Our study found that the global IEI-EMF prevalence rate has declined in recent years, not increased as predicted by Hallberg and Oberfeld [20]. They collected 17 estimates of the prevalence rates from 1985 to 2004 in seven countries and plotted them over time in a normal distribution diagram, which showed that the prevalence rate would be 50% in 2017. The prediction was based on the assumption that the trend over time will not change direction, and it is proven wrong by the data collected in our study, which extended the data collection to 2013 and observed a change in the direction around 2007. It is true though that there was a consistent increasing trend during the study period of Hallberg and Oberfeld. In our study, three countries had more than one estimate after 2007 (Taiwan, Germany, and the Netherland), and all of them showed a declining trend over time. Among the three countries, two were not included by Hallberg and Oberfeld. Nonetheless, even in Germany, which was covered by their study, there was a decline in the prevalence rate from 2009 to 2013.
Scientific evidence bridging EMF and IEI-EMF symptoms has been scarce [1, 11, 37]. Many double-blind provocation studies have been conducted to determine whether people with IEI-EMF can detect the existence of EMF and whether EMF is the cause of their symptoms. However, the results indicated that IEI-EMF sufferers are unable to precisely detect the existence of EMF and that short-term exposure to EMF cannot provoke the IEI-EMF symptoms [6]. Although some believe that the level of environmental EMF exposure is associated with the prevalence of IEI-EMF, a study in European cities found that while the environmental EMF exposure was increasing annually, the EHS prevalence rate was declining [38].
Since we applied the same method as used in the 2007 survey, we could assess the changes in the prevalence of IEI-EMF over time, which has rarely been achieved. Although our study and the survey in 2007 were both nationwide telephone interviews, a strength of our study is that the number of participants was nearly triple the number as in the 2007 survey (3303 vs. 1197). In both surveys, women had a higher prevalence than men, and the respondents reporting IEI-EMF were mainly in the age range of 35 to 64 years, around 70% of all participants. Our study found a higher prevalence of reporting impairment in daily activities by respondents with IEI-EMF (adjusted OR = 2.17, 95%CI: 1.24—3.78), which was not observed in the 2007 survey (adjusted OR = 1.3, 95%CI: 0.8—2.1). However, the proportion of respondents with IEI-EMF who reported being unable to work was 3.9% in our study, much less than the 20% proportion in the 2007 survey. In fact, while the 2007 survey reported that the IEI-EMF group had a higher risk of being unable to work (adjusted OR = 1.8, 95%CI: 1.1—3.2), we did not observe such an increased risk in our survey (adjusted OR = 0.9, 95%CI: 0.3—2.5). The question to inquire participants if their health status contributing to the impairment in daily activities was not classified into more detail items, such as impairment in walking, dressing, writing, reading, and so on. Therefore, the severity of impairment in daily activities cannot be mirrored to the work ability. In addition, although the risk of IEI-EMF sufferers in our study who reported their health status as very poor (adjusted OR = 5.4, 95%CI: 1.4—20.6) was higher than that in the 2007 survey (adjusted OR = 4.9, 95%CI: 1.6—15.2), the differences between the two surveys did not reach statistical significance.
It is possible that very serious IEI-EMF sufferers could not be contacted by our telephone interviews, and consequently, the prevalence rate could be underestimated. However, this should have occured in both surveys and is unlikley to account for the large difference in the prevalence rates. The phonebook of Chunghwa Telecom does not include mobile phones, and therefore our survey might over-estimate the prevalence rate because sufferers of IEI-EMF are less likely to use mobile phones and thus more likely to subscribe landline phones due to the fear of EMF. Since the 2007 survey also used the phonebook and we observed a decrease instead of an increase, our conclusion of a decreasing trend should still hold even if the sample was biased.
Media reports affecting public awareness may partly explain the trend. The media reports on IEI-EMF have focused on precaution and could influence reader’s perception, even in lack of scientific evidence [39,40,41]. When the media pays less attention to IEI-EMF, the number people attributing their discomfort to EMF may decrease. For example, a study in the Netherlands found that the number of newspaper articles decreased from 87 in the first year (March 2008 to March 2009) to 68 in the second year (March 2009 to March 2010), and a decline in the prevalence of IEI-EMF from 7.0% in 2009 [29] to 3.5% in 2011 [31] was observed (Fig. 2). An alternative explanation for the declining trend is the effects of efforts in managing the public fear of EMF. Beginning with the early reports of IEI-EMF symptoms observed in the 1930s, public health workers have made efforts to verify the causality of EMF exposure to IEI-EMF and to alleviate the tense conflict among the government, general public, scientific community, and industries by setting EMF exposure guidelines and monitoring envionmental EMF [42]. The WHO has compiled many fact sheets, and therefore public knowledge of environmental EMF exposure might have been altered by various efforts in the world.
The declining IEI-EMF prevalence trend might also be attributed to the public’s concern having been turned to other environmental issues such as particulates in the air. Furthermore, it is also possible that humans may develop tolerance of EMF after a period of exposure. A double blind crossover study that investigated the potential effects of mobile phone-like RF-EMF on pain threshold perception in response to thermal stumli observed a reduced desensitization effect between repeated stimulations [43].
Our study showed that women had a higher prevalence of IEI-EMF and constituted 63.6% of the sufferers, and this is consistent with findings in previous studies. For example, Schreier et al. [13] conducted a questionnaire survey in Switzerland and found the proportion to be 54.5%, and Röösli et al. [28] conducted another survey in Switzerland and found the proportion to be 72.3%. In addition, women were found to have a lower perception threshold than men in detecting a 50-Hz electric current [44]. Our meta-analysis also showed that women are more likely to report IEI-EMF than men. Some reserachers believe that women are emotionally more sensitive than men and thus are likely to misattribute their idiopathic symptoms to the exposure of EMF [29, 45]. Women supposedly stay indoors for longer than men because of the nature of their work. Stratifed analysis of job contents between men and women could probably test the hypothesis of static daily activity being correlated with IEI-EMF for further study. An alternative explanation of this finding might be the masculine gender role discourages the expression of pain.