HEAT STRESS PREVENTION ON ONTARIO CONSTRUCTION SITES

← Back to Blog

Every summer, Ontario construction workers face a hazard that does not involve scaffolds, harnesses, or heavy equipment — heat. When the Humidex climbs past 35 and you are doing heavy physical work in direct sunlight, your body can overheat dangerously fast. Heat-related illness kills construction workers in Ontario, and most of those deaths are preventable.

This guide covers how heat stress works, how to recognize it, what Ontario law requires, and the practical steps that keep workers safe when temperatures soar.

Heat Exhaustion vs. Heat Stroke: Know the Difference

These are two stages of the same problem, but the difference between them is the difference between a medical concern and a life-threatening emergency.

• Heat exhaustion happens when the body is losing the battle to cool itself. Symptoms include heavy sweating, weakness, dizziness, nausea, headache, cool and clammy skin, fast but weak pulse, and muscle cramps. The worker is still sweating — the body's cooling system is overwhelmed but still functioning. Heat exhaustion is serious and must be treated immediately, but it is not usually fatal if you act quickly.
• Heat stroke is a medical emergency. It happens when the body's cooling system has completely failed. Core body temperature rises above 40 degrees Celsius (104 degrees Fahrenheit). Symptoms include hot, dry skin (sweating has stopped), confusion, slurred speech, loss of consciousness, seizures, and a rapid, strong pulse. Heat stroke can cause permanent organ damage and death within minutes. Call 911 immediately.

The critical warning sign is when a worker stops sweating in hot conditions. That means the body has lost its ability to regulate temperature. This is heat stroke, and it is a 911 call, not a water break.

Ontario's Legal Framework for Heat Stress

Ontario does not have a specific regulation that sets a maximum working temperature for construction sites. However, the OHSA's general duty clause (Section 25(2)(h)) requires employers to "take every precaution reasonable in the circumstances for the protection of a worker." When those circumstances include extreme heat, employers must take action.

• General duty clause: This is the legal basis for heat stress prevention. An employer who fails to protect workers from foreseeable heat-related hazards is violating the OHSA. MOL inspectors have issued orders and fines under this clause for inadequate heat stress programs.
• JHSC and worker input: On projects with a Joint Health and Safety Committee (JHSC) or health and safety representative, heat stress should be a regular agenda item during warm months. Workers have the right to raise heat concerns.
• Right to refuse: If a worker believes that heat conditions create a danger to their health, they have the right to refuse unsafe work under Section 43 of the OHSA.

Heat stroke is a medical emergency that can kill in minutes. If a worker stops sweating, becomes confused, or loses consciousness on a hot day, call 911 immediately and begin cooling them with water and shade. Do not wait to see if they recover on their own.

Humidex-Based Action Values

The Ontario Ministry of Labour, Immigration, Training and Skills Development (MLITSD) provides guidelines for managing heat stress based on the Humidex — a measure that combines air temperature and humidity to reflect how hot it actually feels. These are not legally binding thresholds, but they represent the standard of care that MOL inspectors expect employers to follow.

• Humidex 35-39 (low risk): Post heat stress alerts. Ensure drinking water is readily available. Encourage workers to hydrate frequently. Monitor workers for signs of heat stress, especially those who are new to the job or not yet acclimatized.
• Humidex 40-44 (moderate risk): Implement a work-rest schedule. Workers performing heavy physical labour should take a 15-minute rest break in a cool or shaded area for every 45 minutes of work. Increase water intake. Assign a buddy system so workers can monitor each other.
• Humidex 45 and above (high risk): Increase rest periods significantly. For heavy work, consider 15 minutes of work followed by 45 minutes of rest, or suspend heavy outdoor work entirely. Provide active cooling (shade structures, misting fans, air-conditioned rest areas). Have emergency response procedures ready.

These values apply to acclimatized workers doing moderate to heavy physical work. For workers who are not acclimatized (first week on the job, returning from vacation, first hot spell of the season), the action values should be lower.

Hydration Rules

Proper hydration is the single most effective defense against heat stress. By the time a worker feels thirsty, they are already partially dehydrated.

