The early manufacture of ethylene oxide was via ethylene chlorohydrin as an intermediate, but this route has been superseded by the direct oxidation of ethylene in the presence of air or oxygen over a silver oxide catalyst. In an attempt to improve the economics of ethylene oxide manufacture, considerable effort is being expended in trying to find ways of reducing energy consumption and prolonging catalyst life. Capacities range from 20,000–400,000 tonnes per year. Because of its hazardous nature, the production of ethylene oxide is frequently integrated with that of its major derivative, ethylene glycol. Ethylene compressed oxygen and recycle gas are mixed and preheated by product gases from the reactor. The reactor gases are cooled and compressed before passing to a scrubber where chilled water absorbs the ethylene oxide. Ethylene oxide is toxic and a potential human carcinogen. Its vapour is extremely irritating to eyes, causing permanent damage, and to the respiratory tract.