program 

are understood and implemented.

For a listing of best practices on worker 

acclimatization, visit the American Conference 

of Governmental Industrial Hygienists (ACGIH®) 

website at www.acgih.org to obtain information on 

the Action Limit (AL) for unacclimatized workers, 

a Threshold Limit Value (TLV®) for acclimatized 

workers and see Heat Stress and Strain: TLV®

Physical Agents 2022 Edition.

Joint Responsibilities

The OSH Act under section 5(a)(1) requires 

employers to provide a safe and healthful 

workplace for all workers. When investigations 

reveal that a temporary worker is exposed to a 

recognized hazard, and the worker is employed 

by both a staffing agency and a host employer, 

OSHA will consider issuing citations to either or 

both of the employers, depending on the specific 

facts of the case. OSHA will make an assessment 

of whether both employers have fulfilled their 

respective compliance responsibilities in each 

individual case. 
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Example Scenario*

The example provided below describes what may occur when temporary workers exposed to heat 

hazards are not provided with a work/rest schedule, cool water, cool break area, and the benefit of 

training on the effects of heat for their job assignments by the staffing agency and a host employer.

Scenario

A commercial laundry facility, Clean Wash, services several businesses in a busy industrial area. 

Clean Wash occupies an old building. The building is supplied with exhaust fans that bring in 

outside air into the facility creating a similar environment inside as outside. The facility’s main floor 

is divided into multiple rooms that hold washers, dryers, ironing machines, soil sorting, and folding 

areas. The basement floor is another area for folding and ironing and this area has neither windows 

nor a cool break area. Clean Wash needed additional workers to fill a backlog of laundering beach 

towels due to a new contract they acquired from a nearby hotel during a summer heat wave. 

Clean Wash has a contract with Weather Staffing, a staffing agency, to provide the extra workers 

to perform ironing and folding in the basement. Weather Staffing hired six workers, and informed 

them of the various jobs operating the laundry equipment, ironing, and folding laundry, and also 

gave them general safety training (e.g., personal protective equipment, hazard communication) 

before sending them to Clean Wash.

When the temporary workers arrived, Clean Wash informed them of the hot (i.e., radiant heat/

steam) environment and told them that fans for cooling off were available at the facility, but did 

not provide site-specific training on heat hazards and heat illness prevention methods or provide 

them with a break schedule, cool water, or a cool break area. Weather conditions included an NWS 

ongoing heat advisory in effect for the local area. On the first day, one of the temporary workers 

began to experience fatigue, thirst, and dizziness. As more days passed, other workers developed 

similar symptoms, and some even experienced muscle cramps and periods of dizziness. When the 

temporary workers complained, Clean Wash provided floor fans. However, on the third day, three 

temporary employees collapsed. They were taken to the hospital where they were diagnosed with 

heat exhaustion. 

Analysis

As the host employer, Clean Wash was responsible for providing site-specific training on 

preventing heat hazards, including, but not limited to, a work/rest regimen, and for providing access 

to cool water, and a cool break area. Clean Wash supervises and controls the day-to-day work of the 

temporary workers t its facility and was clearly aware of the growing risk for heat-related illness. It 

is in the best position to provide the site-specific training and implement procedures to control heat 

hazards applicable to its workplace. For this failure to provide appropriate site-specific heat-related 

information and training, and not implementing a work/rest regimen with access to cool water and 

cool break area, Clean Wash may be subject to OSHA citations.

Staffing agency Weather Staffing fulfilled some of its OSHA responsibilities by providing the 

general safety training but did not provide heat-related training addressing issues such as health 

effects associated with heat stress, symptoms of heat induced illnesses, methods of preventing 

such illnesses, and the need to be acclimatized. Since the staffing agency knew the workers would 

be working in a heat-generating environment, it should have inquired about what provisions the 
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Under OSHA’s Heat NEP, CSHOs will evaluate 

worker exposure to heat hazards and contributing 

factors and inquire about any heat illness and injury 

programs. The host employer’s Heat Illness and 

Injury Program should outline the procedures that 

are to be implemented to protect all workers from 

suffering heat-related illnesses and injuries. In cases 

where temporary workers work for multiple host 

employers for short periods of time (i.e., seasonal, 

monthly), the staffing agency should coordinate 

employee protections with all host employers. 

