HYDRATION AND HEAT ILLNESS PREVENTION — A SUPERVISOR'S CHECKLIST

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Every summer, Ontario construction sites become some of the most dangerous workplaces in the province — not because of falling objects or trench collapses, but because of heat. Heat-related illness sends hundreds of construction workers to emergency rooms each year, and in the worst cases, it kills. As a supervisor, you have a legal duty under the Occupational Health and Safety Act to take every reasonable precaution to protect your workers from heat stress. This checklist will help you do exactly that.

Heat illness is not a matter of toughness. It is a physiological response that can escalate from mild discomfort to organ failure in under an hour. Understanding the difference between heat exhaustion and heat stroke — and knowing how to intervene — is the single most important thing you can do for your crew during a heat wave.

Heat Exhaustion vs. Heat Stroke — Know the Difference

These two conditions are on the same spectrum, but they require very different responses. Confusing them can cost a life.

• Heat exhaustion — the worker is sweating heavily, skin is pale and clammy, they may feel dizzy, nauseous, or fatigued. Core body temperature is elevated but typically below 40°C. The worker is still conscious and responsive. This is the warning stage — if you catch it here, recovery is straightforward.
• Heat stroke — sweating has stopped, skin is hot and dry or red, the worker is confused, disoriented, or has slurred speech. Core temperature exceeds 40°C. Seizures and loss of consciousness can follow. This is a medical emergency. Call 911 immediately. Heat stroke can cause permanent brain damage or death within minutes if untreated.

Train every worker on your site to recognize these signs — not just in themselves, but in their coworkers. A buddy system where workers check on each other every 15 to 20 minutes during extreme heat is one of the simplest and most effective interventions available.

Water Intake Guidelines

Dehydration is the gateway to every form of heat illness. By the time a worker feels thirsty, they are already dehydrated. Your hydration protocol needs to be proactive, not reactive.

• Minimum intake — workers should drink at least 250 ml (one cup) of water every 15 to 20 minutes during hot conditions, even if they do not feel thirsty. This translates to roughly one litre per hour.
• Water availability — potable water must be available within easy reach at all times. Do not rely on workers bringing their own supply. Provide coolers with cold water at multiple locations across the site, especially near active work areas.
• Electrolyte replacement — for shifts lasting more than two hours in high heat, supplement water with electrolyte drinks or oral rehydration solutions. Plain water alone can dilute sodium levels if consumed in very large quantities without food or electrolytes.
• Avoid caffeine and energy drinks — these act as diuretics and accelerate fluid loss. Workers should be reminded that coffee, energy drinks, and carbonated soft drinks do not count toward their hydration intake.

Work-Rest Cycles by Humidex

The Humidex — a Canadian index that combines air temperature with humidity — is the standard measure for determining heat stress risk on Ontario construction sites. As the Humidex rises, the ratio of work to rest must shift dramatically.

• Humidex 30-34 — low risk for acclimatized workers, but new or returning workers should take a 10-minute break every hour. Ensure water is available and encourage regular intake.
• Humidex 35-39 — moderate risk. All workers should take a 15-minute rest break in the shade for every 45 minutes of work. Monitor workers for early signs of heat exhaustion.
• Humidex 40-44 — high risk. Work-rest cycles should shift to 30 minutes of work followed by 30 minutes of rest in a shaded or air-conditioned area. Consider rescheduling heavy physical tasks to early morning or evening.
• Humidex 45+ — extreme risk. Consider suspending outdoor work entirely, especially tasks involving heavy physical exertion, direct sun exposure, or PPE that restricts airflow. If work must continue, limit shifts to 15 minutes with 45 minutes of rest.

Post the day's Humidex reading at the site entrance every morning and update it at midday. Make work-rest schedules visible to every crew on site.

Acclimatization for New and Returning Workers

A worker who has been off-site for a week or more, or who is new to outdoor construction work, is significantly more vulnerable to heat illness. The body needs time to adapt to working in high temperatures.

• New workers — limit exposure to 20% of a normal workload on the first day, increasing by 20% each subsequent day over a five-day period. By day five, the worker should be fully acclimatized.
• Returning workers — workers returning after an absence of one week or more should follow a three-day acclimatization schedule: 50% workload on day one, 75% on day two, and full workload on day three.
• Monitor closely — new and returning workers should be paired with experienced crew members and checked on more frequently during their acclimatization period.

Shade and Cooling Strategies

Rest breaks are only effective if workers have somewhere cool to recover. Standing in direct sunlight during a break does not count as rest from a heat stress perspective.

• Shade structures — erect canopies, tarps, or pop-up shelters near active work areas. If natural shade is available, designate it as a rest area and keep it clear of equipment and materials.
• Cooling stations — provide fans, misting stations, or air-conditioned trailers where workers can bring their core temperature down during breaks.
• Cooling PPE — consider providing cooling vests, damp neck towels, or broad-brimmed hard hat attachments that shade the neck and ears. These are inexpensive and highly effective.
• Schedule adjustments — on extreme heat days, start shifts earlier (6:00 AM) and end before the peak heat window (11:00 AM to 3:00 PM). If work must continue through the afternoon, rotate crews and increase rest frequency.

Emergency Response Steps

When a worker shows signs of heat stroke, every second counts. Your crew should know this protocol before the first hot day of the season, not during an emergency.

• Step 1 — Call 911 immediately. Heat stroke is a life-threatening emergency. Do not wait to see if the worker improves.
• Step 2 — Move the worker to shade or an air-conditioned space. Get them out of direct sunlight and away from heat sources.
• Step 3 — Cool aggressively. Remove excess clothing. Apply cold water or ice packs to the neck, armpits, and groin — areas where major blood vessels are close to the skin surface. Fan the worker continuously. If ice baths are available, immerse the worker up to the chest.
• Step 4 — Do not give fluids if the worker is unconscious or vomiting. If they are conscious and alert, offer small sips of cool water.
• Step 5 — Monitor and document. Stay with the worker until paramedics arrive. Record the time symptoms were first noticed, the interventions taken, and the worker's response. This information will be critical for the medical team.

Conduct a heat illness emergency drill at the start of every summer season. Practice the response steps with your crew so that when a real incident occurs, no one is guessing.

Your Legal Obligation as a Supervisor

Under Section 27 of Ontario's Occupational Health and Safety Act, supervisors must take every precaution reasonable in the circumstances to protect workers. During periods of high heat, this means implementing a heat stress program, monitoring conditions, adjusting work schedules, providing water and shade, and training workers to recognize symptoms. Failure to do so can result in fines, stop-work orders, and personal liability in the event of a serious injury or fatality.

Heat illness is entirely preventable. The tools are simple — water, shade, rest, acclimatization, and awareness. Use them.