One study controlling for confounding factors found an association ofPM with MS [39], where hospital admission for MS relapse was linked to elevation of PM10 [39]. However a recent large prospective study in men and women found no association between MS risk and traffic air pollution [41•]. Together these two studies suggest that there may be air pollution-induced exacerbation of MS symptoms independent of an association of air pollution exposure with increased MS risk. Thus, while early research supports a possible association between MS and outdoor ambient air pollution, there are only a handful of controlled studies and additional research will be necessary to accurately define the relationship