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COPENHAGEN -- Exposure to small particulate matter, of the sort in tobacco smoke and industrial effluents, was associated with increased rates of immune-mediated diseases such as rheumatoid arthritis, Italian researchers found.

In areas where levels of PM₁₀ (particulates smaller than 10 microns) regularly reached or exceeded 30 μg/m³, prevalence of seven autoimmune diseases was 12% greater than in areas with lower chronic exposures, reported Giovanni Adami, MD, PhD, of the University of Verona.

Prevalence of these conditions was also 13% higher in areas where levels of particulates smaller than 2.5 microns (PM_2.5) averaged 20 μg/m³ over long periods, Adami told attendees at the European Alliance of Associations for Rheumatology's (EULAR) annual meeting here. He also noted a dose-response relationship, with each 10-μg/m³ increment in chronic exposure tied to a 7% higher risk for autoimmune diseases.

Prior research on air pollution naturally has focused on lung diseases such as asthma and chronic obstructive pulmonary disease, although many studies have also found adverse cardiovascular risks. But asthma, of course, is also an autoimmune condition. Moreover, Adami's group had previously identified associations between pollution levels and rheumatoid arthritis flares and weaker responses to biologic drugs for rheumatologic illnesses.

At a EULAR press conference where the study was highlighted, commentator Hendrik Schulze-Koops, MD, PhD, of the University of Munich in Germany, joked that Christopher Columbus was to blame. Schulze-Koops said rheumatoid arthritis was unknown in Europe until Columbus brought tobacco back from his Western Hemisphere explorations and triggered a continent-wide obsession with smoking that is only now receding.

For his part, Adami stuck to his group's research, which drew on data from more than 81,000 individuals in Italy who provided health histories as part of screening for osteoporosis. The researchers correlated their reports of autoimmune diseases with air pollution monitoring data from nearly 620 stations around the country.

Because osteoporosis screening provided the research opportunity, the sample's demographics were skewed accordingly: some 90% were women, mean age was 65, and almost all were postmenopausal.

About two-thirds lived in northern Italy, the more heavily industrialized part of the country, with 12% from the central portion and nearly 20% from the south. Mean chronic exposures to particulates were 25.0 μg/m³ for PM₁₀ and 16.0 μg/m³ for PM_2.5, which Adami said was fairly heavy. (World Health Organization and Italian government standards set limits of 20 μg/m³ for PM_2.5 and 30 μg/m³ for PM₁₀; in the U.S., the maintains a 12-μg/m³ standard for PM_2.5 and 35 μg/m³ for PM₁₀.)

Not all of the autoimmune diseases counted in the study were classically rheumatologic. Multiple sclerosis and inflammatory bowel disease (IBD) was included, too, alongside rheumatoid and psoriatic arthritis, systemic lupus erythematosus, systemic sclerosis, and other connective tissue diseases.

Rheumatoid arthritis cases seemed to drive much of the association with particulate exposure. It was the only one of the seven conditions for which its independent association reached statistical significance. (It's also the most common of the seven in general.) Several others seemed to have no correlation with pollution exposure -- multiple sclerosis and lupus both showed odds ratios of 1.0 or less both for PM_2.5 and PM₁₀ -- although there were trends toward numerically important associations for systemic sclerosis and IBD. The study's statistical analyses included adjustments for a host of co-factors including age, steroid treatment, comorbidities, and the specialty of physicians providing the diagnoses.

Adami's conclusion was that, somehow, exposure to particulate matter in the lungs triggers an autoimmune reaction in other body systems. More research will be needed to clarify those pathways, if indeed the associations reflect genuine causation.

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