When heatwave warnings don't tell the whole story

Does heat kill? "I don't think anyone doubts it any more, do they?", was the reply from Julio Díaz Jiménez, a researcher and co-director of the Climate Change, Health and Urban Environment Observatory at Madrid's Carlos III Institute of Health (ISCII).

The figures speak for themselves. "High temperatures killed 8,352 people in Spain last year, mainly in the four months of summer," warned a recent study undersigned by ISGlobal scientists (Barcelona Institute for Global Health). "If we know that heat affects our health, why don't we warn about heatwaves using health criteria?"

Here are two maps from a hot Friday, 9 August, the day the latest heatwave began. The first one belongs to Aemet, Spain's national weather agency and the second is from ISCII's study for the Ministry of Health. The difference? One focuses on health and the other on the weather.

"This work has taken us three years," explained Díaz, the driving force behind the Ministry of Health project together with Cristina Linares Gil. The heart of this project is their published report entitled 'Determination of heatwave mortality thresholds according to isoclimatic regions in Spain'. For each of the isoclimatic zones (IZ), rising from 52 to 182, the study established the maximum daily temperature above which the death rate increases in a statistically significant way. Herein lies the other major divergence with Aemet's colour-coded warning system of yellow, amber, red.

Aemet's heat warnings are based only on the temperatures from historic weather records and are activated when the expected heat is above normal for the summer season. In contrast, the new line of work undertaken by Linares and Díaz for the Ministry of Health covers socio-economic aspects such as income levels and the demographic characteristics of the population and their vulnerability to certain temperatures. It also takes into account "the rural or urban character", "the frequency with which they are subjected or not to heatwaves", "urban infrastructures with the existence or not of green areas" and "the quality of housing." "It is a better representation of the possible impact of heat on our health," says Díaz.

The impact of heat on the body is the same for everyone, but 36C in Seville is not the same as 36 degrees in Gijón. During the last heatwave warning issued on Friday 9 August, the northern third of Spain was free of risk according to Aemet data. Using Ministry of Health criteria, the risk to health was red.

Aemet's warnings are activated when temperature limits are exceeded. In Santander, for instance, the yellow alert is activated when the maximum temperature reaches 34C, amber at 37 and red at 40 degrees. The health threshold, however, is lower at 26.1C - a difference of 7.9 degrees. "I am not saying that we should not give voice to and make known the weather alerts because they are needed, but if we know that heat kills, I repeat, kills, we must do it with health criteria, right?"

Díaz argues for using criteria beyond just the meteorological ones, because "perhaps it is not only the heat that kills." It is also influenced by how the heat is then handled. The map above shows the deaths attributed to heat and recorded on the system for monitoring daily mortality from all causes (MoMo), developed by ISCII.

Last summer, Galicia accumulated more deaths attributable to excess temperatures than Murcia. Oddly enough, A Coruña is the Spanish province with the highest mortality rate for high temperatures. The level above which deaths soar in A Coruña is 27.5C, while in Murcia it must reach 38.8 degrees to match. "I don't understand why the city is not warned with health alerts," Díaz repeated several times in our telephone interview. "It's enough to drive me crazy!" However, he acknowledged that "there is a culture of heat. We know its risks and its impact." Although, according to the team's studies, it is also underestimated.

In the record-breakingly hot summer of 2023, had the system designed by Linares and Díaz been active, heat warnings would have coloured the map red on many more occasions. One of the main conclusions reached by these researchers in their published research is that, in 52.6% of the cases in which the new mortality trigger temperatures detected for these areas are exceeded, the Aemet alerts would not be activated. In other words, the heatwave prevention plan would not have been activated despite there being a risk to public health.

"We citizens have to hold our leaders accountable," says Diaz. "It is up to us to decide whether to plant more trees or concrete slabs. This is also fighting the heat."

At present, and until 30 September, the Ministry of Health has a free subscription service for alerts for each of the isoclimatic zones on its website. First thing each morning, registered users receive a text message or email on their mobile devices with information on their area and the alert level for the day. So, why is it that, if there is a red alert, it does not reach our mobile phones like civil protection notifications do, even without subscription? "I don't know, I'm just a researcher," said Díaz.