Exposure to environmental tobacco smoke is associated with increased risk for multiple sclerosis

Background: Tobacco smoking has consistently been associated with increased risk for multiple sclerosis. However, data has been inconsistent regarding the influence of passive smoking. Objective: The aim was to estimate the influence of passive smoking on the risk for multiple sclerosis. Methods: A population-based case-control study using incident cases of multiple sclerosis was performed in Sweden, and the study population was restricted to subjects who had never smoked (695 cases, 1635 controls). The incidence of multiple sclerosis among never-smokers who had been exposed to passive smoking was compared with that of never-smokers who had never been exposed by calculating the odds ratio with a 95% confidence interval employing logistic regression. Results: The risk for multiple sclerosis was increased among never-smokers who had been exposed to passive smoking (OR 1.3, 95% Cl 1.1-1.6) compared to never-smokers who had never been exposed. The risk increased with increasing duration of exposure (p = 0.003). Conclusions: Exposure to environmental tobacco smoke is associated with an increased risk for multiple sclerosis. Since smoking, but not usage of oral tobacco in the form of moist snuff, is associated with increased risk for multiple sclerosis, we consider that the critical effects of passive smoking may be the result of irritations in the lungs. Hence, further studies would be valuable in order to clarify whether other forms of lung irritation, such as air pollution, contribute to the triggering of multiple sclerosis.