Investigating heat, cold and air pollution effects on mortality in a coastal Mediterranean city

The impact of various environmental factors on human health is often assessed in a non-interactive way, yet emerging evidence suggests synergy among multiple risk factors on health outcomes. In Thessaloniki, frequent exposure to air pollution, primarily O₃ in summer and PM10 in winter, coupled with elevated air temperatures, has been linked to high mortality risks, exacerbated under the influence of an Urban Heat Island. In the present we apply an innovative statistical framework to investigate the combined effect of thermal stress (maximum apparent temperature, Tapp_max) and air quality (PM10, NO₂, and O₃ concentrations) on daily mortality. We estimate heat and cold effects as the percentage change in all-cause, cardiovascular disease (CVD) and respiratory disease (RD) mortality between a) the 75^th and 99^th, and b) the 1^st and 25^th percentiles of Tapp_max, respectively, and their variation across low, medium, and high levels of pollutants, defined as the 5^th, 50^th, and 95^th of the sample percentiles.

Overall, heat exhibits a more pronounced impact than cold, as exposure to outdoor conditions is limited during winter. High levels of PM10, NO₂ and O₃ are associated with increases in heat-related all-cause mortality by 43.7%, 37%, and 31%, respectively, while the corresponding increases for cold are 14%, 19%, and 1.7%. A consistent increase in the effect of heat on mortalities is observed, particularly at high pollutant levels. It is worth noting that RD mortality decreases with increasing PM10 and NO₂ levels, potentially reflecting protective adaptations among urban residents during adverse conditions. Only the highest pollutant concentrations impact mortalities in the cold part of the Tapp_max range, albeit at lower levels than heat. Notably, cold-related RD mortality is prominently affected by high NO₂ levels (39%), followed by PM10 (30%), indicating a more pronounced influence of these pollutants on the respiratory system.

The present research captures the impact of concurrent exposure to environmental stressors over time, providing insight into the effects of extended periods of poor air quality and heat stress on human health. By understanding these results, public health interventions, such as the enhancement of heat resilience and targeted healthcare services to sensitive populations, can be effectively tailored to address the specific health risks stemming from exposure to extreme temperature conditions and poor air quality.