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Heat Stress Requirements for Safety in the Workplace
Monday, June 4, 2012 at 11:23:34 AM

Introduction to the Legal Requirements:

Employers have a duty under section 25(2)(h) of the Occupational Health and Safety Act to take every precaution reasonable in the circumstances for the protection of the worker. This includes developing hot environment policies and procedures to protect workers in hot environments due to hot processes or hot weather. For compliance purposes, the ministry of Labour recommends the Threshold Limit Values (TLVs) for Heat Stress and Heat Strain published by the American Conference of Governmental Industrial Hygienists (ACGIH). These values are based on preventing unacclimatized workers’ core temperatures from rising above 38C.

This Guideline is intended to assist employers, workers and other workplace parties in understanding heat stress, and in developing and implementing policies to prevent heat-related illness in the workplace.

What is Heat Stress?

Working or playing where it is hot puts stress on your body’s cooling system. When heat is combined with other stresses such as hard physical work, loss of fluids, fatigue or some medical conditions, it may lead to heat-related illness, disability and even death.

This can happen to anybody – even the young and fit. In Ontario, heat stress is usually a concern during the summer. This is especially true early in the season, when people are not used to the heat.

Heat exposure may occur in many workplaces. Furnaces, bakeries, smelters, foundries and heavy equipment are significant sources of heat inside workplaces. For outdoor workers, direct sunlight is the main source of heat. In mines, geothermal gradients and equipment contribute to heat exposure. Humidity in workplaces also contributes to heat stress.

How We Cope With Heat

Your body is always generating heat and passing it to the environment. The harder your body is working, the more heat it has to lose. When the environment is hot or humid or has a source of radiant heat (for example, a furnace or the sun), your body must work harder to get rid of its heat.

If the air is moving (for example, from fans), and it is cooler than your body, it is easier for your body to pass heat to the environment.

Workers on medications or with pre-existing medial conditions may be more susceptible to heat stress. These workers should speak to their personal physicians about work in hot environments.

Controlling Heat Stress - Acclimatization

The longer you work hard in the heat, the better your body becomes at adjusting to the heat. If you are not used to working in the heat then you should take a week or two to get used to the heat. This is called “acclimatization.” If you are ill or away from work for a week or so you can lose your acclimatization.

There are two ways to acclimatize:

1. If you are experienced on the job, limit your time in hot working conditions to 50 per cent of the shift on the first day, 60 per cent of the shift on the second day, and 80 per cent of the shift on the third day. You can work a full shift the fourth day.

If you are not experienced on the job (if you are, for example, a summer student), you should start off spending 20 per cent of the time in hot working conditions on the first day and increase your time by 20 per cent each subsequent day.

2. Instead of reducing the exposure times to the hot job, you can become acclimatized by reducing the physical demands of the job for a week or two.

If you have health problems or are not in good physical condition, you may need longer periods of acclimatization. Hot spells in Ontario seldom last long enough to allow acclimatization. However, exposure to workplace heat sources may permit acclimatization.

Modifying Work and the Environment

Heat exposures may be reduced by several methods. Selection of appropriate workplace controls will vary, depending on the type of workplace and other factors. Some measures may include:

Engineering Controls

• Control the heat at its source through the use of insulating
and reflective barriers (e.g. insulate furnace walls).
• Exhaust hot air and steam produced by operations.
• Reduce the temperature and humidity through air cooling.
• Provide air-conditioned rest areas.
• Provide cool work areas.
• Increase air movement if temperature is less than 35C (fans).
• Reduce physical demands of work task through mechanical
assistance (hoists, lift-tables, etc.).

Administrative Controls

• The employer should assess the demands of all jobs and have
monitoring and control strategies in place for hot days and
hot workplaces.
• Increase the frequency and length of rest breaks.
• Schedule strenuous jobs to cooler times of the day.
• Provide cool drinking water near workers and remind them to
drink a cup every 20 minutes or so.
• Caution workers to avoid direct sunlight.
• Assign additional workers or slow down the pace of work.
• Make sure everyone is properly acclimatized.
• Train workers to recognize the signs and symptoms of heat
stress and start a “buddy system” since people are not
likely to notice their own symptoms.
• Pregnant workers and workers with a medical condition should
discuss working in the heat with their doctor.
• First Aid responders and an emergency response plan should
be in place in the even tof a heat-related illness.
• Investigate any heat-related incidents.

Personal Protective Equipment

• Light summer clothing should be worn to allow free air
movement and sweat evaporation.
• Outside, wear light-coloured clothing.
• In a high radiant heat situation, reflective clothing may
• For very hot environments, air, water or ice-cooled
insulated clothing should be considered.
• Vapour barrier clothing, such as chemical protective
clothing, greatly increases the amount of heat stress on the
body, and extra caution is necessary.

Managing Heat Stress from Process Heat

For an environment that is hot primarily due to process heat (furnaces, bakeries, smelters, etc.), the employer should follow the guidance of the American Conference of Governmental Industrial Hygienists (ACGIH) as outlined in its booklet and documentation for the recommended Threshold Limit Values (TLVs), and set up a heat stress control plan in consultation with the workplace’s joint health and safety committee or worker health and safety representative.

Further information on the ACGIH TLVs, and on the development of heat stress control plans, may be found at the following websites:


U.S. Occupational Safety and Health Administration (OHSA)

Managing Heat Stress Induced by Hot Weather

Most workplaces don’t have “hot processes” but working in hot weather can pose health risks to their workers. For hot work environments due to hot weather, a hot weather plan is appropriate. A hot weather plan is a simplified heat stress control plan. A hot weather plan should establish the implementation criteria, or “triggers,” to put the plan into effect. The criteria may include:

Weather/environmental indicator triggers such as:

• Humidex reaching or exceeding 35 degrees Celsius
• Environment Canada Humidex advisory (air temperature
exceeding 30 degrees Celsius and Humidex exceeding 40
degrees Celsius) or Ontario Ministry of the Environment smog
• Environment Canada weather reports; and/or
• Heat waves (three or more days of temperatures of 32 degrees
Celsius or more)

Generally, plans related to hot weather should be in place between May 1 and September 30 of each year.

Note: Remember that while complying with occupational health and safety laws, you are also required to comply with applicable environmental laws.