Smoking Duration and Intensity Unrelated to RA Risk in Women

Joe Barber Jr, PhD

April 22, 2013

Among older women, the relationship between smoking and rheumatoid arthritis (RA) persists after smoking cessation, and the risk may be unrelated to the duration or intensity of smoking, according to the findings of a prospective study.

Daniela Di Giuseppe, MSc, from Karolinska Institutet in Stockholm, Sweden, and colleagues present their findings in an article published online April 22 in Arthritis Research and Therapy.

The authors note that although an association between smoking and RA has been established, little is known about how the duration or intensity of smoking affects RA risk. "A large number of case-control studies and fewer cohort studies have shown that cigarette smoking is directly associated with the risk of developing [RA]," the authors write. "Previous studies have mainly focused on establishing this association and assessed smoking status (current, former, and never smoker) and pack-years of smoking, while less attention was given specifically to intensity and/or duration of smoking."

In the study, the researchers assessed the risk for RA among women smokers between 54 and 89 years of age, using data from 34,101 women included in the Swedish Mammography Cohort study and excluding women with missing values on smoking status, those with a diagnosis of RA, and those who died before the start of follow-up. In multivariate analysis adjusted for confounders such as menopausal status, parity, and alcohol use, current smokers had an elevated risk for RA compared with never-smokers (relative risk [RR], 2.20; 95% confidence interval [CI], 1.58 - 3.04).

In addition, in multivariate analysis adjusted for the aforementioned variables, former smokers had a 1.68-fold higher risk of developing RA than never-smokers (95% CI, 1.19 - 2.38). Compared with never-smokers, women who smoked 1 to 7 cigarettes/day had a 2.31-fold higher risk of developing RA (95% CI, 1.59 - 3.36), and an increased risk of developing RA was also observed among women who smoked for between 1 and 25 years (RR, 1.60; 95% CI, 1.07 - 2.38).

Concerning the combined effect of smoking intensity and duration, duration appeared to be the more important factor, as light smokers (1 - 10 cigarettes/day) who smoked more than 30 years had a 2.54-fold higher risk of developing RA than never-smokers (95% CI, 1.76 - 3.69). Neither the number of years since smoking cessation nor the age at smoking cessation appeared to alter RA risk, as both women who quit smoking before 1997 (RR, 1.99; 95% CI, 1.23 - 3.20) and those who quit smoking before the age of 41 years (RR, 1.94; 95% CI, 1.18 - 3.18) had an approximately 2-fold higher risk for RA than never-smokers.

The limitations of the study include a lack of data on younger women and men, the collection of information on smoking habits on only a single occasion, and a lack of information on shared epitope genes.

"In conclusion, our study indicated that even light smoking is associated with increased risk of RA for women," the authors write. "The clearly increased risk of RA development even among former smokers is another reason to persuade women not to start smoking."

The study was supported by funding from the Swedish Research Council/Committee for Medicine, the Swedish Research Council/Committee for Research Infrastructure, and the Swedish COMBINE inflammation research consortium. Di Giuseppe received funding from the Karolinska Institutet's Award for PhD students. The other authors have disclosed no relevant financial relationships.

Arthritis Res Ther. Published online April 22, 2013. Full text