This study is investigating ambient air pollution as a contributor to rheumatoid arthritis (RA) incidence and exacerbation, with a focus on exposure to particulate matter. We will utilize data from a cohort study in the Georgia Air Basin of British Columbia. The cohort was enumerated using linked BC registration and health records, and includes 870,041 adults, ages 45-84, who had lived in the region for five years before 1999. Air pollutant exposures were assigned to each subject by their individual geographic codes of residence, using both air pollution regulatory monitoring networks and land-use regression modeling methods. This study will follow at-risk subjects to identify RA outcomes during the follow-up period (1999-2003). The purported biologic mechanisms by which these risk factors cause RA – inflammation, systemic immune effects, and oxidative stress – have been correlated with air pollution in previous studies, lending biologic plausibility to a possible association between air pollution and RA.