Extreme Heat Is No Longer a Climate Risk Alone, but a Growing Health System Crisis

Rising temperatures are already increasing emergency visits, hospital admissions, and health system costs across OECD countries, with older adults and Mediterranean regions facing the greatest risks. Without stronger adaptation, climate change is set to intensify pressure on hospitals as extreme heat becomes more frequent and severe.

Researchers at the OECD Health Division, working with national health authorities in 16 OECD countries and using climate data from institutions such as the European Climate Assessment, the US National Oceanic and Atmospheric Administration, and the EU’s Copernicus Climate Change Service, show that rising temperatures are already putting visible pressure on hospitals. The study links local temperature records with real hospital data to demonstrate that extreme heat is no longer just an environmental concern but a growing health system challenge.

Over the past two decades, exposure to hot days has increased sharply across the OECD. On average, nearly one-third of people now experience hot days each year, up from just over one-fifth in the early 2000s. In countries such as France, Belgium, the Netherlands, and Poland, exposure has more than doubled. As heat becomes more frequent, hospitals are seeing more patients during the hottest periods of the year.

What Happens to Health When Temperatures Rise

The study focuses on conditions most clearly linked to heat, including heatstroke and dehydration, general symptoms such as malaise and fever, and kidney-related diseases. These are conditions where heat plays a direct role, meaning the findings likely underestimate the full health impact of rising temperatures, which are also known to worsen heart and lung diseases.

Across most countries studied, emergency department visits and hospital admissions for these heat-sensitive conditions have increased over the past decade. The effects are strongest during periods that fall within the hottest 10% of temperatures for each country. Southern and Mediterranean countries such as Portugal, Spain, Italy, and Israel show the largest increases, while cooler countries such as Finland and Iceland generally see smaller or statistically weaker effects.

Who Is Most at Risk

Age is one of the most important factors shaping how heat affects health. Children account for a large share of emergency visits linked to malaise and fever during hot periods, reflecting their vulnerability to temperature changes. Older adults face more serious risks. In many countries, people aged 65 and over make up the majority of hospital admissions directly linked to heat, especially for kidney-related conditions.

These kidney-related admissions are particularly concerning because they are more likely to require hospital stays rather than simple emergency treatment. The findings highlight how heat disproportionately affects ageing populations and raises concerns for countries where the share of older people is increasing.

Unequal Impacts Across Regions and Countries

The impact of heat is not evenly distributed, even within the same country. The study finds large regional differences in heat-related hospital use, especially in larger countries. In some cases, people living in the hottest regions experience more than twice the rate of emergency visits compared with those in cooler areas. Emergency departments show sharper regional differences than hospital admissions, suggesting they are often the first point where heat-related stress becomes visible.

At the national level, the financial costs are substantial. Direct hospital costs linked to heat-related emergency visits and admissions range from a few hundred dollars per 1,000 people in some countries to more than USD 5,000 per 1,000 people in France. Hospitalisations account for more than 70% of these costs in almost every country because inpatient care is far more expensive than emergency treatment alone.

Why the Costs Will Likely Rise Further

Looking ahead, the study warns that rising temperatures will increase pressure on health systems. Using international climate projections, researchers estimate that by 2050, many countries will experience far more weeks of extreme heat than they do today. In Mediterranean countries, heat-related hospital use and costs could rise by 15–20%, and even more in places such as Israel.

These projections may still underestimate future impacts. They do not account for population ageing, which will increase vulnerability, or for exposure to unprecedented heat levels beyond experience. At the same time, they also do not include potential adaptation measures, such as better housing, cooling systems, early warning systems, or improved health planning, which could reduce harm if implemented effectively.

The core message of the study is clear: many countries already have the health data needed to understand how rising temperatures affect hospitals in real time. Using this information more effectively could help health systems prepare for a hotter future, protect vulnerable populations, and reduce avoidable strain on hospitals.