It's not just the weather: Understanding heat stress at work

While hot outdoor environments can increase the risk, heat stress can occur in any workplace where the body generates or absorbs more heat than it can effectively remove. Employees who work near heat-producing equipment, perform physically demanding tasks, spend time in poorly ventilated areas, or who move between cool indoor spaces and hot outdoor conditions can all be at risk. Even those who work in air conditioned areas can be affected.

The key factor isn't simply the temperature—it's whether the body can keep itself cool.

A person’s age, health, fitness level, clothing all play factors in how one’s body responds to heat. Physical exertion, dehydration, fatigue and some medications—such as diuretics, antihistamines, stimulants, mood stabilizers and some antibiotics can all increase a worker’s risk.

Signs and symptoms of heat stress

There are early warning signs of someone experiencing heat stress. You should look out for sweating, excessive thirst, and painful muscle cramps. Be on the watch for darker than usual urine: It is often a sign of dehydration.

Signs of advanced heat stress

If you or your staff are experiencing fatigue, headache, light-headedness or dizziness, it could signal a more advanced stage of heat stress. 

Fainting, nausea or vomiting, blurred vision and trouble focusing or experiencing confusion is a sign of extreme heat distress. Also keep an eye out for excessive sweating or ceasing to sweat altogether.

Left unchecked, heat stress can progress to heat exhaustion, heat stroke, heart attack, and other serious health effects. It can also impact your business through lost productivity, disability costs, fines, and penalties.

Employers’ legal duties regarding workplace heat stress

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.

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

The Ministry of Labour, Immigration, Training and Skills Development (MLITSD) recommends using guidelines from the American Conference of Governmental Industrial Hygienists, which are designed to prevent workers’ core body temperature from rising above 38°C (100.4°F).

Five heat stress prevention tips

Heat stress can be prevented through a combination of engineering and administrative controls. 

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