The risk of developing anti-citrullinated protein antibody-positive rheumatoid arthritis is high among current smokers exposed to silica (OR 7.5, 95% CI 4.2–13.2), with the two risk factors having a synergistic effect, according to findings from the Swedish Epidemiological Investigation of Rheumatoid Arthritis (EIRA) study. The additive interaction between silica exposure and smoking increased with pack-years of smoking, with the highest attributable proportion due to interaction (AP) seen with ≥28 pack-years of smoking (AP 0.7, 95% CI 0.4–0.9).