Air pollution by particulate matter PM may trigger multiple sclerosis relapses.

Background: Seasonal variation of relapses in multiple sclerosis (MS) suggests that season-dependent factors, such as ambient air pollution, may trigger them. However, only few studies have considered possible role of air pollutants as relapse's risk factor.

Objective: We investigated the effect of particulate matter of aerodynamic diameter smaller than 10µm (PM) on MS relapses.

Methods: In total, 536 relapsing MS patients from Strasbourg city (France) were included, accounting for 2052 relapses over 2000-2009 period. A case-crossover design was used with cases defined as the days of relapse and controls being selected in the same patient at plus and minus 35 days. Different lags from 0 to 30 days were considered. Conditional logistic regressions, adjusted on meteorological parameters, school and public holidays, were used and exposure was considered first as a quantitative variable and second, as a binary variable.

Results: The natural logarithm of the average PM concentration lagged from 1 to 3 days before relapse onset was significantly associated with relapse risk (OR =1.40 [95% confidence interval 1.08-1.81]) in cold season. Consistent results were observed when considering PM as a binary variable, even if not significant.

Conclusion: With an appropriate study design and robust ascertainment of neurological events and exposure, the present study highlights the effect of PM on the risk of relapse in MS patients, probably through oxidative stress mechanisms.