Exposure to particulate matter air pollution and risk of multiple sclerosis in two large cohorts of US nurses

Abstract

Background: Air pollution is thought to raise the risk of neurological disease by promoting neuroinflammation, oxidative stress, glial activation and cerebrovascular damage. Multiple Sclerosis is a common auto-immune disorder, primarily affecting young women. We conducted, to a large prospective study of particulate matter (PM) exposure and multiple sclerosis (MS) risk in two prospective cohorts of women: the Nurses Health Study (NHS) and the Nurses Health Study II (NHS II).

Methods: Cumulative average exposure to different size fractions of PM up to the onset of MS was estimated using spatio-temporal models. We used multivariable Cox proportional hazards models to estimate the hazard ratios (HR) and 95% confidence intervals (CI) of MS associated with each size fraction of PM independently. Participants were followed from 1998 through 2004 in NHS and from 1988 through 2007 for NHS II. We conducted additional sensitivity analyses stratified by smoking, region of the US, and age, as well as analyses restricted to women who did not move during the study. Analyses were adjusted for age, ancestry, smoking, body mass index at age 18, region, tract level population density, latitude at age 15, and UV index.

Results: We did not observe significant associations between air pollution and MS risk in our cohorts. Among women in the NHS II, the HRs comparing the top vs. bottom quintiles of PM was 1.11 (95% Confidence Intervals (CI): 0.74, 1.66), 1.04 (95% CI: 0.73, 1.50) and 1.09 (95% CI: 0.73, 1.62) for PM₁₀ (≤10μm in diameter), PM_2.5 (≤2.5μm in diameter), and PM_2.5-10 (2.5 to 10μm in diameter) respectively, and tests for linear trends were not statistically significant. No association between exposure to PM and risk of MS was observed in the NHS.

Conclusions: In this study, exposure to PM air pollution was not related to MS risk.

Keywords: Cohort studies; Epidemiology; Incidence studies; Parkinson disease.

Figure 1.

Exposure to PM10, PM2.5 and PM2.5–10, and risk of MS in the Nurses Health Study II (N = 102,583 women at baseline) and Nurses Health Study (N = 112,595 women at baseline). Adjusted for age, smoking (never/past/current and pack years), body mass index (BMI) at age 18, population density, region (east, north, south, west), ethnicity (European Caucasian, Scandinavian, other Caucasian and non-white), latitude at age 15 tier (northern, middle, southern states), tract level income, and UV index.

Figure 1.

Exposure to PM10, PM2.5 and PM2.5–10, and risk of MS in the Nurses Health Study II (N = 102,583 women at baseline) and Nurses Health Study (N = 112,595 women at baseline). Adjusted for age, smoking (never/past/current and pack years), body mass index (BMI) at age 18, population density, region (east, north, south, west), ethnicity (European Caucasian, Scandinavian, other Caucasian and non-white), latitude at age 15 tier (northern, middle, southern states), tract level income, and UV index.

Figure 2.

Risk of MS associated with 10ug/m³ increase in PM₁₀, PM_2.5 and PM_2.5–10, stratified by region of the US in the NHS II and NHS. *all models stratified by region are adjusted for smoking (ever/never and pack years), and population density, with the exception of the model for west region in NHS, which is adjusted for smoking only (as addition of population density in the model resulted in loss of convergence).