Objective: To study the association between silica exposure, separately as well as combined with smoking, and the risk of developing Rheumatoid Arthritis (RA) with or without presence of antibodies against citrullinated peptide antigens (ACPA).
Methods: This Swedish population based case-control study analysed 577 incident RA cases and 659 randomly selected controls, all men aged 18-70 years, included during May 1996 – May 2006. Self-reported silica exposure, defined as exposure to stone dust, rock drilling or stone crushing, and cigarette smoking was registered. ACPA status among cases was analysed.
Results: Silica exposed persons were observed to have a moderately increased risk of ACPA positive RA (odds ratio (OR) adjusted for age and residency = 1.67 (95% confidence interval (95% CI) 1.13-2.48)), but not of ACPA negative RA (OR = 0.98 (95% CI 0.57-1.66)), compared with persons unexposed to silica. Persons exposed to rock-drilling were observed to have a somewhat more markedly increased risk of ACPA positive RA (OR = 2.34 (95% CI 1.17-4.68)). A high risk of developing ACPA positive RA was observed among silica exposed current smokers (OR = 7.36 (95% CI 3.31-16.38)), exceeding what was expected from the separate effects of silica exposure and current smoking, indicating an interaction between these exposures (attributable proportion due to interaction = 0.60 (95% CI 0.26-0.95)).
Conclusion: Silica exposure combined with smoking among men is associated with increased risk of developing ACPA positive RA. These results suggest that different inhalation exposures may interact in the aetiology of ACPA positive RA.