The effects of exposure to different noise frequencies on the prevalence of metabolic syndrome

Over the past couple of decades, a considerable number of studies have demonstrated that shortand long-term exposure to noise can increase the risk of cardiovascular disease (CVD) such as hypertension, ischaemic heart disease and finally mortality. However, the effects of exposure to different noise frequencies on the human body have not been well established. The results of a limited number of previous studies that investigated the effects of exposure to different noise frequencies on the cardiovascular system have revealed that the effects of some frequencies are more than others. One study showed that exposure to high levels of noise at 2, 4 and 8 kHz frequencies increased the risk of hypertension, although these risks were not significant. However, another study found, in addition to high noise frequencies (2, 4 and 8 kHz), exposure to noise at 250 and 1000Hz frequencies significantly raised the risk of hypertension. In addition, another study reported a significant relationship between exposure to traffic noise at 63, 125 and 1000Hz frequencies and hypertension. To date, not only a limited number of studies has assessed the effects of exposure to different noise frequencies on the cardiovascular system but also the results are inconsistent. Apart from the impact of exposure to different noise frequencies, the prevalence of the metabolic syndrome (MetS) has not been studied. This study aimed to quantify and clarify the effects of exposures to different noise frequencies on the prevalence of MetS. This cross-sectional study was carried out in the thermal power plant industry in the west of Iran during 2017. A total number of 518male workers, including 214 office and 304 production line workers, was assessed. The diagnosis of MetS was made according to the National Cholesterol Education Program (NCEP) Adult Treatment Panel III (ATP III) definition. In addition, the noise measurements were performed by two trained occupational health engineers according to ISO 9612, 2009 (Acoustics – determination of occupational noise exposure – engineering method) using a Taiwanese TES-1358 sound analyser that was calibrated by the TES-1356 calibrator. In the present study, because subjects work in different work sections during 8 hours of working time, we calculated the 8-hour equivalent A-weighted sound pressure level (8-hour LAeq) for each participant. According to the 8-hour LAeq and types of work, the participants were divided into three groups at each octave band frequency. One group consisted of office workers and other groups included production line workers. The median of noise distribution between production line workers was considered as a cut-off point (see Figure 1). Accordingly, production line workers who had exposure to noise levels less than the median and greater or equal to the median were assigned to low and high exposed groups, respectively. The average of noise levels at each octave band frequency is listed in Table 1. As demonstrated by Table 1, we pointed out per 1-dBA increase in noise levels in the high versus low subgroup the risk of MetS significantly raised at 4 and 8 kHz frequencies (P< 0.05). However, we did not find any significant increased risk of MetS in the low