On September 30, the World Health Organization released a package of evidence reviews and planning tools for heat-related illness at mass gatherings. The central operational finding is encouraging: when on-site medical systems are well organized, more than 90 percent of patients in the studies WHO reviewed could be treated at the event venue instead of being transferred to hospitals.
That number is useful. It is not a promise.
WHO says the evidence review covered studies published from 1980 through 2025 and documented nearly half a million medical encounters, including more than 22,000 heat-related illnesses. Those observations span different events, climates, medical systems and reporting practices. A pooled finding can show that capable on-site care often prevents unnecessary transport. It cannot tell an organizer that a particular concert, pilgrimage, tournament or festival will achieve the same result.
What the evidence supports
The review supports treating heat as an operational risk, not merely a personal-responsibility problem. WHO’s implementation guidance recommends combining safe drinking water and toilets, cooling infrastructure, schedule adjustments, crowd-flow management, worker protections, public communication, real-time surveillance and prepared on-site medical services. None of those measures is especially glamorous. Together, they form a system that can reduce exposure, identify illness earlier and keep local ambulances and emergency departments from becoming the event’s default heat plan.
The guidance also broadens who counts in preparedness. Spectators matter, but so do volunteers, security staff, vendors, performers, transportation workers and medical teams. Their exposure can be longer than a visitor’s, and their ability to seek cooling or water may be constrained by a job assignment.
What remains uncertain
The new package does not establish one universal temperature at which every event becomes unsafe. Heat risk changes with humidity, direct sun, crowd density, event duration, exertion, clothing, acclimatization, age, medication use, chronic disease and the capacity of local services. Even a strong venue plan cannot erase those differences.
The reported on-site treatment rate also should not be read as evidence that heat illness was mild or inevitable. Treating a patient at the venue may reflect excellent early care; it does not by itself reveal how much illness could have been prevented through earlier scheduling, shade, water access or crowd redesign. Nor does it mean hospitals can be excluded from planning. Severe heat illness can progress rapidly, and transfer pathways remain essential.
The practical takeaway
The most defensible conclusion is narrower and more useful than a slogan: heat-related harm at major events can often be reduced, and much of the resulting medical demand can often be managed on site, when organizers build heat protection into the entire event cycle.
That means planning before the forecast becomes alarming. Organizers need thresholds tied to specific actions, clear authority to modify schedules, accessible water and cooling, trained staff, medical escalation routes and surveillance that can detect rising demand while there is still time to respond. Public messages help, but they work best when the environment makes the safer choice possible.
Evidence does not remove uncertainty. It gives decision-makers a better way to act before uncertainty becomes an emergency.