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 μm (PM(10)) and ≤ 2.5 μm 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 PM(10) and O(3) (adjusted β = 0.04 (95%CI 0.001–0.08), aβ = 0.04 (95%CI 0.009–0.07) per 10 μg/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 NO(2), NOx and PM(2.5) (aOR ranging from 0.62 to 0.94), and increased with exposures to PM(10), 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 μg/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.