Fine particulate matter components and interstitial lung disease in rheumatoid arthritis

BACKGROUND: Exposure to ambient fine particulate matter with an aerodynamic diameter <2.5 μg·m-3 (PM2.5) is a risk factor for pulmonary and systemic autoimmune diseases; however, evidence on which PM2.5 chemical components are more harmful is still scant. Our goal is to investigate potential associations between major PM2.5 components and interstitial lung disease (ILD) onset in rheumatoid arthritis (RA). METHODS: New-onset RA subjects identified from a US healthcare insurance database (MarketScan) were followed for new onset of RA-associated ILD (RA-ILD) from 2011 to 2018. Annual concentrations of ambient PM2.5 chemical components (i.e. sulfate, nitrate, ammonium, organic matter, black carbon, mineral dust and sea salt) were estimated by combining satellite retrievals with chemical transport modelling and refined by geographically weighted regression. Exposures from 2006 up to 1 year before ILD onset or end of study were assigned to subjects based on their core-based statistical area or metropolitan division codes. A novel time-to-event quantile-based g (generalised)-computation approach was used to estimate potential associations between RA-ILD onset and the exposure mixture of all seven PM2.5 chemical components adjusting for age, sex and prior chronic obstructive pulmonary disease (as a proxy for smoking). RESULTS: We followed 280 516 new-onset RA patients and detected 2194 RA-ILD cases across 1 394 385 person-years. The adjusted hazard ratio for RA-ILD onset was 1.54 (95% CI 1.47-1.63) per every decile increase in all seven exposures. Ammonium, mineral dust and black carbon contributed more to ILD risk than the other PM2.5 components. CONCLUSION: Exposure to components of PM2.5, particularly ammonium, increases ILD risk in RA.