Out-of-Hospital Cardiac Arrest and Airborne Fine Particulate Matter: A Case-Crossover Analysis of Emergency Medical Services Data in Indianapolis, Indiana

Background: Previous studies have found particulate matter (PM) < 2.5 μm in aerodynamic diameter $({\rm PM}_{2.5})$ associated with heart disease mortality. Although rapid effects of ${\rm PM}_{2.5}$ exposure on the cardiovascular system have been proposed, few studies have investigated the effect of short-term exposures on out-of-hospital cardiac arrest (OHCA). Objectives: We aimed to determine whether short-term ${\rm PM}_{2.5}$ exposures increased the risk of OHCA and whether risk depended on subject characteristics or presenting heart rhythm. Methods: A case-crossover analysis determined hazard ratios (HRs) for OHCAs logged by emergency medical systems (EMS) versus hourly and daily ${\rm PM}_{2.5}$ exposures at the time of the OHCA and for daily and hourly periods before it. Results: For all OHCAs (n = 1,374), exposures on the day of the arrest or 1-3 days before arrest had no significant effect on the incidence of OHCA. For cardiac arrests witnessed by bystanders (n = 511), OHCA risk significantly increased with ${\rm PM}_{2.5}$ exposure during the hour of the arrest (HR for a $10\text{-}\mu {\rm g}/{\rm m}^{3}$ increase in ${\rm PM}_{2.5}$ exposure = 1.12; 95% confidence interval, 1.01-1.25). For the subsets of subjects who were white, 60-75 years of age, or presented with asystole, OHCA risk significantly increased with ${\rm PM}_{2.5}$ during the hour of the arrest (HRs for a $10\text{-}\mu {\rm g}/{\rm m}^{3}$ increase in ${\rm PM}_{2.5}$ = 1.18, 1.25, or 1.22, respectively; p < 0.05). HR generally decreased as the time lag between ${\rm PM}_{2.5}$ exposure and OHCA increased. Conclusion: The results suggest an acute effect of short-term ${\rm PM}_{2.5}$ exposure in precipitating OHCAs, and a need to investigate further the role of subject factors in the effects of PM on the risk of OHCA.