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Heat Stress

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Heat Stress

Ministry of Labour

What Is Heat Stress?

Heat stress occurs when a combination of hot, humid conditions and physical activity strains and overcomes the body's natural cooling system.  It can cause symptoms ranging from cramps and fainting to serious heat exhaustion or even heat stroke.  Heat stroke is a form of heat stress that can kill quickly.

Environmental factors that affect heat stress include air temperature, humidity, air movement and sources of radiant heat such as working in the sun or near large, hot objects. Factors that affect heat stress are the physical demands of the job and the frequency and length of breaks.

Heat stress can happen in many workplaces.  Furnaces, bakeries, smelters, foundries and heat-generating equipment inside workplaces are significant sources of heat.  For outdoor workers, direct sunlight is the main source of heat.  In mines, heat from surrounding rock and nearby equipment contributes to heat exposure.  Humidity in workplaces also contributes to heat stress.

Signs and symptoms of heat stress can include:

  • excessive sweating
  • headache
  • rashes
  • cramping
  • dizziness
  • nausea
  • fainting, and
  • confusion

What Can Workers Do To Protect Themselves?

Some things workers can do to protect themselves from heat stress include:

  • Drink lots of fluids to replace perspiration. Try to drink a cup of water about every 20 minutes - or more frequently - to stay hydrated
  • Avoid working in direct sunlight (to reduce heat gain and risk of sunburn)
  • Reduce the pace of work
  • Increase the number of breaks and take breaks in cool or shaded areas
  • Schedule heavy work for cooler periods
  • When working outdoors, wear light-coloured and/or light-weight clothing, preferably long-sleeve shirt and pants, and cover head to prevent exposure to direct sunlight.
  • Reduce the physical demands of work by using aides, e.g., hoists, etc.

What Can Employers Do To Protect Workers?

Employers have a duty under the Occupational Health and Safety Act to take every precaution reasonable in the circumstances to protect workers. This includes developing hot environment policies and procedures to protect workers in hot environments due to hot processes or hot weather.

Employers can protect workers from heat stress by giving consideration to one or more of the following:

  • Reducing the temperature and humidity through air cooling
  • Providing air-conditioned rest areas
  • Increasing the frequency and length of rest breaks
  • Scheduling strenuous jobs for cooler times of the day
  • Providing cool drinking water near workers and remind them to drink a cup of water about every 20 minutes
  • Assigning additional workers or slow down the pace of work
  • Training 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

Additional Resources

For more detailed information on heat stress, consult the ministry's heat stress webpage and the Heat Stress Guideline.

A wealth of resources and links is also available from Ontario's health and safety system partners, including a guidebook, poster and a tool to determine humidex guidelines.

These websites have information on weather reports, smog alerts, and humidex,:

Environment Canada - Weather and Meteorology
Environment Canada Weather Office
Air Quality Ontario Smog Advisories
Occupational Health Clinic for Ontario Workers (OHCOW) - humidex-based heat response plan

Health and safety information, training materials, and consulting services are offered by Health and Safety Ontario.

The Infrastructure Health and Safety Association (IHSA) provides links to resources on heat stress prevention, including an e-Learning video.


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