Twenty-Five Years of Heat Index (HI) Variability and Evolution in Twelve Cities Overlooking the Mediterranean Basin

The correlation between heat exposure in outdoor working environments and adverse effects to the health of workers has been widely investigated. Recent epidemiological studies, combined with direct observations, highlight an increase in work-related risks and hazards, which in turn leads to an increase in accidents at work. In this research, we investigated the tendencies and variability of such hazards in the Italian cities of Cagliari, Genoa, Lamezia Terme, Lecce, and Venice, as well as other cities overlooking the Mediterranean Basin, namely Beirut, Cairo, Gibraltar, Istanbul, Larnaca, Tunis, and Valencia.Specifically, we evaluated the Heat Index (HI), which is not considered the most accurate and reliable heat stress indicator, but can be applied on a wide scale as it depends entirely on temperature (Ta) and relative humidity (RH) data.Data products released by NOAA and based on local measurements across the Mediterranean Basin, performed between 2000 and 2024, were collected and analyzed with Jamovi 2.6.19, R v. 4.4.2 and MATLAB v. 2023. The HI provides two threshold values, Danger and Extreme Danger, which correspond to specific combinations of relative humidity and temperature considered hazardous for human health. We calculated the number of hours that, every year between May and October, exceed these thresholds. All trends lines show an increase over time, compatible with anthropogenic climate change. From this, we infer a percentage increase in the order of 60% with respect to productivity hour loss over the time span of two decades. The analysis also showed an extension of the boreal summer to September. Data confirm a constant increase in the number of HI-Danger which, in Tunis for example, increased from 49 days in 2000 to 156 days in 2024.Furthermore, the threshold of 35 °C as a limit for work activities often used in absence of national regulations does not appear to be conservative in more than one third of the studied locations. Despite the limitations of this methodology, this work is intended to set up new mitigation policies, at various scales, in the absence of more detailed methods accounting for additional parameters such as clothing and specific body acclimatation.