Two studies recently published in JACC explore the relationship between short-term exposure to ambient fine particulate matter (PM_2.5) and cardiovascular mortality and the impact of long-term exposure to PM_2.5 from wildfire smoke on hospitalizations due to cardiovascular disease.

The first study, authored by Yongkang Yang, MPH, et al., identifies a significant nonlinear relationship between PM_2.5 exposure and cardiovascular mortality. They included 100 epidemiologic studies in their analysis, using a three-stage meta-regression model to determine the relationship between air pollution exposure and cardiovascular mortality, and then a receiver-operating characteristic-like curve analysis to identify an optimal public health alert threshold.

For every 10 μg/m³ increase in short-term PM_2.5 concentration, cardiovascular mortality risk was 1.0090 (95% CI 1.0074-1.0106). The authors identified a supralinear trend, where "marginal risk was high at low concentrations, flattened at moderate concentrations (~75-150 μg/m³), and rose sharply again above 150 μg/m³." The optimal public health alert threshold, balancing health protection benefits with societal disruption, was 136 μg/m³ (95% CI 129-148). They estimate that using this threshold would prevent 73% of attributable deaths but only affect 32% of at-risk person-days.

"The 136 μg/m³ value should not be interpreted as a universal, one-size-fits-all recommendation," write the authors. "It is a global average derived from global data. The true optimal threshold for any specific city or region is critically dependent on its own unique, local PM_2.5 exposure distribution."

In an accompanying editorial comment, María Neira, MD, MPH, adds: "For [the World Health Organization] and its Member States, the policy implications are profound: Countries can now build evidence-based alert systems, grounded in local exposure distributions and population characteristics...Governments can quantitively balance health benefits and societal costs, rather than relying on historical or politically convenient thresholds."

Contributing additional findings on this topic, a recent study published in the European Heart Journal found an association between short-term PM_2.5 exposure and an increased risk of acute myocardial infarction. Masanobu Ishii, et al., included 23,037 patients with myocardial infarction with nonobstructive coronary arteries from Japan's JROAD-DPC database, comparing pre- and post-pandemic subgroups.

Another study in JACC reveals the association between long-term, cumulative wildfire smoke PM_2.5 exposure with cardiovascular disease hospitalization, finding an elevated risk that varied by exposure levels and individual characteristics. Siqi Zhang, PhD, et al., included 65.2 million Medicare beneficiaries from 2017 to 2022 and estimated wildfire smoke PM_2.5 exposure by beneficiaries' residential ZIP codes.

The authors found a nonlinear pattern where risk of hospitalization due to cardiovascular disease peaked at moderate exposure levels. For wildfire smoke PM_2.5 concentrations between 0.26 and 0.32 μg/m³, estimated relative risk for overall cardiovascular disease was 1.04 (95% CI 1.03-1.05), 1.07 for ischemic heart disease (95% CI 1.06-1.09) and 1.06 for arrhythmias (95% CI 1.04-1.07). Zhang and colleagues note that hospital admissions for cerebrovascular disease exhibited an increasing trend across exposure levels.

In addition, individuals of lower socioeconomic status appeared to be more vulnerable. "The higher risk observed in socioeconomically disadvantaged individuals could be attributable to higher prevalence of preexisting conditions, restricted access to health care, greater personal exposure to outdoor work, and limited awareness and resources for protective measures such as air filtration or temporary relocation," state the authors.