The study population consisted of 70 taxi drivers and 70 office employee who referred to a fertility clinic during 2020–2021. The participants had to meet the entry criteria of the study while also being willing to recruit. The participants were required to be 20 to 50 years of age with at least 2 years of employment [19]. Those who had used steroids, prednisolone, testosterone (prior to clinic referral), anti-oxidants (such as selenium), vitamins B, E or C (prior to clinic referral) or bodybuilding supplements (prior to clinic referral) were excluded. Participants who had a familial history of infertility or organic disorders affecting reproductive performance such as diabetes, kidney disorders, angina pectoris, heart disorders, arterial blood pressure, disorders of the pituitary gland, CPOD, testicular infection, testicular inflammation, varicocele or a history of chemo or radio therapy [19] and also people who change the car during the past year and their car had the serious damage during the past year were deemed to meet exclusion criteria. The taxi driver drove at least 8 hours a day or two shifts of 4 hours and 4 days a week. In the case of administrative staff, the requirement to enter the study was to have at least 1 year of work experience with a maximum of 8 hours of daily work and at least 4 hours of daily sitting work. Participants who did not complete the questionnaires or were unwilling to continue at any stage in the study and excluded from the study. The written informed consent was obtain from all subjects before their participation. It has been approved by the ethics committee of Shahid Beheshti University of Medical Sciences (IR.SBMU.PHNS.REC.1398.164).
Semen samples were taken for sperm analysis (Spermogram) via masturbation into a sterile plastic container after 3 days of celibacy. The samples were collected at a fertility clinic in the city of Tehran in order to prevent the effects of heat, contamination by secretions of the female reproductive organ and faster testing. Sperm parameters such as concentration, pH, color, viscosity, motility, total sperm count, and morphology were measured as per the guidelines of the World Health Organization (WHO) [20] using computer-assisted sperm analysis (CASA). This involves the use of a phase contrast microscope (NikonTM Eclipse E-200, Japan) connected to a camera (BaslerTM, A312FC, Germany) aided by software (SCA Microptic S.L., Spain). According to the WHO in 2010, a normal semen analysis conforms to the following: the sample must contain a minimum volume of 2 ml, semen must have a pH higher or equal to 7.2, the sperm concentration (number of sperm per milliliter) must be equal or above 15 million/ml, total sperm count must be larger than 39 million per ejaculation, at least 40% of the sperm must have movement until 1 hour after ejaculation, a minimum of 32% of sperm must have rapid progressive movement and 4% must be have normal morphology as per Kruger’s criteria [20].
The next stage involved the measurement of vibration acceleration in three axes via a calibrated WBV meter (Bruel and Kjaer model 4447) which has a thin disk and a plastic cover and is placed on the buttocks area of the driver seat (Fig.1). Measurements were taken in m/s2 for 30 minutes and in all three axes (X, Y and Z) as per the ISO 2631 standard [21] when participants were driving certain ways around the infertility clinic. The overall vector sum and the acceleration level were calculated using the relevant equations. The root mean square (RMS) of acceleration for each of the axes was obtained using Eq. 1 (measurement time of 30 minutes) from the exposure group (cars of taxi drivers) and the control group (cars of office employees).
$${a}_{w=}{\left[\frac{\mathbf{1}}{\boldsymbol{T}}\int_{\mathbf{0}}^{\boldsymbol{T}}{{\boldsymbol{a}}_{\boldsymbol{w}}}^{\mathbf{2}}\left(\boldsymbol{t}\right)\boldsymbol{dt}\right]}^{\frac{\mathbf{1}}{\mathbf{2}}}$$
(1)
In the above equation, aw is the root mean square (RMS) frequency-weighted acceleration in each axis, aw (t) is the frequency-weighted acceleration at the moment of (t) and T is the measurement duration. The vibration acceleration vector sum was calculated using Eq. 2 for each of the three axes in m/s2.
$${a}_{v=}\sqrt{{\left(\mathbf{1}.\mathbf{4}{\boldsymbol{a}}_{\boldsymbol{wx}}\right)}^{\mathbf{2}}+{\left(\mathbf{1}.\mathbf{4}\ {\boldsymbol{a}}_{\boldsymbol{wz}}\right)}^{\mathbf{2}}+{\left(\mathbf{1}.{\mathbf{a}}_{\boldsymbol{wz}}\right)}^{\mathbf{2}}}$$
(2)
Here, av is the vector sum of the RMS frequency-weighted accelerations while ax, ay and az represent the RMS frequency-weighted accelerations in the specific direction. The daily 8-hour frequency weighted exposure to vibration acceleration is determined using Eq. 3.
$$A(8)={\boldsymbol{a}}_{\boldsymbol{v}}\sqrt{\frac{\boldsymbol{T}}{{\boldsymbol{T}}_{\mathbf{0}}}}$$
(3)
In the above equation, av is the frequency-weighted acceleration during exposure, T0 is the average exposure duration (8 hours for taxi drivers and 2 hours for office employees) and A(8) is the 8-hour equivalent RMS frequency-weighted acceleration in m/s2 [22].
Participants also filled the demographic questionnaire (age, driving experience, body mass index, education, smoking habits, and physical exercise), the general health questionnaire.
The General Health Questionnaire (GHQ) was devised by Goldberg and Hillier in 1979 [23] with 28 items and four aspects of distress which include somatic symptoms, anxiety and insomnia, social dysfunction and severe depression. Taghavi et al. assessed the viability of the Farsi translation of the GHQ questionnaire and reports a reliability of 0.70 as per the Test-Retest method, 0.90 as per Cronbach’s alpha and 0.93 as per the Split-Half method [24]. A 4-point Likert scale is used for scoring (0, 1, 2 and 3) with a maximum obtainable score of 84. Scores between 0 and 22 are indicative of health (none or minimal distress) and scores above 22 are indicative of psychological distress and are further divided into 3 ranges namely weak, medium or extreme distress.
After data was collected in the form of questionnaires, vibration measurements and sperm analysis, data distribution normality was determined using the Shapiro-Wilk test. Various tests including t-test, ANOVA, crosstab, Mann-Whitney, Chi-square, Pearson and Spearman test were used to compare normal and non-normal quantitative and qualitative data in the control and the exposure groups. Descriptive statistics are presented in the form of mean, median, standard deviation and interquartile range. The relationship between exposure intensity and reproductive parameters was determined using partial correlation and univariate analysis of variance. Data analysis was performed using SPSS v26 (Chicago, Illinois, USA) with a P-value bellow 0.05 considered to be statistically significant.