Ethylbenzene is the prime feedstock used for the production of styrene. Between 85–90% of styrene is produced from ethylbenzene, the remainder being obtained as a by-product of propylene oxide manufacture. Ethylbenzene is separated from styrene in the second column by distillation under vacuum and recycled. Styrene can be produced as a by-product of the oxidation of propylene followed by the dehydration of the a-methylbenzyl alcohol formed. The importance of styrene has increased rapidly since World War II until today it is a major monomer. The most important outlet for styrene, which accounts for 65% of total demand, is in the manufacture of general purpose, high impact and expandable polystyrene. Styrene vapour is mildly toxic and has an irritating effect on eyes and respiratory tract. Styrene polymerizes readily and must not be stored unless it is inhibited and then only for short periods. Styrene is highly inflammable and forms explosive mixtures in air.