Heat-attributable Mortality in the Summer of 2022 in Spain

To the Editor:

In July 2022, Spain experienced an excessive heat episode during which many cities broke temperature records. According to the Spanish Meteorological Agency, it has been the warmest month in Spain ever, at least since 1961.¹ The average mean temperature was 2.7ºC higher than the long-term average. The same heatwave event later affected other parts of Europe. For example, the United Kingdom recorded the hottest day ever, with a maximum temperature of 40.3ºC in the east of England. Likewise, August has also been very warm in Spain, with a mean temperature of 2 ºC above the historical average for this month.

We estimated the mortality attributable to heat using the observed daily mortality by the Spanish daily mortality surveillance system (MoMo) and the nationwide mean temperature averaged from the 52 capital cities by calculating the temperature–mortality association during the summer months (June to August) between 1999 and 2018 (eAppendix 1; https://links.lww.com/EDE/C3).² Briefly, we modeled the temperature–mortality association nationwide using the daily mortality counts for all natural causes (ICD-9: 1–799 and ICD-10: A00-R99) for the 52 Spanish capital cities and their average daily mean temperature. We analyzed data using Poisson regression with distributed lag-nonlinear models.³ We estimated heat-attributable mortality as the sum of the contributions for days in the summer of 2022 with temperatures between the optimal minimum mortality temperature as a reference, and the 95th percentile of the temperature distribution in the summer months between 1999 and 2018. We defined deaths attributable to extreme heat as the sum of days above the 95th percentile as reference.⁴ The choice of the 95th percentile as extreme heat threshold is based on the definition of heatwave intensity from the heat-health warning system by the Spanish Ministry of Health.⁵

The cumulative deaths estimated to be attributable to moderate heat were 1,798 in June, 2,588 in July, and 2,352 in August (Table). In June, the extreme heat threshold (26.5 ºC) exceeded five consecutive days causing 706 deaths. However, July and August were strongly affected by extreme heat as conditions exceeded this threshold for 2 weeks, almost consecutively, between 10 and 25 July, and continued for another 2 weeks, between 31 July and 13 August. Consequently, we estimated attributable deaths to extreme heat of 3,204 in July and 1,406 in August.

TABLE. - Number of Deaths Attributable to Moderate and Extreme Heat in Spain Between 1 June and 31 August 2022

	Temperature (ºC)		Moderate heat a			Extreme heat b
Month	Mean	(Range)	Days	Deaths	(95% CI)	Days	Deaths	(95% CI)
June	22.9	(19.7; 28,2)	25	1,798	(281; 3,621)	5	706	(668; 909)
July	26.2	(21.7; 29.0)	17	2,588	(1,629; 4,042)	14	3,204	(3,011; 4,079)
August	25.7	(21.7; 28.1)	19	2,352	(1,474; 3,812)	12	1,406	(1,371; 1,837)

^aModerate heat: days between the MMT (15 ºC) and the extreme heat threshold (26.5 ºC).

^bExtreme heat: days above the extreme heat threshold.

Excess high temperatures and heatwaves are relevant events due to their role in heat-related mortality, social vulnerability, and economic impact in the agriculture and energy sectors. Although the apparent impact of heat on health has decreased over time,⁶ a sensitivity analysis using the exposure–response function with data from the last decade, 2009–2018, estimated a similar impact on the magnitude of heat-attributable mortality (eAppendix 2; https://links.lww.com/EDE/C3). The summer of 2022 has shown the largest extreme heat impact on mortality compared with previous years (eAppendix 3; https://links.lww.com/EDE/C3). The extreme heat-attributable deaths have been almost five times larger than in 2018 and 2020, and the number of days exceeding the extreme heat threshold has tripled. Nevertheless, this summer’s extreme heat impact is at the same level as the August 2003 heatwave, which in Spain was estimated to have killed 6595 people, according to official data.⁷ However, it is essential to clearly define the reference scenario for assessing and interpreting heat-attributable mortality.⁴ The minimum mortality temperature allows quantifying the overall impact of heat from an optimal temperature of minimum risk for health. In contrast, the reference for extreme heat threshold can be considered an added dimension of heatwave intensity.

Beyond quantifying the heat-attributable mortality, our study shows not only the magnitude but exceptionality of summer 2022, compared with recent years and similar to the 2003 heatwave. Unfortunately, according to the 6th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) projections, this exception will become the usual scenario in Spain, one of the countries most affected by increasing temperatures.⁸ Moreover, one in three current heat-related deaths is already known to be related to human-induced climate change.⁹ Our findings shed light on the increasing burden of heat-related mortality among vulnerable populations.