Heat stress happens when the body cannot get rid of heat fast enough, because of hot or humid conditions, hard physical work, heavy clothing or all three. It ranges from rashes and cramps to heat stroke, which can kill. Canadian summers are short but increasingly hot, and many indoor workplaces such as mechanical rooms, kitchens and plants are hot all year. Employers must assess heat as a hazard and control it.
What drives heat stress
- Air temperature and humidity. High humidity stops sweat from evaporating, which is the body's main cooling method.
- Radiant heat from the sun, furnaces, boilers, hot pipes and roofs.
- Air movement, which helps cooling unless the air is hotter than the skin.
- Workload. Heavy work generates a lot of internal heat.
- Clothing and PPE. Coveralls, chemical suits and respirators trap heat.
- The worker. Acclimatization, fitness, hydration, some medications and medical conditions all affect heat tolerance.
Measuring the risk
Two common approaches are used in Canada:
- Humidex, which combines temperature and humidity. Humidex-based heat stress plans, with action levels adjusted for workload and clothing, are widely used as a practical screening tool, especially in Ontario.
- Wet bulb globe temperature (WBGT), which also accounts for radiant heat and air movement. ACGIH threshold limit values for heat stress use WBGT together with workload and work-rest cycles, and some provinces, including British Columbia, reference them.
Check which method your province expects. For hot indoor processes or workers in protective suits, assessment by a competent person using WBGT is usually more reliable than humidex.
Heat-related illness
| Condition | Signs | What to do |
|---|---|---|
| Heat rash | Red, itchy bumps where sweat collects | Keep skin cool and dry |
| Heat cramps | Painful muscle cramps during or after work | Rest in a cool place, drink fluids |
| Heat exhaustion | Heavy sweating, weakness, dizziness, headache, nausea, cool clammy skin | Move to a cool place, loosen clothing, cool down, give fluids if alert, get medical advice |
| Heat stroke | Confusion, slurred speech, collapse, seizures; skin may be hot and dry or still sweating | Medical emergency: call 911, cool the person rapidly while waiting |
Confusion is the red flag. Any worker in the heat who becomes confused, behaves strangely or collapses should be treated as having heat stroke. Call emergency services and start cooling right away. Do not wait to see if they improve.
Controls
Engineering
- Shade over outdoor work areas and rest areas.
- Ventilation, spot cooling and air conditioning in enclosed areas.
- Shielding and insulation on hot surfaces and equipment.
- Mechanical aids to reduce physical workload.
Administrative
- A written heat stress plan with action levels tied to your chosen measure.
- Schedule heavy work for cooler times of day.
- Work-rest cycles that lengthen rest as conditions worsen.
- Cool drinking water close to the work, and encouragement to drink regularly rather than waiting for thirst.
- Acclimatization for new and returning workers, starting with shorter exposures and building up over several days.
- A buddy system and supervisor checks during hot periods.
- Training on signs, symptoms and first aid.
Clothing and PPE
- Light-coloured, breathable clothing where the hazard assessment allows.
- Hats or hard hat sun shades, and sunscreen for UV exposure.
- Cooling vests and other cooling garments where suitable, particularly with impermeable protective clothing.
Protective clothing that must be worn for another hazard, such as chemical suits or arc-rated clothing, increases heat load. Plan for that rather than letting workers remove the protection. See choosing the right PPE.
Writing a heat stress plan
A short written plan makes the response consistent from one supervisor to the next. It usually covers:
- Who checks the forecast or takes measurements, how often, and where the readings are recorded.
- Action levels, adjusted for workload and clothing, and what changes at each level: more water and rest, shorter work periods, rescheduling heavy tasks, or stopping non-essential work.
- How new and returning workers are acclimatized.
- Where water, shade and cool rest areas are, and who keeps them supplied.
- First aid and emergency steps, including who calls for help and how a worker is cooled while waiting.
Review the plan before each summer, and after any heat-related illness or near miss.
Indoor heat in buildings
Boiler rooms, mechanical penthouses, attic spaces and rooftops can be far hotter than outdoor conditions suggest. Maintenance staff working in these spaces in summer need the same planning as outdoor crews. Ventilation and cooling problems in occupied spaces may also point to building system issues worth investigating; see ventilation and indoor air quality. Hot, poorly ventilated spaces may also need to be assessed as confined spaces.
How ehZee can help
ehZee helps employers write heat stress plans, include heat in their hazard assessments and train supervisors, as part of a wider safety program. See safety program development and review.
Common questions
Should workers drink sports drinks?
Water is usually enough for most work. For long, heavy work in heat with a lot of sweating, drinks with electrolytes or regular meals help replace salt. Avoid relying on energy drinks or large amounts of caffeine.
Does acclimatization last?
It fades after a period away from hot work, such as a vacation or a stretch of cool weather. Workers returning to heat should be eased back in.