Heat Stress at Work: Catching It Before the Collapse

Heat illness is one of the few occupational hazards where a person can go from working normally to dying in under an hour, and where the difference is usually a decision a colleague makes in the first ten minutes.

It is also almost entirely preventable. What follows is how the body handles heat, how to tell the dangerous version from the unpleasant one, and the numbers a workplace should run on: WBGT thresholds, acclimatisation schedules, work-rest ratios, fluid volumes.

What heat actually does to the body#

Your body defends a narrow temperature range with two main mechanisms.

The first is skin blood flow. Blood is diverted to the skin so heat can radiate away. That is not free: it moves a substantial volume away from the core, so the heart beats faster and harder to hold blood pressure up. This is why heat is a cardiovascular stress, and why people with heart disease are at higher risk.

The second is sweating. Evaporating sweat carries away a lot of heat, but only if it evaporates. In high humidity it drips instead and does almost nothing. This is why humidity matters as much as temperature, and why a sweat-soaked coverall in humid air can be worse than the same job in drier heat.

Heavy work generates internal heat too: several times the resting load. Add radiant heat from the sun, a furnace or hot machinery, add impermeable PPE that stops evaporation, and both cooling mechanisms can be overwhelmed while the air temperature reading still looks unremarkable.

Past about 40°C (104°F) core temperature, cells begin to fail, the gut wall becomes leaky, an inflammatory cascade starts and the brain stops working properly. That is heat stroke, and from there damage accumulates by the minute.

Heat exhaustion versus heat stroke#

The reliable dividing line is the brain. A worker who feels terrible but can tell you their name, where they are and what they were doing has heat exhaustion. A worker who is confused, behaving strangely, slurring or collapsed has heat stroke until proven otherwise.

What to do for suspected heat stroke#

Call emergency services, then cool immediately. Cool first, transport second. Survival tracks closely with how quickly core temperature comes down, and cooling started on site beats cooling started at hospital.

In order of effectiveness:

1. Cold water immersion. Get the person into a tub, trough, drum or any container of cold water up to the neck, holding their head clear. By far the fastest method; athletic programs using immediate immersion report survival approaching 100%.
2. Tarp-assisted cooling. No tub? Lay the person on a tarpaulin, have several people lift the edges into a basin, and pour in cold water and ice.
3. Dousing, ice packs, airflow. Strip outer clothing and PPE, soak continuously with cold water, ice packs to the neck, armpits and groin, fan hard. Continuous wetting plus moving air is what matters.
4. Shade alone. Better than nothing, nowhere near enough.

Do not give fluids by mouth to anyone confused or not fully alert. The choking risk is real. Do not delay cooling to arrange transport. Keep cooling until the person is clearly improving or paramedics take over. Everyone with suspected heat stroke needs hospital assessment even if they seem to recover, because kidney, liver, muscle and clotting complications can appear hours later.

The rest of the spectrum#

Heat illness is not two conditions. Watch for heat cramps (painful spasms, often salt-related, in heavy sweaters), heat syncope (fainting on standing, common in the unacclimatised), heat rash, and exertional rhabdomyolysis. Muscle breakdown causing severe pain, weakness and dark cola-colored urine, which can damage the kidneys and needs urgent assessment.

There is also a slower harm: repeated days of heavy work in heat with inadequate rehydration is linked to progressive kidney damage. Epidemics of chronic kidney disease of unknown cause among sugarcane, construction and agricultural workers in hot regions have made this an important emerging occupational issue. It does not announce itself with a collapse; it shows up years later on a blood test.

The numbers: WBGT and exposure limits#

Air temperature alone is a poor guide. The standard occupational measure is Wet Bulb Globe Temperature (WBGT), which folds four things into one index:

• Outdoors with solar load: WBGT = 0.7 × natural wet bulb + 0.2 × globe temperature + 0.1 × dry bulb
• Indoors or no solar load: WBGT = 0.7 × natural wet bulb + 0.3 × globe temperature

The heavy weighting on wet bulb is deliberate: humidity dominates, because it determines whether sweat can evaporate.

Limits are then set as a WBGT value varying with workload and acclimatisation status. Widely used threshold limit values, for a full working hour with no rest:

Above those values, work must be broken up with rest. The higher the WBGT, the more of each hour spent resting in a cool area.

Crucially, clothing changes the number. WBGT is measured on a nearly nude reference worker, so clothing adjustment factors are added: roughly +2°C for cloth coveralls, +4°C for double-layer clothing, and more for vapour-impermeable suits, where WBGT stops being valid at all and physiological monitoring is needed instead. A team in Tyvek suits in a 26°C WBGT environment is not working at 26°C.

Work-rest cycles#

The practical output is a schedule. As WBGT rises for a given workload, the ratio shifts:

• Full hour of work, no scheduled rest, at the lowest end
• 45/15, then 30/30, then 15 minutes work to 45 minutes rest at the top end
• Above that, reschedule, re-engineer or stop the task

Rest must be real rest: genuine shade or air conditioning, PPE loosened, fluids available. Sitting in the sun next to the job is not a rest break, and a break spent walking 400 meters each way to the water point is mostly not a break.

Acclimatisation: the number that saves lives#

Here is the statistic that should drive every heat program. Analyses of workplace heat fatalities repeatedly find the large majority occur in a worker's first few days on the job, and a substantial share on the very first day. New, transferred and returning workers are the ones who die.

