Abstract
January 2025 brought devastating wildfires to Los Angeles (LA) County, California, causing poor air quality, destroying homes and businesses, and displacing thousands of people. We used electronic health record data from 3.7 million Kaiser Permanente Southern California members to promptly determine if the 2025 LA Fires increased outpatient acute healthcare utilization. We created exposure categories using the maximum wildfire burn zone reached by an LA or Ventura County wildfire as of January 16, 2025. Highly-exposed members resided in census tracts located <20km from burn zones and moderately-exposed members lived in tracts ≥20km but within LA County. We identified daily outpatient and virtual acute care visits in five categories: all-cause, cardiovascular, injury, neuropsychiatric, and respiratory. We conducted 2-stage interrupted time-series analyses using machine-learning algorithms to determine if and by how much the 2025 LA Fires increased acute healthcare utilization. Across the week following the January 7 LA Fires ignitions, virtual respiratory visits were 41% (95% empirical confidence interval [eCI]: 26%, 56%) higher and 34% (95% eCI: 17%, 52%) higher than expected in highly- and moderately-exposed groups, respectively, totaling 3,221 excess visits. Similarly, both exposure groups had approximately 35% more virtual cardiovascular visits than expected over the same period. Among highly-exposed members, outpatient and virtual injury visits and outpatient neuropsychiatric visits were ≥ 18% higher than expected on January 7. Substantial increases in acute healthcare utilization driven primarily by virtual care-seeking were observed following the LA Fires. As disruptive climate events increase, such data are essential to inform healthcare preparedness and response.