The Role of Ozone as a Mediator of the Relationship Between Heat Waves and Mortality in 15 French Urban Areas.

This article investigates the role of tropospheric ozone as a mediator in the relationship between heat waves and mortality across 15 French urban areas during summers from 2000 to 2015. Using a causal mediation analysis under the potential outcomes framework, the study decomposed the total effect (TE) of heat waves on mortality into natural direct effects (NDE) and natural indirect effects (NIE) mediated by ozone, adjusting for confounders such as nitrogen dioxide. Results indicate that ozone partially mediates the impact of heat waves on nonaccidental, cardiovascular, and respiratory mortality, with mediation effects varying by geographic location and cause of death; for example, ozone mediated up to 57% of respiratory mortality risk in some areas. These findings highlight the importance of considering ozone formation in public health interventions and heat-health warning systems, especially as climate change may increase the frequency and intensity of heat waves and associated ozone levels.