Subdaily PM_2.5 exposure and cardiorespiratory risks: data and findings from Southern California, 2018–2020

As hourly PM_2.5 measurements become more accessible, health impacts from subdaily exposures can be evaluated to develop health guidance. We obtained hourly PM_2.5 concentrations covering Southern California from May 2018 through March 2020 and daily emergency department visits (EDVs) for cardiorespiratory-related conditions at ZIP Code Tabulation Area (ZCTA) levels. ZCTAs were aggregated into 35 clusters based on similar geographic and sociodemographic features. Daily exceedance concentration hours (DECH) above 9, 12, and 15 µg/m³, daily maximum, and average PM_2.5 concentrations were calculated for each cluster-day. Two-stage time-series analyses were conducted to estimate excess risks of daily EDVs. DECH metrics exhibited the same direction but smaller effects on cardiovascular and respiratory EDVs compared to daily average metrics. Excess risks for cardiovascular EDVs were 1.77% (95% CI: 1.20, 2.34), 1.04% (0.61, 1.47), and 2.67% (1.98, 3.37) per interquartile range increase of DECH-9, DECH-12, and daily average PM_2.5 during 7-day lag period, respectively. Excess risks of respiratory EDVs increased by 6.34% (4.25, 8.48), 4.39% (2.85, 5.95), and 6.61% (4.78, 8.47) per IQR increase of DECH-9, DECH-12, and daily average PM_2.5 during a 3-day lag period, respectively. Elevated excess risks were observed among older adults (65+), children (0–17), and low-poverty neighborhoods on both subdaily and daily metrics. In summary, subdaily PM_2.5 exposures above the current standards exhibited excess risks in cardiorespiratory-related EDVs but no greater than those derived from the daily average metric. Health guidance based on the daily average metric provides sensible protection to the public in Southern California.