Hedstrom AK, et al. - Researchers estimated the effect of passive smoking on the risk of developing anti-cyclic citrullinated peptide antibodies (ACPA)-positive and ACPA-negative rheumatoid arthritis (RA). They performed a population-based case–control study using incident cases of RA in Sweden, restricting the study population to include never-smokers. Findings did not suggest any relationship between exposure to passive smoking and RA risk. This could be explained by a threshold below which no relationship between smoke exposure and RA occurs. Also, authors did not find any suggestion of a trend between duration of passive smoking and RA risk.