Last Update: 04/05/2026 at 2:50 PM EST
Climate Change Raises Summer Air Alerts
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Executive Summary
Modeling shows warming could make unhealthy summer air routine for about 100 million Americans by 2100, especially in California and the East
- One in three Americans could face air unhealthy for sensitive groups by 2100
- About 100 million people may live in areas triggering summer air alerts
- Unmitigated warming adds about 28 alert days for sensitive groups by 2100
- Limiting warming to 2 C or 2.5 C cuts alert days by about 30 percent
- The biggest increases are projected in California and the eastern United States
- Ozone and PM2.5 drive most of the projected air quality alerts
- Full compliance with alerts offsets only about 15 percent of economic impacts in the worst case
Quick Facts
- What: Climate change is projected to raise summer air quality alerts
- Where: Across the United States, especially California and the East
- Why: Higher warming increases ozone and PM2.5 pollution levels
- Who: University of Waterloo researchers and vulnerable Americans
- When: By 2100 during the May to September smog season
Featured Article
University of Waterloo research projects more US summer air quality alert days by 2100 under unmitigated warming, driven by ozone and PM2.5 and concentrated in California and the eastern United States.
Additional Articles
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A University of Waterloo study estimates climate change will expand US smog-season air quality alerts to about 100 million people by 2100.
University of Waterloo researchers project that climate change will expand U.S. smog-season unhealthy air alerts to about 100 million people by 2100.
University of Waterloo researchers project U.S. summer ozone and PM2.5 air quality could deteriorate by 2100 without climate action, increasing alert days for sensitive groups in California and the eastern U.S.
University of Waterloo modeling projects more unhealthy ozone and PM2.5 summer air alert days in the United States by 2100 unless climate warming is reduced.
A University of Waterloo study projects increased U.S. summer ozone and PM2.5 air-quality alert days by 2100 without climate action, with largest burdens in California and the eastern United States.
A University of Waterloo-led study projects that by 2100 about 100 million Americans could live in areas where smog-season air quality routinely triggers unhealthy alerts.