Recent modeling indicates that by the year 2100, nearly one in three Americans will regularly inhale air deemed unhealthy, particularly for sensitive groups, as a consequence of climate change. This represents a staggering seven-fold increase from the year 2000.
This study, led by researchers from the University of Waterloo, reveals that approximately 100 million individuals in the United States will reside in regions where the average air quality during smog season is sufficiently poor to warrant alerts advising at-risk populations to remain indoors. The findings are published in the journal Environmental Science & Technology.
This figure rises from an estimated 14 million people affected in 2000, with the majority of the increase anticipated in California and the eastern regions of the United States. Smog season encompasses the period from early May to the end of September.
"Climate change could transform occasional poor air quality days into a regular occurrence," stated Dr. Rebecca Saari, a professor of civil and environmental engineering and the Canada Research Chair in Global Change, Atmosphere and Health at the University of Waterloo.
"For those particularly susceptible to air pollution—such as the elderly, children with asthma, and individuals with preexisting health conditions—there will be a daily uncertainty regarding air quality alerts, with nearly equal chances of receiving an alert each day that could necessitate behavioral adjustments to minimize exposure."
This study expands on earlier research projecting a fourfold increase in air quality alerts across the United States. By the century's end, it is anticipated that vulnerable individuals will need to spend an additional 142 days indoors to mitigate the health risks associated with worsening air quality due to climate change.
The new research also considers the dual impact of ozone and particulate matter pollution, which are responsible for nearly all air quality alerts and are major contributors to illness and premature mortality.
Furthermore, the research team evaluated the economic implications of escalating air pollution, along with the potential benefits of implementing policies aimed at its reduction over the next 75 years.
Dr. Saari emphasized a noteworthy finding: seniors gain substantially greater benefits than other at-risk groups from precautions designed to limit their exposure to poor air quality—such as staying indoors, wearing masks, and enhancing indoor air filtration systems.
"We were taken aback by the extent to which sensitive populations could regularly encounter air quality deemed unhealthy," Dr. Saari noted.
"We aspire for our work to guide more precise recommendations and to emphasize the necessity for pollution reduction and long-term adaptations, including ensuring access to clean indoor environments."
Collaborating researchers included experts from Harvard University, North Carolina State University, and the University of California, Davis.
Publication details
Matt S. Sparks et al, Air Quality Alerts, Health Impacts, and Adaptation Implications Under Varying Climate Policy, Environmental Science & Technology (2026). DOI: 10.1021/acs.est.5c12522
Share this story