Heat Stress and Strain Program


Working in hot environments (indoors or outdoors) can put strain on the body.  Whether this strain will cause a health effect depends on the following factors:

  • Temperature and humidity
  • Radiant heat sources
  • The physical demands of the work being performed; and,
  • The condition and clothing of the worker

This heat stress program has two basic elements:

  1. Anticipating conditions when heat stress may occur; and,
  2. Implementing controls to minimize worker risk in these situations.


The following definitions are essential to the proper implementation of this plan:

Acclimatization - is the gradual physiological adaptation that improves an individual's ability to tolerate heat stress.  The Ministry of Labour considers that most outdoor workers in Ontario are not acclimatized to work at hot temperatures because the elevated hot conditions do not last long enough for workers to become acclimatized. 

Heat Stress - is the net heat load due to physical demands, clothing requirements, and the environment (air temp, humidity, radiant heat, and air movement) to which a worker may be exposed.

Heat Strain - is the body's physiological response to the heat stress experienced.  A normal physiological response is dedicated to the dissipation of excess heat from the body.

Temperature - is a measure of how hot something is.  For air, it is normally expressed in degrees Celcius (0C) or degrees Fahrenheit (0F).

Relative Humidity - humidity refers to the amount of water vapour present in air.  Relative humidity is the current amount of water vapour in air in comparison to the maximum amount of water vapour air at a given temperature.  Relative humidity is expressed as a percent.

Humidex - is a calculated value combining the effects of temperature and humidity together.  Its intent is to provide a measure of how one feels based on the given humidity and temperature.  Table 1 provides a summary of how Environment Canada relates the humidex value to the comfort of an average adult:

Table 1: The relationship between degree of comfort and the humidex.  Take from Environment Canada August, 2017.

Image result for humidex comfort levels

Program Plans

In developing this Working in Hot Environments Program, the Safety Office has sub-divided work into two categories - those that occur indoors, and those that occur outdoors. 

We have currently identified various outdoor working tasks that have the potential to cause workers excess heat stress, and therefore have created a Working in Hot Outdoor Environments Control Plan. 

This plan can be accessed using the following link:


A Working in Hot Indoor Environments Control Plan will be created if, after assessment, an indoor work task or area is found to have the potential to cause a worker excessive heat stress.  If there is an indoor work task, or indoor work area at UW that you feel has the potential to cause heat stress, please contact the Safety Office at:


Introductory video

Heat Stress and Outdoor Worker Fact Sheet

Heat Stress, Signs and Symptoms Fact Sheet

Legal Requirements in Ontario

Off-the-Job Sun Safety Fact Sheet

Personal Protection Fact Sheet

Photosensitizing Substances Fact Sheet

Shade Fact Sheet

Solar UV Protection in Vehicles Fact Sheet

Solar UV & Outdoor Work Fact Sheet

Sunscreen Fact Sheet


  1. ACGIH [2016]. TLV Guide - Heat Stress and Strain.  Cincinnati, OH: American Conference of Governmental Industrial Hygienists.
  2. OHSCO [2014].  Heat Stress Awareness Guide.  Occupational Health and Safety Council of Ontario.  Retrieved from
  3. OHCOW [2014].  Humidex Based Heat Response Plan.  Occupational Health Clinics for Ontario Workers.  Retrieved from:
  4. SunSafety at Work (August 2017).  Various Fact Sheets.  Retrieved from