Occupational Heat Illness

Occupational heat illness is a spectrum of disorders from heat strain at work, ranging from heat rash, cramps, oedema, syncope and exhaustion to heat stroke. Heat stroke is a life threatening emergency with core temperature over 40 degrees Celsius and central nervous system dysfunction.

Epidemiology. Outdoor and hot indoor workers (agriculture, construction, mining, foundry, firefighting, military, and athletes) are most affected. Deaths cluster in the first days of work in the heat and during heat waves.

Risk factors. High temperature, humidity and workload, lack of acclimatisation (usually 7 to 14 days of graded exposure), protective clothing, dehydration, obesity, age, poor fitness, prior heat illness, intercurrent illness, and drugs (diuretics, anticholinergics, antipsychotics, stimulants, alcohol).

Types of heat stroke. Exertional (young active workers, rapid onset, often with rhabdomyolysis) and classic (older or ill people in heat waves, slower onset).

Complications. Rhabdomyolysis, acute kidney injury, disseminated intravascular coagulation, liver injury, ARDS, seizures and multi-organ failure. Mortality rises with delay in cooling.

Pathophysiology.

Heat production from work plus environmental load exceeds heat loss by sweating and skin blood flow. Rising core temperature causes direct cellular injury, endothelial damage, gut barrier failure with endotoxaemia and a systemic inflammatory response.

Cardiac output cannot meet both skin and organ perfusion, so ischaemic injury and rhabdomyolysis follow.

• Heat exhaustion: fatigue, headache, dizziness, nausea, tachycardia, profuse sweating and core temperature under 40 degrees Celsius, with intact mental state.
• Heat stroke: confusion, delirium, ataxia, seizures or coma with core temperature over 40 degrees Celsius; skin may be sweaty (exertional) or dry (classic).
• Muscle pain, dark urine, vomiting and diarrhoea.

Red flags.

• Any altered mental state with heat exposure.
• Core temperature over 40 degrees Celsius, seizures, hypotension or oliguria.

• Core (rectal or oesophageal) temperature; oral, axillary and tympanic readings are unreliable.
• Glucose, sodium (hyponatraemia can occur in exertional illness), potassium, creatinine, creatine kinase, liver tests, coagulation screen, lactate, arterial gas and urinalysis for myoglobin.
• ECG and troponin if chest pain or arrhythmia.
• Repeat creatinine, creatine kinase and liver tests at 24, 48 and 72 hours; liver enzymes peak at 24 to 72 hours.

Differential diagnosis. Sepsis, meningitis and encephalitis, thyroid storm, neuroleptic malignant syndrome, serotonin syndrome, malignant hyperthermia, anticholinergic and sympathomimetic toxicity, alcohol or drug withdrawal, hypoglycaemia, malaria, cerebrovascular event.

• Heat stroke: cool first, transport second. Remove clothing and use cold or ice water immersion (1 to 5 degrees Celsius) to bring core temperature to 39 degrees Celsius, aiming for a cooling rate over 0.15 degrees Celsius/minute. When immersion is not possible use continuous cold water spraying with fanning, ice packs to the neck, axillae and groin, and cold wet towels. Stop active cooling at 38 to 39 degrees Celsius to avoid overshoot. Check rectal temperature continuously.
• Antipyretics (paracetamol, aspirin) do not work and are not advised.
• Intravenous 0.9% sodium chloride 1 litre over 1 hour for hypovolaemia, then by urine output, blood pressure and sodium. Take care with large volumes in exertional hyponatraemia.
• Rhabdomyolysis: isotonic crystalloid to maintain urine output over 200 mL/hour; monitor potassium, calcium, creatinine and fluid balance; renal replacement therapy for refractory hyperkalaemia, acidosis or oliguria.
• Seizures, severe agitation or shivering: lorazepam 2 to 4 mg intravenously over 2 minutes, repeated once after 10 minutes if needed. Maximum 8 mg. Monitor breathing and blood pressure; caution in respiratory depression and hepatic impairment.
• Heat exhaustion: rest in a cool place, remove excess clothing, oral rehydration with fluids containing sodium (about 0.5 to 1 litre over the first hour), and intravenous 0.9% sodium chloride 1 litre over 1 hour if vomiting or unable to drink. Observe until symptoms settle and temperature is normal.
• Prevention: acclimatisation over 7 to 14 days, work and rest cycles directed by wet bulb globe temperature, water access and drinking about 250 mL every 15 to 20 minutes in heavy work (not over 1.5 litres/hour), shade, light clothing, buddy checks and early recognition.
• Return to work after heat stroke is graded and individualised: no work in heat for at least 1 week, then gradual re-acclimatisation, with occupational physician clearance, and longer if organ injury occurred.

Monitoring. Core temperature, mental state, urine output and vital signs continuously during cooling; laboratory tests at 24, 48 and 72 hours.

Referral and escalation. Emergency transfer for any altered mental state; intensive care for heat stroke with organ dysfunction; occupational physician for return to work and workplace controls; notification and compensation where required.