Structures in petrochemical and similar industrial plants can act as inadvertant receiving aerials, leading to a risk of ignition of flammable vapours when strong radio waves are present. The hazard is greatest when the structure is tuned to the transmission frequency. When several transmissions are present, the structure is unlikely to be tuned to more than one transmission at any given time and will tend to discriminate against those to which it is not tuned. The detuning effect may be quantified by a parameter Q. Values of Q have been calculated for a variety of structures from their measured impedance/frequency characteristics.