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Knave et aP5-have found no similar problems associated with electric field exposure in either switchyard workers or linesmen.Nevertheless, if there is one general criticism that might be made of all studies so far carried out it is that exposures to electric fields have been in varying degrees estimated rather than measured directly. In the present study we have attempted to remedy this deficiency by including individual exposure measurements for the whole study population. Our aim has been to relate these exposure measurements (together with estimates of exposure produced by local management) to several possible indicators of Received 30 January-1984 Accepted 20 February 1984 ill health as elicited by a questionnaire interview.The groups chosen for study consisted of the non-clerical staff of three Central Electricity Generating Board (CEGB) transmission districts in the south west of England and South Wales, together with a group of similar staff of the South Wales Electricity Board (SWaEB). The CEGB staff o,prated and maintained transmission lines and substations running at 132, 275, and 400 kV. The SWaEB staff worked with distribution systems running at 11, 33, 66, and 132 kV; the geographical area they covered overlapped with that of CEGB South Wales district. From the nature of their duties, it was expected that the CEGB groups were more likely to be exposed to high electric fields than the SWaEB group.From a general occupational health point of view, the interview results contain much interesting material, particularly on the health differences between different jobs. In the present report, however, we concentrate on the relationships with exposure to electric fields. Methods and measurementsFor a two week period, closely preceding his questionnaire interview, each subject wore a simple single channel electrochemical exposure meter strapped to his upper arm. On interrogation at the end of the period, it gave a single measure of integrated exposure to all field strengths above about 75

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Thirty-five patients fitted with 16 different pacemaker models (from 6 manufacturers) were exposed to 50 Hz electric fields up to a maximum of 20 kV/m. Four different response patterns were encountered: (1) normal sensing and pacing in all Medtronic and some Vitatron units; (2) reversion to the fixed (interference) rate in all Telectronics, all Pacesetter, some Vitatron and CPI units; (3) slow and irregular pacing in one CPI and in all Cordis units; (4) mixed behavior over a critical range of field strengths in which slow and irregular pacing preceded reversion to fixed-rate, in some Telectronics and Pacesetter units. The field strengths required to induce such behavior varied from unit to unit and from model to model, with Telectronics being the most sensitive. In general, the interference threshold depended on the magnitude and distribution of induced body current relative to the pacemaker as well as field strength and thus varied with patient height, build and posture. While only a small proportion of pacemaker patients are likely to encounter electric fields strong enough to interfere with pacemaker behavior, this possible hazard should be recognized.