Working in the intense heat of the summer sun can put workers at risk of heat stress, but heat stress can also hit you in places you wouldn't expect.

Any job that causes your body temperature to rise has the potential to cause heat stress. Even jobs that are performed in air-conditioned environments. Enclosed spaces with poor ventilation, physical exertion, or heat-generating equipment can all contribute to rising body temperatures and increase the risk.

About heat stress

What is heat stress?
Heat stress occurs when the body can't get rid of excess heat. When this happens, the body's core temperature rises and the heart rate increases. Left unchecked, heat stress can lead to heat exhaustion, heat stroke, heart attack, and other serious health effects.

Who can be impacted?
Any worker can be affected depending on their environment (e.g., air temperature, humidity, direct sunlight) or job duties (e.g., physical activity, exposure to radiant heat, or contact with hot equipment).

How hot does it need to be for heat stress to occur?
There is no specific temperature. Tools like OHCOW’s Heat Stress Calculator and Response Plan offer helpful guidance.

Individual risk factors
Age, health, fitness level, clothing, and acclimatization all affect susceptibility. Certain medications—such as diuretics, antihistamines, stimulants, mood stabilizers, and some antibiotics—can also increase risk.

Signs and symptoms of heat stress

Those experiencing heat stress may experience the following:

Early warning signs include:

• Sweating
• Excessive thirst
• Painful muscle cramps
• Darker urine (a sign of dehydration)

More serious symptoms include:

• Fatigue
• Headache
• Light-headedness or dizziness
• Fainting
• Nausea or vomiting
• Blurry vision
• Trouble focusing or confusion
• Excessive sweating or ceasing to sweat altogether

Left unchecked, heat stress can lead to heat exhaustion, heat stroke, heart attack, and other physical health effects., It can also be damaging to your business, causing lost productivity, disability costs, and fines and penalties.

What the law says: Employer obligations and legal duties

Under Section 25(2)(h) of Ontario’s Occupational Health and Safety Act (OHSA), employers must “take every precaution reasonable in the circumstances for the protection of a worker.” This includes developing policies and procedures to protect workers exposed to hot environments, radiant heat sources, or hot weather.

The Ministry of Labour, Immigration, Training and Skills Development (MLITSD) recommends using the Threshold Limit Values (TLVs) for heat stress and heat strain published by the American Conference of Governmental Industrial Hygienists (ACGIH). These TLVs aim to prevent unacclimatized workers’ core temperatures from exceeding 38°C (100.4°F) 

Employers should implement a heat stress prevention program that includes:

• Worker training on heat-related hazards and prevention
• Monitoring methods (e.g., humidex, WBGT, Environment Canada alerts)
• A sampling plan for when and where to monitor conditions
• Preventive measures (e.g., more frequent breaks, reduced workloads, shaded areas)
• A hydration plan (e.g., 1 cup of water every 20 minutes)
• First aid and emergency response protocols

5 heat stress prevention tips

Sun-related heat stress is a well-recognized hazard that can be prevented through a combination of engineering and administrative controls. 

Indoor heat stress can occur anytime the body can't cool itself, even in cooler environments. If protective clothing traps heat, workers may be at risk.

Here are five cost-effective prevention tips: 

1. Wear breathable clothing. Cotton or other loose, woven fabrics help wick moisture and allow evaporation to cool the body.
2. Provide hydration stations. Small amounts of cool (not cold) liquids help prevent dehydration and maintain the body’s ability to sweat.
3. Train your team to recognize symptoms. Supervisors and coworkers can often spot signs of heat stress before the affected person does.
4. Promote a healthy lifestyle. Physically fit workers are better able to regulate body temperature and handle exertion.
5. Review risk factors regularly. Age, medications, and health conditions can all increase vulnerability to heat stress.

Resources

OHCOW Toolkit

• Visit the OHCOW Heat Stress Tool Kit, courtesy of the Occupational Health Clinics for Ontario Workers (OHCOW) and the Centre for Research in Occupational Safety & Health (CROSH). This tool kit is designed to support, empower and protect workers while guiding workplaces for both indoor and outdoor use. You'll find:
  • New guides on Heat Stress Awareness, Prevention Tools and Strategies, and Physiological Monitoring
  • Posters and Infographics
  • Videos
  • Humidex-based heat stress calculator

Resources

• Safety Connection Webinar - Heat Stress (pre-recorded)
  
• Heat Stress (general article)
• What is heat stress? What are the signs and symptoms? (video)

Agriculture resources

• Staying safe in the field: Heat hazard management for farm workers and landscapers (article)
• Farm Safety Roundup Podcast - The heat is on: Protecting farm workers from the dangers of heat stress (podcast)
  
• Agricultural Safety Topic - Heat Stress (guide)
• 5 Tips For Heat Stress Prevention (poster)

Restaurant and kitchen resources

• 10 ways to prevent heat stress in retail kitchens (article)

The information in this article is accurate as of its publication date.