Every summer, Ontario construction workers face a hazard that doesn't involve scaffolds, harnesses or heavy equipment: heat. When the Humidex climbs and you're doing heavy physical work in direct sun, your body can overheat dangerously fast. Heat illness can kill, and almost every case is preventable.
This guide covers how heat illness develops, what Ontario law requires, the Humidex response plan Ontario workplaces use, hydration and acclimatization, what to do in an emergency, and a daily checklist supervisors can run on hot days.
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 worker who needs to cool down and a life-threatening emergency.
Heat exhaustion happens when the body is losing the battle to cool itself. Signs include heavy sweating, weakness, dizziness, nausea, headache, intense thirst, muscle cramps, and pale, cool, moist skin. It is serious and needs treatment right away, but caught early, recovery is usually straightforward.
Heat stroke is a medical emergency: the body's cooling has failed and core temperature is often above 40°C. Signs include confusion, irrational behaviour, slurred speech, collapse, seizures and loss of consciousness. Heat stroke can cause permanent organ damage or death. Call 911 immediately.
Don't wait for a worker to stop sweating before you act. That is a common myth: when heat stroke comes on during hard physical work, sweating is usually still present. The warning sign that matters is a change in behaviour. Confusion, clumsiness, slurred speech or collapse on a hot day is a 911 call, not a water break.
What Ontario Law Requires
Ontario has no regulation that sets a maximum working temperature or a heat stress standard for construction. The duties come from the Occupational Health and Safety Act (OHSA):
Employers must “take every precaution reasonable in the circumstances for the protection of a worker” (section 25(2)(h)). On a hot day, that means a heat stress plan: water, shade, rest, training and monitoring.
Supervisors have the same duty under section 27(2)(c), and must tell workers about any danger they know of.
Workers have the right to refuse work they believe is likely to endanger them, under section 43.
Joint Health and Safety Committees and health and safety representatives should review the heat plan before summer, and workers can raise heat concerns with them at any time.
To judge what's reasonable, the ministry points employers to the heat stress and strain threshold limit values (TLVs) published by the American Conference of Governmental Industrial Hygienists (ACGIH). OHCOW's Humidex plan, below, turns those TLVs into Humidex readings anyone on site can use.
Dedicated heat rules have been proposed but are not law. The ministry consulted on a stand-alone heat stress regulation in 2023, and a private member's bill, Bill 36 (the Heat Stress Act, 2025), was introduced in May 2025. As of October 2026 the bill has not gone past first reading, so the general duties above are still what inspectors enforce.
Heat stroke is a medical emergency that can kill in minutes. If a worker becomes confused, starts acting strangely or collapses on a hot day, call 911 immediately and start cooling them with water and shade. Do not wait to see if they recover on their own.
The Humidex Response Plan
The Humidex combines air temperature and humidity into one number for how hot it actually feels. OHCOW's Humidex Based Heat Response Plan turns the ACGIH TLV into actions at each Humidex level. It assumes moderate work by workers who are not acclimatized, and in Ontario, OHCOW treats workers doing moderate work as unacclimatized unless they regularly work around significant heat sources such as ovens or foundries.
Humidex 25–29: supply water to workers as needed.
Humidex 30–33: post a Heat Stress Alert notice, encourage workers to drink extra water, and start recording the temperature and relative humidity every hour.
Humidex 34–37: post a Heat Stress Warning notice, tell workers they need to drink extra water, and make sure everyone is trained to recognize the symptoms.
Humidex 38–39: work can continue with 15 minutes of rest per hour. Provide cool water (10–15°C), at least one cup (240 mL) every 20 minutes. Workers with symptoms should get medical attention.
Humidex 40–41: work can continue with 30 minutes of rest per hour, plus everything above.
Humidex 42–44: if feasible, work can continue with 45 minutes of rest per hour.
Humidex 45 and over: only medically supervised work can continue.
Two adjustments matter on construction sites. First, the plan works from an adjusted Humidex: its steps tell you to add to the measured reading for radiant heat, such as working in direct sun, and for extra clothing like coveralls. Second, the plan is built for moderate work, so heavier work such as digging, carrying heavy loads or placing concrete needs more rest than the table shows. Near extra sources of heat or humidity, OHCOW's steps tell you whether the plan still fits; where it doesn't, follow the ACGIH TLV directly.
