Two North American studies have explored the association between PM2.5 and JIA. From a patient population in Utah, 338 cases were identified based on a clinical examination by a rheumatologist. Exposure determination was based on monitoring sites data and no-intercept regression models. RR of 1.60 per 10 μg/m3 (95% CI: 1.00 to 2.54) for disease onset was found for children < 5.5 years of age but the results were imprecise when all ages were included in the analysis (RR: 1.11; 95% CI: 0.85–1.45) . The results were not replicated when studying a broader population in America and Canada with systemic-onset JIA .