Heat stress at work: building a prevention plan before summer

In August 2025, the World Health Organization and the World Meteorological Organization published Climate change and workplace heat stress, a technical report and guidance on the health and productivity risks of heat for workers. Their joint announcement lists heatstroke, dehydration, kidney dysfunction and neurological disorders among the effects, and calls for occupational heat action plans tailored to each industry and region, developed with employers, workers, unions and public health experts.

The International Labour Organization adds a finding that changes how a plan is built: nine out of ten workers exposed to excessive heat worldwide were exposed outside a heatwave, and eight in ten heat-related occupational injuries happened outside heatwaves. It calls for year-round heat action plans. A kitchen, a laundry or a warehouse under a metal roof can be dangerous with no weather warning at all.

No single temperature tells you when work becomes unsafe. The risk depends on temperature, humidity, sunlight, effort, clothing, duration, acclimatization and the person: EU-OSHA notes that severity also depends on individual characteristics such as age, health, socioeconomic status and even gender.

France offers a worked example of turning this guidance into law. A decree of 27 May 2025 added a chapter on intense heat episodes to the French Labour Code, applicable since 1 July 2025 according to INRS, the national occupational safety institute. The episode is defined by reference to the heat warning system of Météo-France, the national weather service (article R. 4463-1): it starts at the yellow level and covers orange and red, as INRS explains.

Employers must assess the risks of these episodes, indoors and outdoors, and define prevention measures where a risk to health or safety is identified (article R. 4463-2). INRS asks for the results to be recorded in the mandatory written risk assessment, the document unique or DUERP, with an action plan prepared in advance.

The following articles list the measures risk reduction relies on: work processes that avoid or reduce heat exposure, changes to workplaces and workstations, adapted work organization and hours to limit exposure and allow rest, technical means against sunlight and heat build-up, more cool drinking water, suitable equipment and personal protective equipment, and information and training. During an episode, the employer provides enough cool drinking water and adapts the measures if the heat intensifies.

French law sets no maximum temperature above which work is banned, INRS points out. But if the employer considers that employees are in danger, work must stop.

Start by observing the tasks. The same job can be barely exposed in the morning and heavily exposed in the afternoon. Note the areas, hours, effort, heat sources, radiating surfaces, protective equipment worn, access to water and room for breaks, without forgetting travel and unplanned call-outs.

Involve employees and their representatives: they know where the air does not move, when the pace leaves no time to drink, and why a shaded area goes unused. Occupational health services can help with the assessment and with protecting vulnerable people. Record the results in your risk assessment and link them to a dated action plan: a risk written down with no owner, deadline or trigger protects nobody.

The order of the measures matters: those that remove or reduce exposure come before those that ask people to cope with it.

Reduce exposure at the source

Moving a task to a cooler time of day, mechanizing a physical operation, shielding against radiant heat or choosing a cooler process acts on the cause. INRS lists mechanizing physical tasks, fans, blinds and shelters from the sun for outdoor work.

Adapt the organization of work

Changing hours for outdoor work, limiting exposure time and rotating tasks where less exposed roles allow it are the levers INRS describes, along with avoiding lone work in favor of teamwork. Targets and pace must follow, or people will skip breaks to keep up.

Fit out workstations and water access

Ventilation, shading, cool areas and water close to workstations reduce exposure. Personal protective equipment is still worn in hot weather, so INRS recommends choosing it in advance with working conditions in mind.

Inform and train

Explain the warning signs, the site’s measures, how to report someone feeling unwell and what to do in an emergency. INRS explicitly includes temporary staff and new hires. Train managers to change the organization of work before anyone collapses, and repeat the information when teams change.

The plan can link each warning level to minimum actions agreed in advance: checking water stocks, sending out adapted hours, suspending certain tasks, reinforcing breaks, checking on lone workers. Measures have to be adapted when heat intensifies; a plan that already sets out these steps avoids improvising on the day.

Do not rely on the weather map alone. Add local triggers, such as conditions in a workshop or a ventilation failure, and let supervisors adapt on the ground: physical work in the sun is not the same exposure as light work in the shade.

Name who checks the warnings, who decides, who informs and who checks that measures are applied. Plan for weekends, night shifts and subcontractors.

The WHO and WMO single out middle-aged and older workers, people with chronic health conditions and those with lower physical fitness. INRS mentions conditions heat can worsen, mainly cardiorespiratory, kidney and endocrine ones such as diabetes, and treatments that affect temperature regulation.

The employer does not need a diagnosis to protect someone. In France, when the employer is informed that a worker is particularly vulnerable, for reasons including age or health, the measures are adapted with occupational health (article R. 4463-5). The company tells everyone they can turn to that service, which is bound by medical confidentiality. How the roles are split is covered in our article on employee health privacy.

Unacclimatized and lone workers

Acclimatization reduces the risk but takes time: usually 8 to 12 days according to INRS, and it is lost within 8 days once exposure stops. New starters, temporary staff and people returning from leave need a gradual increase in workload and close follow-up.

For lone or remote workers, French law requires procedures to report warning signs or someone feeling unwell and to get help quickly, shared with workers and with occupational health (article R. 4463-6).

Fatigue, headache, intense thirst, cramps, dizziness and nausea are signs of heat-related illness described by INRS. Dry, red, hot skin, confusion, unusual behavior, loss of consciousness or a core temperature above 39 °C point to heatstroke, which is rare but can be fatal: it is a life-threatening emergency.

The first step is to call your local emergency number (15 or 112 in France, 911 in the US, 999 in the UK), then move the person to a cool, shaded place, cool them down and stay with them until help arrives. The site procedure states who calls, how responders reach the area and where the equipment is.

After an incident or a near miss, the debrief is there to revise the assessment and the measures, not to blame the person for struggling in the heat.

Track organizational indicators: workstations assessed, actions completed, water availability, ventilation working, breaks actually taken, training, incidents and time to fix them. Anonymous feedback from teams shows the gap between the plan and reality. Our method to measure a workplace health program helps choose them.

A physiological wristband does not fix an excessive pace or an exposed workstation. Measuring each employee’s body temperature or other health data only makes sense for a specific prevention question, within a medical framework, and never for management.

With MyTwin for employers, the organization offers preventive health modules; employees who want to can follow a personal journey, coordinated by a MyTwin physician or by the company’s own medical team, and the employer only tracks results in aggregate.

1. Lead. Name an owner and involve employee representatives, staff and occupational health.
2. Map. Identify the tasks, places, hours and people exposed, outside heatwaves included.
3. Assess. Update the risk assessment and rank the risks.
4. Choose. Favor technical and organizational measures over personal equipment.
5. Escalate. Set actions for each warning level, plus local triggers.
6. Respond. Organize information, reporting, emergency response and checks on lone workers.
7. Test. Try the plan before summer and fix its weak points.
8. Review. Check how the plan is applied and revise it after every episode or major change.