Post the day's Humidex at the site entrance at the start of the shift and update it at midday, so every crew knows which level applies.
Hydration
Water is the cheapest heat control on any site. By the time a worker feels thirsty, they are already behind.
Drink before you're thirsty: from Humidex 38, OHCOW's plan calls for at least one cup (240 mL) of cool water every 20 minutes, thirsty or not.
Keep it close and cool: put coolers of cool water near each work area, not just at the trailer, and don't rely on workers bringing their own. If workers have to walk ten minutes for a drink, they won't drink enough.
Electrolytes for long, sweaty shifts: over several hours of heavy sweating, alternate water with electrolyte drinks and make sure workers eat. Very large amounts of plain water without food or salt can dilute the body's sodium.
Skip the alcohol: drinking the night before a hot shift means starting the day dehydrated, and energy drinks are no substitute for water.
Check the colour: workers should be urinating regularly. Dark or infrequent urine is a sign of dehydration.
Acclimatization
The body adapts to heat over one to two weeks of gradually increasing exposure. Many fatal heat illnesses happen in a worker's first few days in the heat, so easing people in is one of the most effective protections you have. NIOSH recommends:
New workers: no more than 20% of the usual time in the heat on day one, increasing by no more than 20% each day.
Workers with previous experience, including those back from a week or more away: no more than 50% on day one, 60% on day two, 80% on day three and 100% on day four.
The first heat wave: even a seasoned crew isn't acclimatized when the first hot spell follows a cool spring. Treat that first week like a break-in period for everyone.
Individual differences: age, fitness, body weight, medications and health conditions all affect how someone copes with heat. Pair new and returning workers with an experienced buddy and check on them more often.
Shade, Cooling and Scheduling
Shade near the work: canopies, tarps or pop-up shelters close to active areas. A rest break in direct sun doesn't count as recovery.
Cooled rest areas: an air-conditioned trailer is the best place for rest breaks. Fans and misting stations help where there isn't one.
Cooling PPE: cooling vests, wet neck towels and hard hat sun shades reduce the heat load. They add to rest and water; they don't replace them.
Move the heavy work: start earlier and schedule the heaviest tasks for the morning, before the afternoon peak. If work has to continue through the hottest hours, rotate crews and add rest.
Emergency Response for Heat Illness
Every site should have a heat illness emergency plan in place before the first hot day. When a worker shows signs of heat stroke:
Call 911 immediately. Don't wait to see if they improve.
Move them to shade or a cool space, away from heat sources.
Cool them fast. Remove outer clothing. If you can, immerse them in cold water up to the chest. If not, soak them with cool water, apply cold, wet cloths or ice to the neck, armpits and groin, and fan them continuously.
Don't force fluids, and give nothing to drink if they are confused, unconscious or vomiting.
Stay with them and write it down: when symptoms started, what you did and how they responded. Paramedics will need it.
For heat exhaustion, move the worker somewhere cool, have them lie down, loosen or remove heavy clothing, cool their skin with water and fanning, and give water or a sports drink in small sips if they are alert and not nauseated. Get medical help, and call 911 if they get worse or don't improve quickly.
A Supervisor's Daily Heat Checklist
Check tomorrow's forecast Humidex and plan the heaviest tasks for the morning.
Measure and post the Humidex at the start of the shift and at midday. Once it reaches 30, record temperature and humidity every hour.
Fill the water coolers and place them near each work area.
Set up shade or a cooled rest area before it's needed.
Put new and returning workers on a reduced schedule with a buddy.
From Humidex 38, post the work-rest schedule and make sure breaks are taken.
Make sure everyone knows the signs of heat stroke and who calls 911.
Run a heat emergency drill at the start of the season.
Keep the day's readings and what you did about them. If an inspector asks how you managed the heat, that record is your answer.
The Bottom Line
Heat stress is predictable and preventable. Track the Humidex, follow the response plan, keep water and shade close, ease new and returning workers in, and make sure everyone can spot heat stroke. These measures cost very little next to a heat-related injury or death.
Heat awareness is part of a broader safety mindset. Workers who are trained to recognize hazards, whether a fall hazard at height or heat on the ground, are safer workers. If your crew needs Working at Heights certification or any safety training, 4 Your Safety Solutions can help.
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