Working when it’s hot puts stress on your body’s cooling system. When ignored, it can lead to heat-related illness, disability and even death. This can happen to anybody.

Heat stress can get worse when combined with:

  • physical work
  • loss of fluids
  • fatigue
  • a pre-existing medical condition

Causes of heat stress

Factors that can cause heat stress include:

  • working in direct sunlight in the summer months
  • humidity in the workplace (more than 50% relative humidity)
  • working in certain workplaces such as foundries, smelters, chemical plants, bakeries and commercial kitchens
  • working in mines, especially deep mines with geothermal gradients
  • working in mines with equipment that radiates heat

Illnesses due to heat stress

Heat rash

Red bumps on skin with severe itching caused by hot humid environments and plugged sweat glands.


Change into dry clothes and avoid hot environments. Rinse your skin with cool water.


Wash regularly to keep skin clean and dry.

Heat cramps

Muscle pain in overworked areas such as arms, legs or stomach caused by a salt imbalance from heavy sweating. This can happen at work or later at home. 


You should:

  • not take salt tablets
  • move to a cool area
  • loosen clothing
  • gently massage and stretch affected muscles
  • drink cool, slightly salted water or a beverage with electrolytes

If the cramps are severe or don’t go away after drinking salt water or a beverage with electrolytes, get medical help right away.


You should:

  • reduce activity levels
  • reduce heat exposure
  • drink fluids regularly
  • check on your coworker(s) for any irregular behaviour


Caused by fluid loss and not enough water intake. Before losing consciousness, you may not experience any warning symptoms but if you do, the common signs are:

  • cool, moist skin
  • weak pulse


You should:

  • get medical attention
  • move to a cool area
  • loosen clothing
  • lie down
  • if awake, sip some cool water


You should:

  • reduce activity levels
  • reduce heat exposure
  • drink fluids regularly
  • check on your coworker(s) for any irregular behaviour
  • avoid standing in one place for too long

Heat exhaustion

Caused by the breakdown of your body’s cooling system. Symptoms can include:

  • heavy sweating
  • cool, moist skin
  • body temperature above 38°C
  • weak pulse
  • low blood pressure
  • tired and weak
  • nausea and vomiting
  • very thirsty
  • panting or breathing rapidly
  • blurred vision


Do not leave the person alone and:

  • get medical attention
  • move to a cool area
  • loosen or remove clothing
  • drink and spray cool water


You should:

  • reduce activity levels
  • reduce heat exposure
  • drink fluids regularly
  • check on your coworker(s) for any irregular behaviour

Heat stroke

Heat stroke is caused by the breakdown of your body’s cooling system and has a high risk of irreversible damage to body organs and organ systems. Some people with heat stroke lose the ability to sweat and are not very physically active when ill (classic heat stroke), while others experience heat stroke while still sweating and active (exertional heat stroke).   

Symptoms include:

  • high body temperature (above 40°C)
  • a fast pulse
  • headache or dizziness
  • passing out
  • weakness, confusion or acting strangely
  • hot, dry, red skin (classic heat stroke) or profusely sweating (exertional heat stroke)


You should:

  • call an ambulance
  • remove excess clothing
  • drink and spray cool water


You should:

  • reduce activity levels
  • reduce heat exposure
  • drink fluids regularly
  • check on your coworker(s) for any irregular behaviour

Ways to manage heat stress in the workplace

Under the Occupational Health and Safety Act, you must take every reasonable precaution in the circumstances for the protection of a worker.  This may include protecting your workers from heat stress. You can do this in several ways:

Design your workplace to reduce heat stress

If possible, start with engineering controls. For example:

  • use machines (for example, hoists and lift-tables) to reduce the physical demands of work
  • control the heat at its source by using insulating and reflective barriers (for example, insulate furnace walls)
  • exhaust hot air and steam produced by operations
  • use air conditioners to reduce the temperature and humidity
  • use fans if the temperature is below 35°C (if fans are used when the temperature is above 35°C they may recirculate the hot air, which can prevent cooling)
  • provide:
    • cool, shaded work areas
    • air-conditioned rest areas

Plan ahead to reduce heat stress

Your workplace policies and procedures, schedule and training can help reduce the risk of heat stress. Administrative and work practice controls can include:

  • assessing the demands of all jobs and putting a plan in place for hot days and workplaces
  • increasing the frequency and length of rest breaks
  • scheduling strenuous jobs to cooler times of the day such as in the early morning, late afternoon or night
  • providing cool drinking water near workers
  • reminding workers to drink a cup of water at least every 15 to 20 minutes to stay hydrated
  • cautioning workers to avoid direct sunlight
  • assigning more workers or slowing down the pace of work
  • making sure workers have time to acclimatize to a modified intensity of work
  • training workers to recognize the signs and symptoms of heat stress
  • starting a “buddy system” because people are not likely to notice their own symptoms
  • investigating any heat-related incidents reported by workers
  • making sure workers trained in First Aid are available and on-site
  • creating an emergency response plan to respond to heat-related illnesses
  • advising workers who are pregnant or have a medical condition to consult their physician about working in the heat and make appropriate accommodations

Help workers adjust to hot environments

The more time a worker has to acclimatize to a hot environment, the better their body handles the heat.

If workers have health problems or are not in good physical shape, they may need more time to adjust to hot environments.

For workers with no experience in hot conditions, there are two ways to help them tolerate the heat:

  1. gradually increase the activity level over one to two weeks
  2. gradually increase the amount of time spent in hot working conditions

For workers with experience in hot conditions, but who may have been ill or away from work for 9 or more days, the worker will need to gradually readjust to the heat.

You can find more information on heat acclimatization from The National Institute for Occupational Safety and Health: Heat Stress – Recommendations.

Encourage workers to wear suitable protective clothing

Workers should:

  • wear light and breathable summer clothing (if applicable)
  • cover their head to prevent exposure to direct sunlight
  • wear reflective clothing in a high radiant-heat situation
  • consider air, water or ice-cooled insulated clothing for very hot environments
  • avoid clothing that isn’t breathable, such as chemical protective clothing. If the workers must wear it, they should pay close attention to symptoms that suggest they may be ill due to heat stress.

Supervisors should be constantly monitoring workers for signs that could suggest a risk of illness due to heat stress.

Creating a heat stress plan

We recommend employers create a heat stress control plan, based on the work environment.

Process heat

For workplaces that are hot primarily due to process heat (for example, furnaces, bakeries and smelters), we recommend employers:

  • follow the guidance in the American Conference of Governmental Industrial Hygienists (ACGIH) booklet, Threshold Limit Value (TLVs)
  • set up a heat stress control plan in consultation with the workplace's joint health and safety committee or worker health and safety representative

Hot weather

A hot weather plan is a simplified heat stress control plan.

Employers should create one to use between May 1 and September 30 of each year.

Consider using the plan when:

  • the humidex on-site reaches or exceeds 35
  • Environment Canada reports air temperature that exceeds 30°C and a humidex of 40
  • heat waves of 32ºC or more are predicted for three or more days
  • the Ontario Ministry of the Environment, Conservation and Parks issues a smog alert

This resource does not replace the Occupational Health and Safety Act (OHSA) and its regulations, and should not be used as or considered legal advice. Health and safety inspectors apply the law based on the facts in the workplace.