The health impact of heat waves on the population is not determined by the daily maximum temperatures alone. It also depends on a range of factors that go beyond the intensity of these events. This distinction arises from the different definitions of a heat wave used in meteorology versus public health.
A study from the Carlos III Health Institute (ISCIII) titled Determination of mortality thresholds for heat waves by isoclimatic regions in Spain shows that health-oriented analyses focus on potential mortality effects, while meteorological analyses concentrate on temperature percentiles, duration, and intensity. The research reveals that in more than half of the cases, specifically 52.6 percent, the hottest months’ maximum temperature series percentile falls below the 95th percentile, which is the meteorological definition of a heat wave.
Researchers warn that using this percentile would mean not activating the Heat Wave Prevention Plan when it is needed in more than half of the isoclimatic zones in Spain, with a resulting toll on mortality that could be avoided by timely plan activation. Conversely, for regions with percentiles above 95, triggering those precautions would occur when not strictly necessary.
The study was conducted by the Climate Change, Health, and Urban Environment Unit at the ISCIII National School of Health. Authors Cristina Linares, Julio Díaz, José Antonio López-Bueno, and Miguel Ángel Navas-Martín highlight that the factors shaping heat-related mortality extend beyond temperature levels and intensity. Demographic characteristics, income levels, socioeconomics, social vulnerability, housing quality, urban infrastructure, and the presence of green spaces all influence how deadly a heat wave can be for a community.
As emphasized by the health minister during the launch of the Health and Climate Change Observatory, this crisis seriously harms health but does not affect all groups equally. Vulnerable populations such as the elderly, pregnant people, children, individuals with disabilities, and low-income households are especially at risk. Efforts to reduce inequalities aim to improve lives with more years of healthy living.
Local factors
The study argues that the variation in heat wave mortality and the timing of temperature percentiles are driven by local conditions. This clear influence of local factors suggests a need to apply prevention plans at a finer scale than the provincial level when temperatures rise, to better reflect actual risk on the ground.
This local specificity could lead to fewer heat-related deaths and a decrease in the number of alerts activated each year due to high temperatures across Spain.
The conclusion is that health-based heat wave definitions should not rely on a single fixed percentile for every geographic area. Instead, a health-focused definition that incorporates vulnerability factors should support the determination and activation of prevention plans tied to high temperatures.