Toxic Heat: WHO Report Links Rising Temperatures and Pollution to Health Crises

The World Health Organization’s 2025 technical brief, "The Synergies of Heat Stress and Air Pollution and Their Health Impacts," presents a stark analysis of how climate change is creating a dangerous interplay between extreme heat and air pollution, threatening lives worldwide. Drawing on research from globally respected institutes, including Helmholtz Zentrum München, German Center for Environmental Health, Ludwig-Maximilian Universität Munich, the University of Tokyo, Peking University, and the University of Southern California, the report underscores how these two environmental stressors are not only escalating in severity but also interacting to magnify their combined health impacts. The consequences are alarming: higher mortality rates, increased hospitalizations, and worsening chronic and infectious diseases, particularly among vulnerable groups such as infants, the elderly, pregnant women, and those with pre-existing conditions.

Heatwaves and Pollutants: A Vicious Cycle

Heatwaves alone are known to cause severe physiological stress, leading to conditions like heatstroke, kidney injury, and complications for people with cardiovascular and respiratory illnesses. Air pollution, particularly from fine particulate matter (PM2.5) and ground-level ozone, compounds this threat. These pollutants penetrate deeply into the body, impacting not just the lungs but also the heart, brain, and even reproductive systems. During periods of extreme heat, the levels of these pollutants rise significantly due to increased photochemical reactions and events such as wildfires and dust storms. As a result, the report shows that the effects of heat and pollution are not merely additive; they are multiplicative. For example, the likelihood of heart attacks or asthma attacks spikes when both stressors are present simultaneously. This co-exposure accelerates disease onset, worsens existing conditions, and increases the risk of premature death.

Cities on the Frontline: Urban Hotspots of Risk

Urban environments, especially in low- and middle-income countries, are particularly susceptible to the twin threats of heat and pollution. The report highlights how poor urban planning, such as the use of heat-retaining materials, a lack of green spaces, inadequate ventilation, and overcrowded housing, intensifies exposure. The urban poor are especially at risk, often living in homes that offer little protection from either heat or pollutants. Gender dynamics also play a role: women who cook indoors using solid fuels in hot weather are exposed to both indoor air pollution and excess heat. Outdoor workers, such as construction laborers and agricultural workers, face daily exposure during peak heat and pollution levels, increasing their susceptibility to exertional heat stress and respiratory conditions.

Solutions in Sight: From Science to Action

Despite the scale of the challenge, the report outlines a number of promising pathways forward. A critical driver is the updated 2021 WHO air quality guidelines, which set ambitious targets for reducing exposure to harmful pollutants. These guidelines are spurring countries to reduce fossil fuel use and expand access to clean, renewable energy. The report also points to successful models of integrated health and climate action, such as China’s Air Health Index, which quantifies risks from both heat and air pollution, and the European Union-funded EXHAUSTION project, which offers practical guidance on addressing co-exposures. Intergovernmental organizations, city authorities, health professionals, and civil society groups are increasingly collaborating to build evidence-based, climate-resilient public health frameworks.

To safeguard public health in the short term, WHO recommends the implementation of early warning systems, robust health surveillance mechanisms, and public health campaigns that target the most vulnerable populations. Emergency shelters should be constructed to protect against both heat and polluted air. Health care facilities must be equipped to respond to extreme weather events, such as heat waves and wildfires. In the long term, the brief calls for deep structural changes in how cities are designed and how sectors like energy, housing, agriculture, and transportation operate. Urban planning must integrate heat mitigation strategies like green roofs, reflective surfaces, and improved ventilation. Agriculture and forestry practices must also adapt to prevent wildfires and dust storms, which are closely tied to both pollution levels and heat waves.

A Call to Integrate Climate and Health Policies

The WHO urges countries to incorporate joint risk assessments of heat stress and air pollution into national and subnational climate adaptation plans. Health indicators from the Lancet Countdown project are recommended to track progress. Furthermore, academic and research institutions are called upon to deepen the evidence base on co-exposure risks, particularly for underexplored areas like interactions between medications and pollution or the effects of long-term dual exposures. Training programs for health professionals should incorporate climate-sensitive medical practices, and health care systems themselves must reduce their carbon footprints. The medical community is also encouraged to develop guidelines for treating patients facing combined environmental threats and to advocate for policies that center health in climate responses.

Ultimately, the WHO emphasizes that every action taken to reduce fossil fuel combustion and mitigate climate change will yield substantial public health co-benefits. The intertwined crises of extreme heat and air pollution demand a unified, cross-sectoral response rooted in science, policy, and equity. As the climate continues to warm, delaying action will only increase the health burden, while proactive, integrated solutions offer a path to safer, healthier, and more resilient communities worldwide.