Investigators note that seasonal variation in relapse in MS has been observed, suggesting that season-dependent factors such as ambient air pollution may be involved.
Air pollution was linked to an increased risk of relapse in multiple sclerosis (MS), in a well-controlled study with hourly data on air pollutants measured by the residential block of each subject over a 10-year period.
The investigators note that seasonal variation in relapse in MS has been observed, suggesting that season-dependent factors such as ambient air pollution may be involved.
"Only few studies have considered possible role of air pollutants in the pathogenesis of MS relapses," the researchers wrote. "All led to significant associations with different magnitudes of risk, but all suffered from several methodological limitations that may question the results."
Emmanuelle Leray (pictured), PhD, Department of Epidemiology and Biostatistics, EHESP French School of Public Health, Rennes, France, and colleagues distinguished their study for using a case-crossover design, and a "robust set of data". The latter referred to their use of the Atmospheric Dispersion Modeling System (ADMS)-Urban air dispersion model, which comprises emissions inventories, background pollution measurements, land use and surface roughness, among other factors.
The case-crossover design was accomplished by each patient serving as their own control, and comparing their course in periods of different pollutant levels. The analysis also accounted for such variances as warm or cold season, meteorological variables of humidity and atmospheric pressure, daily pollen count and weekly regional influenza case counts.
536 patients with relapsing MS were identified from a regional MS network based in the city of Strasbourg, France. These patients had experienced a total of 2,052 episodes of relapse over the period of 2000-2009 (1,043 during hot season and 1,009 in cold season). They were living among 151 of the defined census blocks, which the investigators differentiated by socioeconomic characteristics but without finding differences in residents mean age at MS onset, or sex ratio.
Levels of air particulates of up to 10 µm in size were tracked, with MS relapse episodes juxtaposed to both daily quantitative levels and to a binary measure of levels above and below the threshold of 50 µg/m3 for 24 hours. The investigators explained that this threshold corresponds to pollution awareness notifications in France, prior to the alerts issued at 80 µg/m3.
The investigators found a positive association of relapse occurring within one to three days after a high air particulate day during cold seasons, and a similar trend without reaching significance during hot seasons. There were also positive association with pollen count in the hot season, and with influenza-like reports in the cold season.
Having distinguished the residential blocks by socioeconomic status, the researchers also found an increased susceptibility in the most, as well as the less deprived categories of socioeconomic position.
"To our knowledge, frequency of relapses was never shown as different, and even never studied according to socioeconomic status," researchers wrote. "Thus, this observation may reflect another aspect of the complex web of causation leading to the development of MS or the triggering of relapses."