While some studies dispute it,^{3, 4} there is preliminary evidence that exposure to organic solvents,⁵ insecticides,⁶ and X-rays⁷ may cause or aggravate MS. This may explain why clusters of multiple sclerosis cases occasionally occur in certain geographical areas or even in work sites.⁸

Swiss researchers found that nicotine temporarily impairs arm movement in people with MS.⁹ In one study, when people with MS smoked cigarettes, movement capacity was diminished for 10 minutes in 76% of them. Although this evidence is preliminary, there are many other adverse health effects of smoking. Smokers with MS should quit smoking.

While the outcome of some research disputes the connection between MS and mercury exposure,¹⁰ other investigations have reported an association between dental amalgams and this disease. One study found that mercury levels in the hair of people with MS are higher than in the hair of healthy people.¹¹ This same report found that people with MS who had their amalgam fillings removed experienced one-third fewer relapses than people who kept their fillings. Another preliminary study found that people having a large number of fillings that had been in place for a long time appeared to be at increased risk for MS compared with those having fewer fillings.¹² Preliminary evidence has also identified an association between tooth decay—as opposed to fillings—and MS.¹³ The importance of the reported links between mercury, tooth decay, and risk of MS has not been clearly established.