Choosing threshold levels for electromagnetic hazards

The concept of electromagnetic hazards is today frequently taken to refer to direct effects on biological tissue. However, several other effects are possible and have been the subject of a substantial amount of research in earlier decades. One of these is the hazard of ignition of flammable gas/vapor mixtures due to sparks resulting from radiofrequency currents flowing in metallic structures. Although the physics of the mechanism linking power from a radio transmitter to ignition of a flammable mixture is relatively straightforward, compared with bioelectromagnetic effects, there are many uncertainties and poorly-understood steps in the process, leading to considerable difficulties for committees tasked with setting safety limits. The author has experience of such committees, notably that concerned with British Standard 6656, and the way in which these difficulties were addressed may give useful insights for the regulation of other hazards.