Various environmental factors – of both infectious and non infectious origin – have been proposed as risk factors for MS. MS is more common in people who live further away from the equator. Why this is the case is not clear, but decreased sunlight exposure has been linked with a higher risk of MS and there is growing evidence that a lack of vitamin D is linked to increasing prevalence in a range of conditions including MS. As we get most of our vitamin D through exposure to sunlight, low sun exposure and subsequent vitamin D insufficiency has been proposed as one explanation of this effect. This effect may also explain the recent observation that there is an excess MS risk in people born in April and May, and a reduced risk in those born in October and November.
Many microbes (particularly Epstein Barr Virus, EBV) have been proposed as potential triggers for MS, but none have been proven. Age at exposure to infection seems to play an important role, and it has been shown that moving at an early age from one location in the world to another alters a person’s subsequent risk of MS.
Another environmental factor that seems to be strongly associated with MS is cigarette smoking.