Associations of fine particulate matter with incident cardiovascular disease; comparing models using ZIP code-level and individual-level fine particulate matter and confounders

Background: PM_2.5 has been positively associated with cardiovascular disease (CVD) incidence. Most evidence has come from cohorts and administrative databases. Cohorts typically have extensive information on potential confounders and residential-level exposures. Administrative databases are usually more representative but typically lack information on potential confounders and often only have exposures at coarser geographies (e.g., ZIP code). The weaknesses in both types of studies have been criticized for potentially jeopardizing the validity of their findings for regulatory purposes.

Methods: We followed 101,870 participants from the US-based Nurses' Health Study (2000-2016) and linked residential-level PM_2.5 and individual-level confounders, and ZIP code-level PM_2.5 and confounders. We used time-varying Cox proportional hazards models to examine associations with CVD incidence. We specified basic models (adjusted for individual-level age, race and calendar year), individual-level confounder models, and ZIP code-level confounder models.

Results: Residential- and ZIP code-level PM_2.5 were strongly correlated (Pearson r = 0.88). For residential-level PM_2.5, the hazard ratio (HR, 95 % confidence interval) per 5 μg/m³ increase was 1.06 (1.01, 1.11) in the basic and 1.04 (0.99, 1.10) in the individual-level confounder model. For ZIP code-level PM_2.5, the HR per 5 μg/m³ was 1.04 (0.99, 1.08) in the basic and 1.02 (0.97, 1.08) in the ZIP code-level confounder model.

Conclusion: We observed suggestive positive, but not statistically significant, associations between long-term PM_2.5 and CVD incidence, regardless of the exposure or confounding model. Although differences were small, associations from models with individual-level confounders and residential-level PM_2.5 were slightly stronger than associations from models with ZIP code-level confounders and PM_2.5.