66
In contrast to ozone, the PM2.5 curve in Figure 2 remains essentially flat across the range of observed 
exposure changes, reflecting the study’s finding that short-term fluctuations in PM2.5 were not 
significantly associated with OHCA risk in this cohort (odds ratio per 5 µg/m³ change ≈ 0.99, p = 0.72). In 
practical terms, this dose-response visualization reinforces the statistical results: ozone shows a positive 
association, where higher levels correspond to incrementally greater odds of OHCA, whereas PM2.5 does 
not exhibit a meaningful change in risk 
over similar changes in concentration. 
Subgroup analyses suggested that certain 
populations may be more vulnerable to 
ozone-related cardiac events, including 
older adults, individuals with preexisting 
cardiovascular disease, and residents 
of urban areas with higher baseline 
ozone levels or greater temperature-
related photochemical activity. These 
results highlight the interaction between 
environmental exposure and population 
vulnerability, showing that routine 
fluctuations in ozone, even within accepted 
air quality standards, can have immediate 
health consequences for high-risk groups. 
By linking high-resolution environmental 
data with national OHCA records, the study 
provided a nuanced understanding of how 
acute ozone exposure can trigger life-
threatening cardiac events.
These findings have important public health implications. Integrating ozone monitoring into early 
warning systems, alongside weather forecasts, could help identify high-risk days and support proactive 
interventions such as public advisories, limiting outdoor activity for vulnerable populations, and strategic 
positioning of EMS resources.
Figure 2. Dose-response relationship between PM2.5 and 
ozone levels from control day to case day and risk of OHCA 
on a continuous scale (results are adjusted for temperature).
From Malik AO, Jones PG, Chan PS. Association of ambient air pollution with risk of 
out of hospital cardiac arrest in the United States. Am Heart J Plus Cardiol Res Pract. 
2022;17:100151. Used with Permission. Licensed under CC BY NC ND 4.0.
Synthesis and Implications
Taken together, these studies demonstrate that OHCA is highly sensitive to environmental conditions, 
from daily weather fluctuations to episodic wildfire smoke and routine ambient ozone. Cardiovascular 
risk is dynamic and can be triggered acutely by exposures sometimes far from the source or days after 
the event. Linking environmental datasets with CARES has allowed researchers to identify patterns of 
vulnerability and forecast elevated risk, providing a foundation for informed interventions.
Preventable environmental exposures can meaningfully contribute to life-threatening cardiac events. 
Proactive strategies such as air quality monitoring, timely public alerts during high-risk weather or 
wildfire periods, and targeted interventions for vulnerable populations are essential. Integrating this 
knowledge into cardiac arrest prevention, EMS preparedness, and public health planning will be critical 
as climate variability and air pollution continue to shape environmental risk and population health 
outcomes.
1. Nakashima T, Ogata S, Kiyoshige E, et al. Development and evaluation of a machine learning model predicting 
out-of-hospital cardiac arrest using environmental factors. NPJ Digit Med. 2025;8(1):789. Published 2025 Dec 22. 
doi:10.1038/s41746-025-02235-4
2. Jones CG, Rappold AG, Vargo J, et al. Out-of-Hospital Cardiac Arrests and Wildfire-Related Particulate Matter 
During 2015-2017 California Wildfires. J Am Heart Assoc. 2020;9(8):e014125. doi:10.1161/JAHA.119.014125
3. Malik AO, Jones PG, Chan PS. Association of ambient air pollution with risk of out of hospital cardiac arrest in the 
United States. Am Heart J Plus. 2022;17:100151. doi:10.1016/j.ahjo.2022.100151

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