Acclimatisation is a real physiological adaptation over 7 to 14 days: you sweat sooner and more, your sweat becomes more dilute so you lose less salt, plasma volume expands, and heart rate at a given workload falls. It cannot be achieved by being told to be careful. It requires graded exposure.

Standard schedules:

New to the job or to the heat: day 1 no more than 20% of a normal workload in heat, then 40%, 60%, 80%, full duty from day 5.

Returning after more than a week away: 50%, 60%, 80%, then full duty from day 4.

Acclimatisation decays. After about a week away from the heat some is lost; after two to three weeks, most people should be treated as unacclimatised again. This is why the Monday after a two-week holiday, and the first hot week after a cool spell, are predictably dangerous.

Hydration that is not just "drink water"#

Sweat rates during heavy work commonly run 0.5 to 1.5 liters per hour and can exceed 2 in extreme conditions. Thirst lags losses, so waiting until you are thirsty means you are already behind.

Volume and timing:

• Roughly 240 mL (one cup) every 15 to 20 minutes during heat work, taken steadily
• Do not exceed roughly 1.5 liters per hour, or about 12 liters a day
• Pre-hydrate before the shift; do not start already short
• Keep drinking afterwards. Much of the deficit is replaced in the evening

Salt, the part everyone forgets. Sweat contains sodium, and heavy sweating over several hours produces meaningful salt loss. If you replace only plain water, blood sodium falls. Symptomatic hyponatraemia causes headache, nausea, confusion and, at the extreme, seizures and death, and its early symptoms look exactly like heat exhaustion, which is why over-drinking sometimes gets treated with more drinking.

Practically: under about two hours in moderate heat, water and normally salted meals are usually enough. For prolonged heavy sweating, use an oral rehydration solution or low-sugar electrolyte drink for part of the intake, and eat salted food at breaks. Avoid high-sugar soft drinks and energy drinks.

Checking yourself, without equipment:

Who is at higher risk#

• Anyone unacclimatised. New starters, transfers, returners from leave or sick leave
• Recent illness. Fever, diarrhea or vomiting in the last 48 hours sharply raises risk. Not someone to put on a hot job today
• Alcohol the night before, and poor sleep
• Older workers, who sweat less efficiently and have less cardiovascular reserve
• Obesity, which raises heat production and reduces heat loss per unit body mass
• Cardiovascular disease, kidney disease, poorly controlled diabetes
• Sickle cell trait, associated with increased risk of exertional collapse and rhabdomyolysis
• Previous heat illness. A strong predictor of another episode
• Certain medicines. Diuretics, some blood pressure medicines, anticholinergics, antihistamines, some antipsychotics and antidepressants, and stimulants can reduce sweating or fluid reserve. Do not stop a prescribed medicine because of heat. Ask your doctor whether it changes your risk.
• Pregnancy

At work: what a workplace should have in place#

OSHA, NIOSH and the ILO converge on the same package. If you are the EHS lead, this is the checklist.

• A written heat illness prevention plan, with named responsibilities and trigger temperatures, not a paragraph in a general policy.
• Measurement. A WBGT meter or validated heat index, a documented action level, and a daily reading logged before the shift.
• A formal acclimatisation protocol for every new starter, transfer and returner, written down and enforced.
• Water, rest, shade. Free, and close enough to the work that a break is actually a break.
• A buddy system. People rarely recognize their own confusion. Pair workers and train each to watch the other.
• On-site cooling capacity: a tub or stock tank, ice, a tarp and a hose at every hot work location. The cheapest life-saving equipment on any site.
• A written emergency plan with the emergency number, the ambulance access point, and the instruction to cool first.
• Training in the languages the workforce actually speaks, repeated before every hot season.
• Explicit authority to stop. Any worker can leave a hot task and rest without permission or penalty. Where people fear losing pay they work through symptoms, and agency, contract and migrant workers are both the most exposed and the most likely to be missed.
• Scheduling as a control. Move heavy work to early morning or night, mechanise the hottest tasks, add shade, ventilate, shield radiant sources. Many Gulf states legally prohibit outdoor work at midday through the hottest months, and the principle is sound wherever you are.

What I actually see#

Two patterns recur.

The person who collapses is almost never the one the supervisor expected. It is rarely the visibly unfit new hire, because everyone is watching him. It is the experienced worker back from two weeks of leave, who knows the job, does not want to be seen taking it easy, and no longer has the adaptation he had a month ago. Acclimatisation status is invisible, which is exactly why it must be tracked on paper rather than judged by eye.

The other is that early symptoms get reported late, and the reason is structural rather than personal. When a rest break costs output, when someone is on a day rate, or when the water point is a five-minute walk away, people push on. Moving the water closer and making rest breaks paid and expected changes incident rates more than any amount of symptom-recognition training.

The question I am asked most is whether someone can "train themselves" to need less water. You cannot. Acclimatisation makes you sweat more, not less: better at cooling, and needing more fluid, not less.

The bottom line#

Heat illness sits on a spectrum, but there is one hard line: if a person working in heat becomes confused, behaves oddly, slurs, staggers or collapses, that is heat stroke, a medical emergency, and you call for help and start cooling immediately rather than waiting for the ambulance. Almost every serious case is preventable through acclimatisation schedules, WBGT-based work-rest cycles, accessible water and shade, and salt as well as fluid. Most fatalities happen in the first days on a hot job, so new and returning workers are where the effort belongs. For the person working in it, the checks are simple: pale urine, steady drinking, a colleague watching you, and an unambiguous right to stop.