No one is immune: neighborhood trends and disparities in air pollution exposure in the city of Chicago during summer 2023 wildfires.

BACKGROUND: In 2023, Chicago experienced unprecedented levels of fine particulate matter (PM2.5) caused by the long-range transport of wildfire smoke (WFS). OBJECTIVE: Examine how time trends in WFS-related fine PM2.5 varied across regions of the city with varied demographic, socio-economic, and health profiles. METHODS: The City of Chicago includes 792 census tracts grouped into nine regions. Hourly PM2.5 concentrations estimated on 500-m grids across the city (May 1-July 31, 2023) were used to calculate daily 24-h average PM2.5 levels at the tract level. To compare across city regions, interrupted time-series (ITS) models estimated PM2.5 levels for: (1) Pre-peak WFS (May 1-June 26); (2) Peak WFS (June 27-June 28); and (3) Post-peak WFS (June 29-July 31). RESULTS: Across Chicago, 24-h average tract-level PM2.5 concentrations during the pre-peak, peak, and post-peak periods were 14.5 μg/m3, 99.7 μg/m3, and 16.6 μg/m3, respectively. Baseline PM2.5 concentrations (May 1, 2023) in the northern and central regions (higher socioeconomic status) were lower than those in the southern regions (lower socioeconomic status). Differences ranged from -2.23 to -1.09 μg/m3. During the 2-day peak WFS period, northern regions of the city (e.g., Northwest Side, North Side) exhibited higher PM2.5 concentrations (differences of 2.29 and 1.81 μg/m3, respectively) compared to the Southwest Side. After the peak, PM2.5 levels in southern regions decreased more slowly than in northern and central Chicago. SIGNIFICANCE: WFS is degrading air quality even in areas far from wildfires. During WFS peak periods, no region was spared from marked increases in PM2.5 from transported WFS. Lower income and higher minority communities had higher baseline PM2.5 levels and took longer to return to pre-peak citywide average PM2.5 concentrations, indicating that although WFS impacts are widespread, some regions may experience more severe effects than others. IMPACT: This is one of the first studies to examine 2023 wildfire smoke exposure patterns in Chicago, a major urban area that is typically distant from wildfires. Using exposure modeling and region-specific temporal analysis, this study reveals that wildfire smoke (WFS) from distant fires exposed all Chicago communities to substantial increases in exposure to fine particulate matter (PM2.5). However, lower-income neighborhoods with a higher proportion of Black or Hispanic residents, shorter life expectancy, and higher burdens of chronic disease faced both higher baseline PM2.5 levels and longer time until PM2.5 concentrations returned to pre-peak levels. These findings underscore the importance of considering within-city variation in pollution levels and recovery patterns when assessing the public health impacts of wildfire-related air pollution events.