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Exposure to second-hand smoke during childhood was associated with an increased risk for developing rheumatoid arthritis (RA) later in life, researchers said.
In a large prospective cohort study involving women followed for up to 3 decades, active smoking, light smoking, and passive exposure to tobacco smoking in childhood were all associated with an increased arthritis risk, reported Marie-Christine Boutron-Ruault, MD, of the Institut Gustave Roussy in Villejuif, France, and colleagues.
Passive smoke exposure during childhood among never-smokers was associated with a borderline increased risk of developing RA (hazard ratio 1.43, 95% CI 0.97-2.11), while ever-smokers also exposed to passive smoke during childhood had a higher RA risk than smokers without early life passive smoke exposure (HR 1.67, 95% CI 1.17-2.39), they wrote in Rheumatology.
The study which included close to 100,000 women living in France who were followed from 1990, is among the first to link childhood secondhand smoke exposure to later-life arthritis and earlier RA onset in adults who have never smoked cigarettes.
"Our data suggest that even in non-smokers, passive exposure to tobacco during childhood tended to increase the risk of RA, with the magnitude of the increase being similar to that associated with active smoking, i.e., approximately 40%, although not statistically significant," the authors stated.
Smoking is a well known environmental risk factor for RA, but the impact of passive smoke exposure on the disease has not been widely studied, they wrote.
Results from the Nurses Health Study (NHS) suggested a potential association between long-term passive smoke exposure (>30 years) and an increased risk of RA, but the findings did not reach statistical significance.
In the NHS cohort, childhood and adult passive second-hand smoke exposures were not considered separately, and active and passive exposures were not combined in a single model.
The newly published study examined data from the French E3N cohort, which is a general population study originally designed to investigate risk factors for cancer and other major non-communicable diseases in women.
The cohort included 98,995 women born between 1925 and 1950, who were enrolled in 1990. Self-administered questionnaires were sent every 2-3 years to collect information on medical events, as well as general, lifestyle, and environmental characteristics.
RA diagnoses were collected in three successive questionnaires and confirmed if women received reimbursement for an RA-specific medication. The risk of incident RA was estimated using an age-adjusted Cox model that considered smoking status as a time-dependent variable.
The mean age at inclusion in the cohort was 49.5, and the mean duration of follow-up after inclusion was 23.8 years.
Among 71,248 women included in the analysis, 371 incident RA cases were confirmed. Ever-smokers not exposed to passive smoking had an increased risk of RA (HR 1.38, 95% CI 1.10, 1.74) compared with never-exposed women (i.e. neither to active nor childhood passive smoking).
Among women with RA, 46.4% were never smokers, 18.6% were current smokers, and 35% were former smokers at enrollment.
Active smoking status was associated with an increased risk of developing RA in both past (HR 1.31 95% CI 1.05-1.63) and current smokers (HR 1.54, 95% CI 1.01-2.14).
When considering age at RA onset according to smoking status, age at onset was greatest among never-smoker women not exposed to childhood passive smoking (mean 64.1). Current smokers and ever-smokers were youngest at RA onset (mean 60.1 and 60.2, respectively).
Study limitations included the self-reporting of RA diagnosis and the inability to examine environmental and genetic interactions.
"Considering that previous reports suggest a stronger association of active smoking with ACPA-positive versus ACPA-negative RA, it is plausible that this could also be the case for passive smoking and that the observed association could therefore be even stronger when separately considering only ACPA-positive RA," the researchers wrote.