Heat Strain at Work with Dr. Denise Koh

Heat Strain at Work with Dr. Denise Koh

​​​​High temperatures can cause excessive stress on the body, especially when combined with high humidity, direct sun and/or a lack of appropriate air movement. These factors can overwhelm the body's ability to cool itself, a condition known as heat strain. It's essential for workers exposed to heat during their work day to know how to recognize and respond to incidents of heat stress exposure, and to take necessary safety measures to stay healthy and comfortable. Manitoba employers should also be aware of their responsibilities when it comes to addressing heat stress in their workplaces.  

Recognizing and responding to heat stress exposure

We tend to associate heat stress with working outdoors; however, indoor workers can also be at risk. Indoor workers who work in hot environments (e.g., foundry workers), or in buildings without air conditioning (especially during the summer), should also take action to protect themselves from the heat.
In any work environment with high temperatures, you should monitor yourself and co-workers, particularly new co-workers, for signs of heat illness, exhaustion or heat stroke. 

Excessive heat stress on the body may result in one or more of the following symptoms of heat strain:

  • heat illness - heat rash, swelling of hands, feet and ankles, headache, dizziness, upset stomach or vomiting
  • heat exhaustion - tiredness or weakness, moist skin, a rapid weak pulse
  • life-threatening heat stroke - hot dry skin, a rapid strong pulse, mental confusion, seizures or convulsions, unconsciousness.

Dizziness/discomfort on the job caused by heat stress can also lead to incidents and injuries.

Pay attention to individuals displaying symptoms of sudden and severe fatigue, nausea, dizziness or light-headedness, rapid breathing and/or a sustained rapid heart rate.

If you see someone showing signs that they are struggling in the heat:

  • Move the person to a cool, shaded area or remove them from the heat source.
  • Loosen or remove heavy clothing.
  • Provide cool drinking water. Thirst isn't a good indicator of dehydration, so drink regularly even when not thirsty.

Call 911 immediately if you think someone may be experiencing heat stroke.

Preventing heat strain

To prevent heat strain from occurring: 

  • work in the shade and away from other heat sources
  • take rest breaks in shaded or cool areas and drink cool water frequently
  • focus on lighter activities when possible and plan the day to tackle the more physically demanding jobs during the coolest hours
  • when returning to work or starting a new position, build up your tolerance to high temperatures (it takes about a week for the body to adjust to heat)
  • wear industry-appropriate clothing that allows free movement of cool, dry air over the skin's surface to maximize heat removal by evaporation and convection (e.g., lightweight, light-coloured clothing, and head coverings when working outside).

Employer responsibilities

The Workplace Safety and Health Regulation (Part 4.12) requires that employers implement safe work procedures and control measures to address the risk to safety and health posed by heat or cold.

Depending upon the humidex, there are also guidelines in the standards that the Workplace Safety and Health Act follows (based on ACGIH threshold limit value or TVL for heat stress) that dictate the length of rest periods (in cool settings) per hour of work, based on how strenuous the work is. 

Manitoba employers should work with their safety and health committees and/or workers to determine heat strain prevention procedures. Depending on the workplace, general measures include:  

  • heat stress exposure training
  • encouraging adequate fluid intake
  • limiting heat exposure
  • watching for symptoms in co-workers.

In addition to general controls, specific measures for hot workplaces or at-risk workers can include:

  • engineering controls to reduce physical job demands
  • shielding of radiant heat
  • increasing air movement
  • reducing heat and moisture emissions at the source
  • adjusting exposure times to allow sufficient recovery
  • adjusting expectations for workers coming back to work after an absence
  • personal body-cooling equipment (e.g., vests with pockets tailored for icepacks) for acclimatized workers doing moderate work.

Did you know?

  • Some workers may be more prone to heat strain. These include older individuals, people with physical impairments or chronic illnesses. Certain medications also affect heat sensitivity and the body's cooling function.
  • Extreme heat in early summer generally results in greater health impacts than those occurring later in the summer. Most people regularly exposed to high temperatures become acclimatized or used to hot environments.
  • Heat impacts on health are worse if high temperatures persist over several days and throughout the night. Elevated night-time temperatures prevent relief from daytime heat and contribute to additional heat stress.
  • Evidence suggests that the combined exposures to air pollution and extreme heat result in worse impacts on health. High temperatures increase the formation of air pollutants, such as ground-level ozone. Air quality can also be affected by forest fires, which occur more often in warmer, dryer weather.
  • Summer driving also requires caution. It is important to note that the strengthening sun angle even in the spring can cause the temperature in vehicles to spike on a sunny day. When outside air temperature is 23°C, the temperature inside a vehicle can be extremely dangerous - more than 50°C. On a hot and sunny day, the temperature inside a vehicle can spike by nearly 20° in just 10 minutes. Ove​r the course of one to two hours, the temperature inside the vehicle can climb as much as 50° higher than the outside air.
Further information on Thermal Stress
Learn more about Humidex guidelines and the impact of humidity on heat stress

About Dr. Denise Koh
Dr. Denise Koh is Manitoba's Chief Occupational Medical Officer as well as a Medical Officer of Health in Environmental Health and Emergency Preparedness.  She is a Public Health specialist with training in Occupational Medicine and Family Medicine.
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