AbstractObjectiveTo investigate associations between air particulate matter of ≤2.5 μm in diameter (PM_2.5) and ovarian cancer.DesignCounty‐level ecological study.SettingSurveillance, epidemiology, and end results from a collection of state‐level cancer registries across 744 counties. Data from the Environmental Protection Agency's network for PM_2.5 monitoring was used to calculate trailing 5‐ and 10‐year PM_2.5 county‐level values. County‐level data on demographic characteristics were obtained from the American Community Survey.PopulationA total of 98 751 patients with histologically confirmed ovarian cancer as a primary malignancy from 2000 to 2016.MethodsGeneralised linear regression models were developed to estimate the association between PM_2.5 and PM₁₀ levels, over 5‐ and 10‐year periods of exposure, and ovarian cancer risk, after accounting for county‐level covariates.Main outcome measuresRisk ratios for associations between ovarian cancer (both overall and specifically epithelial ovarian cancer) and PM_2.5 levels.ResultsFor the 744 counties included, the average PM_2.5 level from 1990 through 2018 was 11.75 μg/m³ (SD = 3.7) and the average PM₁₀ level was 22.7 μg/m³ (SD = 5.7). After adjusting for county‐level covariates, the overall annualised ovarian cancer incidence was significantly associated with increases in 5‐year PM_2.5 (RR = 1.11 per 10 units (μg/m³) increase, 95% CI 1.06–1.16). Similarly, when the analysis was limited to epithelial cell tumours and adjusted for county‐level covariates there was a significant association with trailing 5‐year PM_2.5 exposure models (RR = 1.12 per 10 units increase, 95% CI 1.08–1.17). Likewise, 10‐year PM_2.5 exposure was associated with ovarian cancer overall and with epithelial ovarian cancer.ConclusionsHigher county‐level ambient PM_2.5 levels are associated with 5‐ and 10‐year incidences of ovarian cancer, as measurable in an ecological study.