On the other hand, chronic airway inflammation is also the hallmark of asthma which involves the interplay of different types of inflammatory cells, including T-lymphocytes, eosinophils, neutrophils, macrophages and cytokines in the airway. Although the pathogenesis of asthma is incompletely understood, studies have shown that it is associated with a state of increased free radical formation, because cells derived from airways and peripheral blood of patients with asthma generate increased amount of reactive oxygen species, the level of which is related to severity of asthma. Besides, airway biomarkers indicating airway inflammation and oxidative stress have been consistently shown to be positively related to asthma severity and asthma control, which can be alleviated with treatment by inhaled corticosteroids. Additionally, recent preliminary studies have indicated that, apart from the presence of chronic airway inflammation (elevated EBC nitric oxide, EBC nitrites/nitrates and EBC 8-isoprostane), asthma may also be associated with chronic low grade systemic inflammation (elevation of plasma hs-CRP) and increased oxidative stress (increased 8-isoprostane; altered erythrocyte superoxide dismutase, catalase, glutathione peroxidase and glutathione).