State Plans

Twenty-eight states and U.S. territories operate 

their own OSHA-approved occupational safety 

and health programs, called State Plans. 

These State Plans have and enforce their own 

occupational safety and health standards that 

must be at least as effective as OSHA’s but may 

have different or additional requirements. A list of 

the State Plans and more information are available 

at: www.osha.gov/stateplans.

How Can OSHA Help?

Workers have a right to a safe workplace. If you 

think your job is unsafe or you have questions, 

contact OSHA at 1-800-321-OSHA (6742) or 

visit OSHA’s main web page at www.osha.gov. 

It’s confidential. We can help. 

For other valuable worker protection information, 

such as Workers’ Rights, Employer Responsibilities 

and other services OSHA offers, visit OSHA’s 

Workers’ page www.osha.gov/workers. 

For information on Temporary Workers visit 

OSHA’s Temporary Workers’ page www.osha.gov/

temporaryworkers.

The OSH Act prohibits employers from retaliating 

against their employees for exercising their rights 

under the OSH Act. These rights include raising 

a workplace health and safety concern with 

either employer, reporting an injury or illness, 

filing an OSHA complaint, and participating 

in an inspection or talking to an inspector. If 

workers have been retaliated or discriminated 

against for exercising their rights, they must file a 

complaint with OSHA within 30 days of the alleged 

adverse action to preserve their rights under 

section 11(c). For more information, please visit 

www.whistleblowers.gov.

OSHA also provides help to employers. OSHA’s 

On-Site Consultation Program offers free and 

confidential advice to small and medium-sized 

businesses in all states and several territories, with 

priority given to high-hazard worksites.

On-Site consultation services are separate from 

enforcement and do not result in penalties or 

citations. Consultants from state agencies or 

universities work with employers to identify 

workplace hazards, provide advice on compliance 

with OSHA standards, and assist in establishing 

and improving safety and health management 

systems. To locate the OSHA On-Site Consultation 

Program nearest you, call 1-800-321-6742 (OSHA) 

or visit www.osha.gov/consultation.

host employer had in place to ensure that Weather Staffing workers were adequately protected. 

Further, the staffing agency failed to ensure and confirm that Clean Wash had provided site-specific 

heat-related training to the temporary workers. Weather Staffing may therefore be subject to OSHA 

citations for not providing general heat-related training.

*Note: The company names in this scenario are fictitious. Any resemblance to real companies is 

entirely coincidental.
Appendix A: Description of Serious Heat-related Illnesses and Common Symptoms

The table below describes serious heat-related illnesses and common signs and symptoms. Please 

note that this list is not exhaustive. For additional information including new campaign products visit: 

www.osha.gov/heat.

HEAT-RELATED ILLNESSES* SYMPTOMS AND SIGNS

Heat stroke • Confusion

• Slurred speech

• Unconsciousness

• Seizures

• Heavy sweating or hot, dry skin

• Very high body temperature

• Rapid heart rate

Heat exhaustion • Fatigue

• Irritability

• Thirst

• Nausea or vomiting

• Dizziness or lightheadedness

• Heavy sweating

• Elevated body temperature or fast heart rate

Heat cramps • Muscle spasms or pain

• Usually in legs, arms, or trunk

Heat syncope • Fainting

• Dizziness

Heat rash • Clusters of red bumps on skin

• Often appears on neck, upper chest, and skin folds

Rhabdomyolysis (muscle breakdown) • Muscle pain

• Dark urine or reduced urine output

• Weakness

* Heat-related injury means an injury linked to heat exposure that is not considered one of the typical symptoms of heat-related illness 

(with the exception of kidney injury), such as a fall or cut.

OSHA 4185-07 2022

Disclaimer: This bulletin is not a standard or regulation, and it creates no new legal obligations. It contains 

recommendations as well as descriptions of mandatory safety and health standards. The recommendations are 

advisory in nature, informational in content, and are intended to assist employers in providing a safe and healthful 

workplace. The Occupational Safety and Health Act requires employers to comply with safety and health standards 

and regulations promulgated by OSHA or by a state with an OSHA-approved state plan. In addition, the OSH Act’s 

General Duty Clause, Section 5(a)(1), requires employers to provide their employees with a workplace free from 

recognized hazards likely to cause death or serious physical harm.