Heat Stress Symptoms and First Aid Guide

Last updated: April 2026

It starts with a headache. Then dizziness. Your worker says he just needs a minute, but his skin is pale and clammy, he is sweating through his shirt, and he cannot remember where he set down his tools. Is this heat exhaustion that a 20-minute break in the shade will fix, or heat stroke that needs paramedics in the next 10 minutes?

Heat stress symptoms are the body's warning signals that its cooling system is overwhelmed by heat, physical exertion, or restrictive PPE. Recognizing these symptoms early is the single most important factor in preventing the 55 heat-related workplace deaths and approximately 28,000 heat-linked injuries that occur every year in the United States. This guide walks you through each stage of heat illness, the symptoms to watch for, and exactly what to do when you see them on your job site.

How Heat Illness Progresses: The 5 Stages

Heat illness does not jump from "fine" to "fatal." It follows a progression, and each stage gives you a window to intervene before the next one hits. The problem is that most workers and supervisors on construction and industrial sites have never been taught to recognize the early stages. They know about heat stroke. They do not know that the cramps their worker complained about 30 minutes ago were the body's second warning.

Here is the full progression, sourced from the Canadian Centre for Occupational Health and Safety (CCOHS) and NIOSH guidelines:

Stage 1: Heat Rash

Small red bumps on the skin, usually in areas covered by clothing or PPE, with intense itching. Heat rash happens when sweat ducts get blocked in hot, humid conditions. It is not dangerous on its own, but it is a sign that the worker's body is struggling to dissipate heat. Workers wearing coveralls, harnesses, and hard hats in humid conditions are especially prone.

What to do: Move to a cooler area if possible. Remove wet clothing. Keep the skin dry. The rash usually resolves when the worker returns to a cooler environment.

Stage 2: Heat Cramps

Sharp, involuntary muscle spasms, usually in the legs, arms, or abdomen. Heat cramps are caused by a salt imbalance from heavy sweating combined with drinking large amounts of plain water (which dilutes electrolytes further). The worker can still function, but their body is telling them to stop.

What to do: Stop physical activity. Move to a cool area. Drink water or a sports drink with electrolytes. Gently stretch the cramping muscles. Do not return to heavy work until the cramps have completely resolved.

Stage 3: Heat Exhaustion

This is where things get serious, and this is the stage most people confuse with heat stroke. Heat exhaustion is the body losing its fight against heat, but the cooling system is still working.

Symptoms include:

• Heavy sweating (the body is still producing sweat, which is key)
• Weakness and fatigue
• Dizziness and lightheadedness
• Nausea, sometimes vomiting
• Headache
• Pale, cool, clammy skin
• Rapid, weak pulse
• Muscle cramps
• Intense thirst

What to do: This is a "stop everything" moment. Move the worker to shade or an air-conditioned area. Remove excess clothing and PPE. Apply cool, wet cloths to the head, face, and neck. Have them sip cool water if alert. Do not leave the worker alone. If symptoms do not improve within 15 minutes, get medical attention because this can progress to heat stroke.

Stage 4: Heat Syncope

Sudden dizziness or fainting, caused by insufficient blood flow to the brain. This happens when blood pools in the legs while the body is also diverting blood to the skin for cooling. Most common in unacclimatized workers who have been standing in heat for extended periods. The worker may collapse without other warning signs.

What to do: Help the worker lie down in a cool area with legs elevated. Cool water and rest. Recovery is usually rapid, but the worker should not return to heat exposure for the rest of the shift. If the worker does not regain full consciousness quickly, treat as heat stroke.

Stage 5: Heat Stroke

Heat stroke is a medical emergency. Call 911 immediately.

Heat stroke means the body's cooling system has completely failed. Body temperature rises above 40°C (104°F), and without immediate intervention, organ damage and death can follow within minutes.

There are two types:

• Classical heat stroke: Hot, dry skin with little or no sweating. More common in elderly and chronically ill individuals.
• Exertional heat stroke: Can occur even while the worker is still sweating profusely. This is the type most common on job sites, because the worker's metabolic heat output from physical labour overwhelms the cooling system even when sweating is present.

Symptoms include:

• Body temperature above 40°C (104°F)
• Confusion, irrational behaviour, slurred speech
• Loss of consciousness or seizures
• Hot skin (dry or sweating, depending on type)
• Rapid, strong pulse

Most people believe that if a worker is still sweating, it cannot be heat stroke. That is a dangerous misconception. Exertional heat stroke, the type most likely to hit your workers on a construction site, frequently presents with sweating. The key indicators are confusion, altered mental status, and extremely high body temperature.

What to do:

• Call 911 immediately
• Move the worker to the coolest available area
• Remove as much clothing and PPE as possible
• Apply ice packs to the neck, armpits, and groin (major blood vessel areas)
• Wet the skin with cool water and fan aggressively
• Do not force the worker to drink if they are confused or unconscious
• Stay with the worker until paramedics arrive

Who Is Most at Risk for Heat Stress?

Not everyone on your crew faces the same risk. Understanding who is most vulnerable helps you target your monitoring and prevention controls where they matter most.

Unacclimatized workers are the highest risk group, period. OSHA data shows that the majority of heat-related workplace fatalities occur in the first few days on the job or the first few days of a sudden heat wave. A new hire who has been working indoors or in cooler weather is physiologically unprepared for intense heat. The NIOSH 20% rule (no more than 20% heat exposure on Day 1, increasing daily) exists specifically because of this pattern.

Other high-risk groups:

• Workers over 45: Reduced cardiovascular efficiency and sweat response
• Workers with chronic conditions: Heart disease, high blood pressure, obesity, kidney disease, uncontrolled diabetes
• Workers on certain medications: Diuretics, beta-blockers, antihistamines, and some psychiatric medications affect the body's heat regulation
• Workers in heavy PPE: Fire-resistant coveralls, full-body harnesses, chemical suits, and respiratory protection all trap heat
• Workers performing high-intensity tasks: Concrete pouring, steel erection, roofing, excavation
• Pregnant workers: NIOSH recommends core temperature not exceed 39 to 39.5°C during the first trimester

Heat Exhaustion vs. Heat Stroke: The Critical Differences

This is the comparison every supervisor needs to have memorized. Getting it wrong costs time in the best case and a life in the worst.

The simplest field test: Can the worker carry on a coherent conversation? If they are confused, cannot answer simple questions (What day is it? What is your name? Where are you?), or their speech is slurred, treat it as heat stroke regardless of other signs. Do not wait for a thermometer reading.

After a Heat Illness Incident: Documentation and Reporting

Every heat illness incident, even mild ones resolved on site, should be documented. Here is why it matters beyond compliance.

In the United States: Under OSHA recordkeeping (29 CFR 1904), heat-related illnesses requiring medical treatment beyond basic first aid must be recorded on the OSHA 300 log. Heat-related fatalities must be reported to OSHA within 8 hours. Hospitalizations must be reported within 24 hours.

In Canada: Report to your provincial workers' compensation board. In Alberta, this is WCB Alberta. In British Columbia, WorkSafeBC. In Ontario, the WSIB. Each province has specific reporting timelines for workplace injuries and illnesses.

Beyond compliance, tracking heat incidents reveals patterns: which crews, which tasks, which times of day, which workers are recurring. Use that data to adjust your heat stress training and summer safety program.

Frequently Asked Questions