Effects of Short-Term Air Pollution Exposure on Venous Thromboembolism: A Case-Crossover Study

Rationale: Exposure to outdoor air pollution is associated with increased cardiovascular disease, respiratory illness, and mortality. The effect of air pollution on venous thromboembolism (VTE) is less certain. Objectives: To test for associations between short-term exposure to air pollution and VTE. Methods: This is a retrospective case-crossover study of adult patients with an objectively confirmed VTE event. Exposure to the mean and maximum particulate matter ⩽2.5 microm in aerodynamic diameter (PM_2.5) and ozone were estimated with inverse distance squared weighting from multiple stationary air quality monitors. Conditional logistic regression with a 7-day individual lag model estimated the odds ratio (OR) of VTE occurrence during the case period relative to the referent period. Prespecified subgroup analysis was performed to further test associations in higher risk patients. Results: A total of 2,803 VTE events met inclusion criteria for analysis. Deep vein thrombosis was identified in 1,966 (70.1%) and pulmonary embolism in 915 (32.6%) subjects. Median age was 57 years. Small negative associations were observed for the maximum PM_2.5 exposure at 1 day (OR, 0.992; 95% confidence interval [CI], 0.986-0.997) and the mean PM_2.5 exposure at 1 day (OR, 0.982; 95% CI, 0.97-0.994), 5 days (OR, 0.987; 95% CI, 0.975-0.999), 6 days (OR, 0.984; 95% CI, 0.972-0.996), and 7 days (OR, 0.982; 95% CI, 0.971-0.994) before VTE diagnosis. Similar negative associations were observed for the 8-hour mean (OR, 0.989; 95% CI, 0.981-0.997) and 8-hour maximum (OR, 0.992; 95% CI, 0.985-0.999) ozone exposure 4 days before VTE diagnosis. Positive relationships (ORs of ∼1.02) between the 8-hour mean and maximum ozone exposures 6-7 days preceding VTE diagnosis were observed in a recently hospitalized subgroup. Conclusions: Short-term exposure to PM_2.5 and ozone does not appear to be associated with an overall increased risk of VTE. Further well-designed studies are needed to test whether previously reported associations between VTE and air pollution exist.