An array of environmental risk factors for MS have been examined over the past century. Of those, the Epstein-Barr Virus, ultra-violet light exposure and vitamin D levels, and cigarette smoking have the clearest supporting evidence (Wingerchuk, 2011).
Evidence pointing to smoking as a risk factor for MS and disease progression includes:
Several prospective studies (Hedstrom et al, 2009; Hernan et al, 2005; Hernan et al, 2001; Thorogood & Hannaford, 1998; Villard-Mackintosh & Vessey, 1993) have demonstrated a link between smoking and an increased risk of developing MS.
Cotinine levels (a measure of smoking exposure) have been shown to be higher in people with MS than in controls (Sundström et al, 2008).
Three studies (Healy et al, 2009; Pittas et al, 2009; Sundström & Nyström, 2008) have demonstrated an impact of smoking on the course of MS. Smoking has been associated with an increased risk of: conversion from CIS to confirmed MS; conversion from relapsing-remitting MS to secondary-progressive MS; and more rapid worsening and clinical disability in progressive MS. A recent cohort study confirmed that regular smoking is associated with more severe disease and faster disability progression. In addition, smoking cessation, whether before or after onset of the disease is associated with a slower progression of disability (Manouchehrinia et al., 2013).
Zivadinov et al (2009) have also confirmed an increase in lesion burden and atrophy as shown on MRI in people with MS who smoke.
Given the apparent relationship between smoking and MS, Wingerchuk (2011) suggests that the recent increase in female smokers in many regions around the world may be a possible environmental explanation for the increasing female-to-male ratio in MS.