Workplace ventilation safety requires neutralising toxic emissions and restricting entry until harmful gas and dust levels are permissible. Employer obligations in factories include precautions against dust, noxious gas, fumes and other impurities in workplace ventilation and confined spaces. Exhaust systems for flammable fumes, gases, mists, vapours or residues must not be connected to other ventilating systems or discharged into chimneys or flues used for combustion gases. Where exhaust discharge may harm health or be noxious, it must be neutralised, sterilised and deodorised. No employee may enter a workplace or confined space until permissible limits are achieved.