Chronic effects of wildfire smoke and criteria air pollutants on cardiovascular hospitalization rates in the contiguous US

Existing studies on the health effects of smoke fine particulate matters (PM_2.5), a primary emission from wildfires, have often lacked comparison with other air pollutants, focused primarily on acute exposures, and not applied causal methods. In this study, we obtained county-level, three-year average cardiovascular hospitalization rates for Medicare beneficiaries across the contiguous US between 2006 and 2016 from the Centers for Disease Control and Prevention. These data were linked with spatio-temporal estimates of smoke PM_2.5, non-smoke PM_2.5, nitrogen dioxide (NO₂), ozone, and county-level confounders. We used a difference-in-differences method to evaluate simultaneous causal effects of three-year moving average exposures (lag 0–2, 1–3, 2–4, or 3–5 year) to the four pollutants on hospitalization rates for total cardiovascular disease (CVD) and its two major subtypes: heart disease and stroke. We found that, for total CVD, the absolute change in hospitalization rate associated with smoke PM_2.5 increased with longer lag periods: from −0.879 (95 % confidence interval [CI]: −2.528, 0.771) at lag 0–2 to 7.538 (95 % CI: 4.594, 10.481) at lag 3–5 per 1 μg/m³ increase in exposure per 1000 people. The effect of non-smoke PM_2.5 was smaller and diminished over time. NO₂ and ozone had even smaller effects per 1 part per billion increases in exposure. Similar patterns were seen for heart disease. For stroke, all pollutants had minimal and mostly non-significant effects. More rural and lower-income counties experienced greater risks. These findings suggested the need to prioritize wildfire management in addition to traditional air quality control strategies.