A study on the possible role of PM2.5 in the systemic onset juvenile idiopathic arthritis (SJIA) in young children did not find a significant association between the two. In a case-crossover study covering five metropolitan areas and PM2.5 exposures of up to 14 days, no statistically significant association with new symptoms was observed. An extended unplanned analysis, however, revealed an increased relative risk (1.76, 1.07-2.89) per 10 mg/m3 increase in 3-day average PM2.5 for children younger than 5 and a half. The latter needs to be considered only suggestive, and larger-scale analyses with greater statistical power could help understand the finding