• Drink before you are thirsty: Workers should drink one cup (250 ml) of water every 15 to 20 minutes during hot conditions, regardless of whether they feel thirsty.
• Water must be accessible: Drinking water must be available on site, close to the work area, and cool (not hot from sitting in the sun). If workers have to walk 10 minutes to get water, they will not drink enough.
• Avoid caffeine and alcohol: Both are diuretics that increase dehydration. Save the coffee for the morning and the beer for after work.
• Electrolytes for extended work: For heavy work lasting more than two hours in high heat, electrolyte replacement drinks can help replace the sodium and potassium lost through sweat. Alternate between water and electrolyte drinks — do not rely solely on electrolyte drinks.
• Track intake: A good rule of thumb is one litre of water per hour during heavy work in hot conditions. Workers should be urinating regularly; dark or infrequent urine is a sign of dehydration.

Acclimatization

Acclimatization is the process by which the body adapts to working in heat. It takes time, and rushing it is one of the leading causes of heat-related illness and death on construction sites.

• New workers: Workers who have not been exposed to hot working conditions need 7 to 14 days of gradually increasing heat exposure to acclimatize. During the first week, limit their workload to 50% of normal and increase by 10% each day.
• Returning workers: Workers returning from a vacation of one week or more, or returning after an illness, lose some of their acclimatization. Give them 2 to 3 days of reduced workload to re-adapt.
• First heat wave: The first hot spell of the season is the most dangerous. Even experienced workers who have been working through a cool spring are not acclimatized when temperatures suddenly jump. Treat the first week of hot weather like a break-in period.
• Individual differences: Some workers acclimatize faster than others. Age, fitness level, body weight, medications, and pre-existing health conditions all affect how well someone handles heat. Do not assume everyone adapts at the same rate.

Work-Rest Schedules

When the Humidex rises, work-rest cycles are essential. The goal is to give the body time to cool down before core temperature reaches dangerous levels.

• Light work (supervision, driving, light assembly): At Humidex 40, take 15 minutes rest per hour. At Humidex 45, take 30 minutes rest per hour.
• Moderate work (walking with loads, masonry, carpentry): At Humidex 38, take 15 minutes rest per hour. At Humidex 42, take 30 minutes rest per hour.
• Heavy work (digging, carrying heavy loads, concrete work): At Humidex 35, take 15 minutes rest per hour. At Humidex 40, take 30 minutes rest per hour. At Humidex 45, consider stopping heavy outdoor work entirely.
• Rest must be in shade or cool areas: Sitting in direct sunlight during a rest break does not count as recovery. Workers need access to shade structures, air-conditioned trailers, or at minimum, a shaded area with air movement.

Shade and Cooling

Employers must provide ways for workers to cool down:

• Shade structures: Canopies, tarps, or purpose-built shade structures should be available near work areas. On large sites, multiple shade stations may be needed.
• Air-conditioned rest areas: An air-conditioned trailer or break room is the gold standard for cooling. If available, this is where workers should take their rest breaks.
• Cooling PPE: Cooling vests, wet bandanas, and misting systems can help reduce body temperature during work. These supplement — they do not replace — work-rest cycles and hydration.
• Modified schedules: Consider starting work earlier in the morning (6 AM instead of 7 AM) and finishing before the hottest part of the day (2 PM to 4 PM). Schedule the heaviest physical tasks for the cooler morning hours.

Emergency Response for Heat Illness

Every construction site should have a heat illness emergency plan during warm months. When a worker shows signs of heat-related illness, act fast:

• For heat exhaustion: Move the worker to a cool, shaded area immediately. Have them lie down and elevate their legs. Remove excess clothing. Apply cool water to their skin — wet towels, misting, or fanning. Give them small sips of cool water if they are conscious and not nauseated. Monitor them closely. If symptoms do not improve within 15 minutes, or if they worsen, call 911.
• For heat stroke: Call 911 immediately. This is a life-threatening emergency. Move the worker to the coolest area available. Remove their clothing. Cool them aggressively — immerse in cold water if possible, or apply ice packs to the neck, armpits, and groin. Fan them vigorously. Do not give them anything to drink if they are confused or unconscious. Stay with them until paramedics arrive.
• Know the signs: Train every worker and supervisor on your site to recognize heat exhaustion and heat stroke symptoms. Post the signs and symptoms in visible locations. Designate someone to monitor weather conditions and alert the crew when Humidex values reach action levels.

The Bottom Line

Heat stress is predictable and preventable. Monitor the Humidex, implement work-rest cycles, provide water and shade, acclimatize new and returning workers gradually, and train everyone to recognize the symptoms. These measures cost very little compared to the cost of a heat-related injury or death.

Heat awareness is part of a broader safety mindset. Workers who are trained to recognize and respond to hazards — whether it is a fall hazard at height or a heat hazard on the ground — are safer workers. If your crew needs Working at Heights certification or any safety training, 4 Your Safety Solutions can help.