Does the oxidative stress play a role in the associations between outdoor air pollution and persistent asthma in adults? Findings from the EGEA study

Background: Evidences that oxidative stress plays a role in the associations between outdoor air pollution and asthma are growing. We aimed to study the role of plasma fluorescent oxidation products levels (FlOPs; an oxidative stress-related biomarker), as potential mediators, in the associations between outdoor air pollution and persistent asthma. Methods: Analyses were conducted in 204 adult asthmatics followed up in the French case-control and family study on asthma (EGEA; the Epidemiological study of the Genetic and Environmental factors of Asthma). Persistent asthma was defined as having current asthma at EGEA2 (baseline, 2003-2007) and EGEA3 (follow-up, 2011-2013). Exposures to nitrogen dioxide, nitrogen oxides, road traffic, particulate matter with a diameter <= 10 mu m (PM10) and <= 2.5 mu m were estimated by ESCAPE models (2009-2010), and ozone (O-3) by IFEN models (2004). We used a mediation analysis to assess the mediated effect by FlOPs levels and the interaction between FlOPs levels and air pollution. Results: FlOPs levels increased with PM10 and O-3 (adjusted beta = 0.04 (95%CI 0.001-0.08), a beta = 0.04 (95%CI 0.009-0.07) per 10 mu g/m(3), respectively), and the risk of persistent asthma increased with FlOPs levels (aOR = 1.81 (95%CI 1.08-3.02)). The risk of persistent asthma decreased with exposures to NO2, NOx and PM2.5 (aOR ranging from 0.62 to 0.94), and increased with exposures to PM10, O-3, O-3-summer and road traffic, the greater effect being observed for O-3 (aOR = 1.78, 95% CI 0.73-4.37, per 10 mu g/m(3)). Using mediation analysis, we observed a positive total effect (aOR = 2.16, 95%CI 0.70-11.9), a positive direct effect of O-3 on persistent asthma (OR = 1.68, 95%CI 0.57-7.25), and a positive indirect effect mediated by FIOPs levels (aOR = 1.28 (95%CI 1.01-2.29)) accounting for 41% of the total effect. Conclusions: Our results add insights on the role of oxidative stress in the association between air pollution and persistent asthma.