The invention relates to a process for preparing a polyurethane foam comprising reacting a polyether polyol and a polyisocyanate in the presence of a blowing agent and an antioxidant, wherein: the antioxidant consists of one or two or more phenolic antioxidants, wherein the one or more phenolic antioxidants contain no atoms other than carbon, hydrogen and oxygen, and the amount of the one or more phenolic antioxidants is of from 2,300 to less than 10,000 parts per million by weight (ppmw) based on the polyether polyol; the isocyanate index is higher than 100; the maximum temperature during the preparation of the polyurethane foam is higher than 140° C.; and the density of the polyurethane foam is lower than 25 kg/m3.