Beyond single-hazard alerts: Rethinking heat and air pollution Early Warnings Systems in high-exposure settings

Early warning systems (EWS) for environmental hazards are increasingly implemented across regions and play an important role in protecting population health. However, most existing systems remain predominantly hazard-based rather than health impact-based, relying on simple threshold exceedances and colour-coded alerts. While such warnings provide a useful first indication of elevated risk, they often lack direct relevance for health-protective decision-making, action and behaviour by, e.g., local authorities, health services, care facilities, employers, and the public. This limitation is particularly important for heat and air pollution, which are typically addressed through separate warning systems despite strong epidemiological evidence that their health effects interact. Numerous studies show synergistic effects on death and disease from concurrent exposure to high temperatures and air pollution (PM₂.₅ and ozone), especially from cardiovascular and respiratory causes, implying that health risks during compound events exceed the sum of individual hazards. Failing to consider these interactions may therefore result in underestimation of risk during such events. Moreover, most available evidence on joint heat and air pollution health risks comes from temperate, high-income settings, while many of the world’s hottest regions are those also experiencing the highest air pollution levels.

Beyond improving risk detection, joint consideration of heat and air pollution offers a major opportunity for health co-benefits. Analyses from the Horizon 2020 RIA project EXHAUSTION show that accelerated air-pollution reduction can function as a powerful adaptation strategy to extreme heat. Integrating heat–air pollution interaction effects into regional mortality and welfare-cost projections revealed that achieving WHO’s annual PM₂.₅ guideline level could reduce heat-related cardiopulmonary mortality by nearly 40% in Europe over the coming decades. Particularly large benefits were found for Balkan and Mediterranean regions where high heat exposure and air pollution coincide and the annual welfare economic costs from heat-related mortality reach billions of Euros.

In the ongoing COPE project in India – a country experiencing increasingly frequent and intense heatwaves and home to many of the world’s most polluted cities – we directly respond to the evidence on joint heat and air pollution effects. Working with local partners, we aim to develop an Early Warning and Decision-support System for heat and air pollution in Delhi and Kolkata. We investigate whether alert thresholds should be dynamically adjusted when heat and air pollution co-occur, and whether vulnerability varies by season, warranting differential season-specific alert thresholds. We draw upon insights from the Horizon CSA project ENBEL, which highlight key technical (data and modelling constraints), structural (institutional capacity and funding), and societal (risk communication and equity) barriers to effective heat-health warning systems, as these lessons are directly applicable to the development of integrated heat–air pollution warnings. In COPE, the research is co-designed and conducted in collaboration with user groups from vulnerable populations and stakeholder partners from the health and governmental sector to ensure that alerts reach all relevant users and include meaningful and tailored actionable guidance for different users. We suggest that integrated, impact-based and action-oriented early warning systems are essential for effective, equitable climate-health adaptation in a warming and, in many places, increasingly polluted, world.