Warm weather and your workday: what heat does to the body

The numbers

• WHO estimates about 489,000 heat-related deaths a year between 2000 and 2019, and says heat-related deaths in people over 65 rose about 85% between 2000–2004 and 2017–2021.
• WHO states that heat reduces work productivity and raises the risk of accidents, and lists outdoor and manual workers among those most exposed.
• An open-access review of 35 studies (2012–2022) found heat-related illness was the most commonly reported outcome. Studies also link heat to occupational injuries. In one Guatemalan study, a wet-bulb globe temperature above 30°C was associated with higher injury risk.
• The same review found only 5 studies on kidney disorders and very little evidence on mental health, so those areas are less well understood.

What it means for your workday

Our reading of the evidence

Heat at work is not just an outdoor-job issue. The review lists bakeries, manufacturing and steel work next to construction, mining and agriculture, so hot indoor workplaces count.

NIOSH's acclimatization schedule shows when the risk is concentrated: at the start. A new worker is limited to 20% of the usual time in the heat on day one, with no more than a 20% increase each day. A worker with prior experience steps up over four days: 50%, 60%, 80%, then 100%.

The water advice differs by situation, and that is not a contradiction. NIOSH, for people working in the heat, suggests a cup (8 oz.) every 15 to 20 minutes during moderate activity lasting under two hours, and no more than about 6 cups an hour. WHO's general-public advice is about a cup an hour and at least 2 to 3 litres a day.

What to do now

• Ease in. If you are new to a hot job, ask for a gradual schedule like NIOSH's. Workers who are not physically fit need more time.
• Keep cool water close (NIOSH suggests under 15°C / 59°F), drink small amounts regularly, and skip alcohol and drinks high in caffeine or sugar.
• Use rest and water breaks whenever you feel heat discomfort. Work periods should get shorter and rests longer as heat, humidity and sunshine rise, and for heavy work or when wearing protective gear.
• Look out for others. WHO advises regularly checking on people over 65 and those with chronic conditions, and finding 2 to 3 hours a day somewhere cool.
• Confusion, fainting or collapse in the heat is an emergency. Call your local emergency number.

How solid is this?

• The review is a short, structured summary of the literature, not a pooled meta-analysis. The authors say the evidence is weakest for cardiovascular, neurological, respiratory and mental health outcomes.
• WHO's death figures are global estimates, not counts of workplace deaths.
• NIOSH gives acclimatization percentages and hydration amounts but does not give exact rest durations, so we do not invent them.