Title: Environmental factors and MS

Abstract

Long-term exposure to airborne particulate matter (PM) has been linked with the development of a broad spectrum of health conditions with neurodegenerative diseases not being an exception. PM is commonly classified based on aerodynamic diameter into PM10 (diameter #10 mm) and PM2.5 (diameter #2.5 mm). Little is known about the effect of air pollution on the prevalence of MS. A study in Turkey found a two-fold prevalence of MS in a high-polluted city in comparison to a coastal city. Conversely, two large cohort studies failed to find any significant association. Another study did not find any association between PM and the risk of MS admissions. On the other hand, a study in France found that MS relapse increase was associated with PM10 during the cold season. A study in Italy found a 42% increase in MS hospital admissions on the days preceded by one week with PM10 levels in the highest quartile. These finding were also replicated by a study in Serbia. High air pollution index during the so-called ‘‘low vitamin D season’’ (January-April) were correlated with an increase in the number of relapses. Air pollution appears to be associated with relapses, but this should be interpreted cautiously since it is also related to the cold season, where several other risk factors such as less sunlight exposure and viral infections. Also, it is well known that pollution levels increase as temperature lowers. Nevertheless, seasonal pollution levels depend on the specific pollutant. Particulate matter may be higher in the winter; while ozone is higher in the summer. The case of Mexico City is relevant for several reasons. The city has drastically reduced air pollutants since the early nineties. In fact, comparison of data between 2006 and 2013 indicates a decrease in PM2.5 in the Metropolitan area of Mexico City. As consequence Mexico City epidemiological data can offer some insight not only the impact of air pollution on neurodegenerative diseases such as Alzheimer, Parkinson’s disease and MS, but also on the effect of reducing air pollution regarding such diseases. In this matter, it is important to consider that different sources (i.e. traffic versus industrial activities) may have a distinct impact on pollution and individual analysis would be desirable. In addition, geographical reasons such as altitude preventing proper burning of fuels, being located at a tropical latitude within a valley surrounded by mountains makes the city more prone to air pollution. Recent evidence suggests that major mechanisms involved in MS pathogenesis, such as inflammatory factors expression, free radicals overproduction, the blood brain barrier (BBB) breakdown, neuroinflammation, vitamin D deficiency and mitochondrial dysfunction could also occur due to exposure to air pollutants. A prospective hypothesis is suggested here in which exposure to air pollutants may initiate destructive mechanisms inducing inflammatory-oxidative cascades, reduction of immunological self-tolerance and neurodegeneration leading to brain autoimmunity.