Exposure to PAHs was high among workers employed in the Production of town gas and industrial gas from destructive distillation of coal, in particular among workers involved in coal distillation and purification using old types of gasifiers ,such as horizontal, vertical and continuous vertical retorts (range of typical benzo pyrene level: 1-10 mg/m) PAH exposure ,however ,also occurs in modern processes (fixed-bed , fluidized-bed ,and entrained-bed) In the older gas-making processes ,gas and tars were produced by the destructive distillation of coal; in modern gasifiers ,oxygen is introduced ,resulting in partial combustion of the coal ;in this case ,all the coal is distilled into gas and no coke is produced.