Over 1,000 people may have died from June heatwave in Spain

The month of June in Spain has been “extremely warm.” This is the term that the national weather service Aemet uses for a month or season that departs markedly from the norm on the high side. The average daily temperature on peninsular Spain was 73.8 degrees Fahrenheit (23.2 degrees Celsius), which is 3.2 degrees above the 1991–2020 average, according to provisional Aemet data obtained by EL PAÍS. That is a large anomaly because most differences are measured in tenths of a degree, not whole degrees. It makes last month the second-warmest June in the state agency’s historical series, which begins in 1961. But you only have to go back 12 months to find the record-holder: June 2025, with an average of 74.5 degrees Fahrenheit (23.6 degrees Celsius) and an anomaly of 3.6 degrees.

But heat is not only a cause of discomfort. It also has serious health impacts. This past June over 1,000 people died in Spain of causes related to the high temperatures — 623 of those deaths occurred during the week of the heatwave — according to a provisional estimate from the daily all-cause mortality monitoring system (MoMo) of the Carlos III Health Institute, a public research agency.

With deaths from the final days of June still to be added, the system had estimated 1.028 deaths as of Wednesday, making it the deadliest month for heat-related fatalities since records began in 2015. Over those 11 years, the average is 330 deaths each June.

The estimate is not a direct count of deaths certified as caused by heat, but a statistical calculation. MoMo compares observed mortality in Spain with the expected mortality for each period and links deviations to episodes of potentially dangerous temperatures.

The role of climate change

The late-June heatwave that struck much of the peninsula and the Balearic Islands between June 22 and 24 contributed to the rise in deaths associated with high temperatures. Until recently, a June heatwave was uncommon. According to Aemet data, between 1975 and 2000 there were only two heatwaves in that month on peninsular Spain. Between 2000 and 2025 there were 10 — five times as many. This is also part of a trend: summers are lasting longer and arriving earlier. In addition to arriving earlier, heatwaves are now also more intense and longer-lasting.

That the two hottest Junes have been the last two is not a coincidence or a statistical fluke. It simply reflects what is happening to the planet: it is overheating because of greenhouse gases emitted by humans, primarily from the use of fossil fuels (oil, gas and coal). Climate change has ushered Earth into an era of fleeting records and a stringing together of historic highs. For example, the last 11 years have been the warmest on average for the planet as a whole.

Making public health plans

To reduce the impacts of heatwaves, prevention is fundamental, chiefly to warn the most vulnerable population groups. Heat-health plans are key for this purpose. The World Health Organization (WHO) released guidance three weeks ago to help develop these tools.

But Julio Díaz, a researcher at the Carlos III Health Institute who has played a decisive role in Spain’s current heat-health alert system, believes there is still significant room for improvement — for example, by refining the relationship between heat and pollution. According to data collected at his center, he says, about 18% of the increase in mortality during heatwaves in Spain is driven by pollution.

They are often two problems that feed each other, as with ozone, which forms from the interaction of high temperatures and traffic pollution. Díaz advocates taking these “joint effects” into account and applying them in heat-health plans — for example, imposing traffic restrictions when high-temperature episodes are forecast.

The mechanisms that cause deaths from high temperatures and from pollution are sometimes similar. Only a few dozen cases each year are due to so-called heatstroke, while the vast majority occur in highly vulnerable people who suffer cumulative effects on their bodies.

Heat causes thermoregulatory strain, dehydration and cardiocirculatory stress, among other physiological problems. Pollution induces oxidative stress, systemic inflammation, endothelial dysfunction, autonomic disruption and thrombogenicity. Both exposures converge on common pathways.

Together they form a “synergistic effect,” according to Hicham Achebak, a researcher at the Institute of Social and Preventive Medicine in Bern. The scientific literature shows how heat and pollutants such as fine particulate matter, ozone and, to a lesser extent, NO₂ (mainly emitted by diesel cars) increase deaths more than the two phenomena do separately.

Spain’s Ministry for the Ecological Transition is already working along these lines, and the new work program of the National Climate Change Adaptation Plan includes development of a specific study on the issue. The idea is to prepare an “integrated health plan on air pollution and heatwaves,” with special attention to Saharan dust and biomass burning. “Identifying temperature thresholds adjusted to air pollution levels is essential to anticipate risks and reduce the health impact on the population,” the ministry document states.

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