A pivotal role for reactive oxygen species (ROS) in all three stages has also been proposed. Various biomarkers of oxidative stress, i.e. exhaled ethane and both 8-isoprostane and oxidized proteins in the bronchoalveolar fluid, are elevated in ILD patients of different clinical stages. However, only little is known about the effect of this elevated oxidative stress on the endogenous antioxidant levels present in ILD. Interestingly, clinical administration of an antioxidant, i.e. N-acetylcysteine (NAC), to IPF patients has recently demonstrated that this slows the deterioration of vital capacity and carbon monoxide diffusing capacity (DLCO) at 12 months. This supports the hypothesis that oxidative stress is involved in ILD and proofs the principle of antioxidant treatment in ILD.