Heat stress can be a killer

Resources updated May 25, 2020 

WSN Hazard Alert Heat Stress - Download and share

WSIB Heat Stress Awareness

What happened?

  1. In 2001, a bakery worker in Barrie died of heat stress during a heat wave. The outdoor temperature that day was 34 C, and the temperature inside the bakery was 46 C. Lack of water and not enough rest breaks were blamed. 
  2. In 2002 in the United States, two mine rescue trainers died from heat stress in an abandoned, unventilated, underground gold mine while under oxygen. The temperature was greater than 39 C with very high humidity. Summer temperatures in Ontario are climbing, and in some underground mines, rock temperatures and humidity are high year-round. 
  3. On the second day of his summer job as a garbage collector, a college student was overcome by heat stress symptoms, according to the Workplace Safety and Insurance Board’s Heat Stress Awareness Guide. Not trained to recognize warning signs, the young worker continued to work until he collapsed and fell into a coma. He died 17 days later. The inquest concluded his death could have been prevented if the employer had a heat stress program that included training to recognize the symptoms.

Why did it happen?

Heat stress can happen to anybody, even the young and fit, and heat exposure may occur in all kinds of workplaces. Industrial furnaces, bakeries, smelters, foundries and worksites with 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 can contribute to exposure.

How could the incident have been prevented?

DO NOT UNDERESTIMATE THE HAZARDS OF HEAT STRESS: When it’s hot you need to drink a lot of fluids, dress appropriately and recognize the signs of heat stress. If heat exposure is an issue in your workplace you need to develop and implement policies to prevent heat-related illnesses.

  • Acclimatization – You should take a week or two to get used to the heat and allow your body to adjust. This is called “acclimatization.” Be aware that if you are away from work for a week you may need to re-adjust to the heat. 
  • Engineering Controls – Air-cooling systems, fans and insulating and reflective barriers around furnaces and machinery can help to reduce heat exposure and control workplace temperatures and humidity. 
  • Administrative Controls – Ensure that there are appropriate monitoring and control strategies in place and be ready to take appropriate action for hot days and hot workplaces. To prevent heat stress, increase the frequency and the length of rest breaks and slow down the pace of work.


Employers have a duty under clause 25(2)(h) of the Occupational Health and Safety Act to take every precaution reasonable in the circumstances for the protection of a worker. This includes developing policies and procedures to protect workers in environments that are hot because of hot processes and/or 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 workers’ core body temperatures from rising above 38 C.

Ideas for preventing heat stress

A WSIB-funded research study on the effects of heat stress on firefighters on the job identified the following findings that may help other workplaces:

  • Work and rest schedules are not always an effective strategy to lower body temperatures. 
  • When ambient temperatures exceed about 30ºC other strategies should be incorporated with work rest scheduling.
  • Active cooling strategies such as air conditioned environments, access to fans and misters and specific actions like forearm submersion may effectively reduce  heat stress during rest periods when the protective clothing can be removed.
  • Fluid replacement strategies can reduce the risk of heat stress.
  • Effective fluid replacement and active cooling strategies during rest periods can help reduce the risk of heat stress to the worker. 

Resources for dealing with workplace heat stress

How to manage heat stress in the workplace - Safety talk and leader guide 

Heat Stress Awareness Guide

Heat Stress Guidelines and Legal Requirements - Ministry of Labour, Training and Skills Development

Heat Safety Tool App real-time guidance for your specific location - OSHA-NIOSH

Managing and preventing heat stress in the workplace - Health and safety conference presentation

Humidex Calculator - Occupational Health Clinic for Ontario Workers

Working in the Heat infographic - Canadian Centre for Occupational Health and Safety (CCOHS)

Sun safety in Canada infographic - CCOHS

How WSN can help you

As your health and safety specialist for Ontario’s mining, forestry, and paper, printing, and converting sectors, WSN offers training, resources, risk assessment, safety auditing, and a free confidential initial consultation.