The domain of the activity taken into account was 132, 230 and 400 KV substations in Kerman city and its suburbs (Baghyn, Kerman, Shahab, Tavakol Abad, Zangy Abad, Rhine, Mahan, Syrach and Nirogah). In order to assess the effect of ELFMF exposure on sleep quality in these substations, a case–control study was carried out. The cases comprised 67 workers who had been exposed to electromagnetic fields in the age range 24–57 years and the controls were 110 people in the age range of 24–50 years. In 132 KV substations, 41 cases of were chosen, 20 person in 230 KV substations; and the rest were in the 400 KV substations. all participants were male. Workers of cement factory, Barez tire and Bahonar copper industries constituted the control group who worked in same geographic space of high voltage substations but were far from them. Since high voltage substation operators were the shift workers; control group was selected from shift workers too. Another reason for choosing shift workers was to moderate the effect of shift work on sleep quality in both groups, because the change in circadian rhythm is one of the main causes of sleep disorder in shift workers [16–19]. Smokers, people with cardiovascular, pulmonary, diabetes and other diseases were excluded by the demographic questionnaire to prevent the interference in sleep. 15 high voltage substations, three 230 KV substations and twelve 132KV substations, and one ultra-high voltage substation (400 kV) were inspected. Then, the stations were determined according to the area of each substation, the amount of time that an operator spent in different parts and the distance from equipment. Finally, number of samples was determined in each substation and electric field intensity and magnetic-flux density were measured based on standard IEEESTD 644 – 1994 .
HI-3604 ELF survey meter is used to measure the magnetic flux density and electric field intensity. This device is designed for frequency range of 30–2000 Hz. Its sensitivity is 0.2 mm to 20 Gauss and 1 V/m to 200 kV/m. The electric field strength under power lines was measured at a height of 1 m above ground level. The probe was oriented to read the vertical E-field, because this quantity is often used to characterize induction effects in objects close to ground level. The distance between the electric field strength meter and operator was at least 2.5 m. The distance between the meter and nonpermanent objects was at least three times the height of the object in order to measure the unperturbed field value. The distance between the meter and permanent objects was ~1 m or more to ensure sufficient measurement accuracy of the ambient perturbed field. A remote control system was used to read the electric field values [21, 22]. The consumption load of each substation was recorded during measurements. Since weather conditions can affect the results, all measurements were done between the hours of 4 to 9 pm in summer and in the sunny days.
Occupational exposure (time-weighted average ;TWA) to ELF electromagnetic fields in a shift work was calculated by following equation:
BC = Occupational exposure
B(t)i= The mean magnetic flux density in different parts of each substation
hi= The mean time spent by operator for a specific duty in different parts of substations(hour)
h=Time shift (24 hours and 12 hours) [23–25].
Finally the results were compared with standard ICNRIP [26, 27].
Data collection tool for investigating the operator's sleep quality was the Pittsburgh Sleep Quality Index questionnaire (PSQI). Validity of this questionnaire using Cronbach's alpha coefficient is 0.83 [14, 28]. It checks the patient attitude about sleep quality in a one-month period. The seven component-scores are then summed to yield a global PSQI score, which has a range of 0–21; higher scores indicate worse sleep quality. These components are: subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleeping medications, and daytime dysfunction. Previous studies have shown a remarkable agreement between the results of PSQI and sleep laboratory studies using polysomnography (PSG). Each questionnaire scale takes score of zero to three. Score of zero, one, two and three respectively in each scale represents the normal situation, mild, moderate and severe problem.Total score five and less means having a good sleep quality and score six and higher means having a poor sleep quality [14, 29–33].
Before handing out the questionnaire the researcher explained the aims of the research and the method of questionnaire completion.Case and control subjects completed the questionnaire on three consecutive days. Totally 531 questionnaires were collected: 330 questionnaires for control group and 201 questionnaires from case group. In addition, the demographic questionnaire was completed by both groups. Finally the collected data were analyzed using descriptive statistical methods, independent-samples t-test, repeated measures (ANOVA), Chi - square and Pearson correlation.