Summary
Climate change harms health through extreme heat and weather, air pollution, flooding, disease vectors, water and food systems and mental health. The Lancet Countdown on Health and Climate Change 2025 report, produced with the World Health Organization, found that heat-related deaths averaged about 546,000 a year between 2012 and 2021, a 63% rise since the 1990s, and that 12 of 20 key health indicators reached record levels. It also found that air pollution from burning fossil fuels causes about 2.5 million deaths a year. The burden falls hardest on older adults, children, outdoor workers and low-income communities. Health systems in Germany, the UK, the US and Canada are adding heat plans and early warning systems, and adaptation finance remains a gap for low-income countries.
Climate change is a major threat to human health around the world, according to the World Health Organization. Rising greenhouse gas concentrations lead to higher temperatures and more frequent heatwaves, changes in precipitation, more frequent and intense extreme weather, and rising sea levels and flooding. These changes affect the air we breathe, the water and food we consume, the weather we experience and our interaction with natural and built environments. The IPCC AR6 Working Group II report confirms that climate change already affects human health in every region, with the burden falling disproportionately on vulnerable people.
What are the main exposure pathways?
- Extreme events (heatwaves, storms, wildfires)
- Air quality deterioration
- Flooding
- Vector-borne diseases
- Water-related issues
- Food security and safety
- Mental health and well-being
How does climate change affect human health?
1. How do extreme events harm health?
They cause illness, injury and death, and heat is the largest direct killer. The 2025 Lancet Countdown estimated about 546,000 heat-related deaths a year on average between 2012 and 2021, and found that the average person experienced 16 days of dangerous heat in 2024 that would not have occurred without climate change. Wildfire smoke was linked to a record 154,000 deaths in 2024. An earlier WHO projection estimated roughly 250,000 additional deaths a year between 2030 and 2050 from heat stress, malnutrition, malaria and diarrhoea.
2. How does climate change worsen air quality?
Heat, drought and wildfires raise levels of ozone and fine particles, which contribute to cardiovascular and respiratory illness, asthma, allergies and premature death. The WHO estimates that ambient air pollution causes about 4 million premature deaths a year and household air pollution about 3 million, and the Lancet Countdown attributes about 2.5 million deaths a year to pollution from fossil fuel burning. See our post on air pollution and respiratory health in urban India.
3. How does flooding affect health?
Rising seas, extreme rainfall and storms flood coastal and urban areas. Health effects include deaths, injuries, mental health disorders, waterborne illness and skin conditions. Much urban infrastructure was not designed for current or projected flood levels.
4. How does climate change affect vector-borne infections?
It widens the range and season of disease-carrying insects and ticks, which increases exposure to Lyme disease, malaria, dengue, encephalitis, hantavirus, West Nile virus and chikungunya. Parts of Europe have seen locally acquired dengue and West Nile virus in recent years, and the CDC Climate and Health programme tracks the trends for the United States.
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5. How does climate change affect water-related health?
It changes the quality and quantity of fresh water and raises the risk of cholera, cryptosporidiosis, campylobacter, leptospirosis, diarrhoeal diseases and malnutrition. A classic US study (Curriero and colleagues, 2001) found that 51% of waterborne disease outbreaks between 1948 and 1994 followed rainfall in the top 10% of events, and 68% followed rainfall in the top 20%.
6. How does climate change affect food safety and security?
Temperature and rainfall changes affect crop yields, food quality and food safety. Floods can contaminate drinking water, disrupt wastewater treatment and raise the risk of infection from bacteria, viruses and protozoa.
7. How does climate change affect mental health?
It causes stress, grief and anxiety through disasters, displacement and loss of livelihoods. The CDC names mental health as a key outcome area, and workplace heat and disaster exposure add pressure. If you are struggling, contact a doctor or local mental health service.
What adaptation and mitigation measures protect health?
- Build the capacity and resilience of health systems for climate-sensitive diseases
- Commit to leadership and governance at national and local levels
- Train the health workforce on climate-related threats
- Strengthen health information systems and early warning systems
- Provide medical products and technologies for climate-sensitive diseases
- Deliver efficient, equitable health services
- Increase adaptation finance, especially for low-income countries
What are Germany, the UK, the US and Canada doing?
- EU and Germany: the EU’s first European Climate Risk Assessment (2024) flagged health among the most exposed sectors, and Germany has a national heat protection plan and an annual heat action day.
- UK: the UK Health Security Agency issues impact-based heat-health alerts, and the Climate Change Act 2008 requires adaptation reporting.
- US: the CDC tracks climate and health, and OSHA proposed a federal heat injury and illness prevention rule in 2024, which was not finalised at the time of writing.
- Canada: Health Canada runs heat and wildfire smoke guidance, and provinces manage heat alert systems.
Related reading: Climate Change and Sustainable Development | Climate Change: Facts and Solutions | Sustainable Development Goals
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