Recognition, Prevention, and Ontario Employer Obligations
When the body cannot cool itself fast enough, heat becomes a hazard. For outdoor workers and those in hot indoor environments, heat stress is a genuine and preventable occupational risk. This resource explains the spectrum of heat-related illness, what Ontario law requires, and how to prevent heat injuries.
Bullivant Health + Safety | bullivant.ca | 905-664-4943 | 158 S Service Road, Stoney Creek, ON
This resource is for general informational purposes only and does not constitute legal advice.
1. The Spectrum of Heat-Related Illness
- ~50: Heat-related worker deaths annually in Canada — majority preventable
- First: Day back after time off is highest risk — acclimatization lost within days
- 28°C: Ontario guideline threshold at which enhanced precautions should begin
| HEAT RASH
Skin irritation from excess sweating. Uncomfortable but not dangerous. Indicates excessive heat exposure that needs to be addressed. HEAT SYNCOPE Fainting from blood pooling in extremities. Worker should be moved to a cool area and rehydrated. HEAT STROKE Body temp above 40°C, hot and dry skin, rapid strong pulse, possible unconsciousness. Life-threatening emergency — call 911 immediately. |
HEAT CRAMPS
Painful muscle spasms from fluid and electrolyte loss. A warning sign that hydration and rest are needed immediately. HEAT EXHAUSTION Heavy sweating, weakness, pale skin, fast or weak pulse, nausea. Requires immediate cooling, hydration, and monitoring. RHABDOMYOLYSIS Muscle tissue breakdown from sustained heat stress. Can cause kidney failure. Associated with prolonged exertion in extreme heat. |
2. The Legal Framework
- OHSA Section 25 — general duty to protect workers applies to thermal hazards
- Ontario Regulation 851 — requires workplaces be maintained at a reasonable temperature with adequate ventilation
- Ontario's Heat Stress Guideline — provides Humidex threshold values and work/rest schedules by workload
ONTARIO HEAT STRESS GUIDELINE — KEY THRESHOLD
When the Humidex reaches 35, employers should take preventive action. At a Humidex of 40 and above, work in hot environments should be suspended unless protective measures are in place. The guideline identifies work/rest schedules based on Humidex and physical workload. (Ontario Ministry of Labour, Immigration, Training and Skills Development)
3. Employer Obligations
| HAZARD ASSESSMENT
Assess heat risk considering temperature, humidity, radiant heat, physical workload, and exposure duration. Reassess seasonally and during heat events. ACCLIMATIZATION New workers and those returning from time off must be gradually acclimatized over 7–14 days. Pushing unacclimatized workers into full heat exposure is a leading cause of heat illness. COOL REST AREAS Provide shaded or air-conditioned rest areas. Rest periods are part of the required work/rest schedule, not optional breaks. |
WRITTEN HEAT STRESS PROGRAM
Develop a program including exposure thresholds, work/rest schedules, hydration requirements, acclimatization protocols, and emergency response procedures. HYDRATION Provide access to cool water at all times. Recommend 250mL every 20 minutes during heat-exposed work. Relying on workers to bring their own water is not sufficient. TRAINING & SUPERVISION Train workers and supervisors on heat illness recognition, prevention, and emergency response. Supervisors must identify early symptoms — not rely on workers to self-report. |
4. Worker Rights
- The right to know about heat hazards and the precautions in place
- The right to refuse work they reasonably believe could endanger their health due to heat
- The right to hydration, rest breaks, and cool rest areas without penalty
- The right to report heat-related symptoms or concerns without fear of reprisal
- The right to file a WSIB claim if they suffer a heat-related injury at work
GENERAL INFORMATION NOTICE
This resource is for general informational purposes only. Workers with heat-related symptoms should seek medical attention. Contact the Ministry of Labour for regulatory guidance.