Ambulatory heart rate of professional taxi drivers while driving without their typical psychosocial work stressors: a pilot study

Background of the field study

A total of 22 professional (21 males and 1 female) taxi drivers participated in a field exposure assessment project about air pollutant exposures inside taxi cabs [11]. The research team ran a recruitment/survey campaign at the Los Angeles Airport (LAX) taxi holding lot in February 2013 in order to recruit study participants and collect basic information about taxi drivers and their cabs. A questionnaire designed by the research team included the questions about sociodemographic, car model, work stress experience, health-related behaviors, health conditions, and the use of medication [11]. We initially handed out 2449 questionnaires and finally collected 316 complete questionnaires. Out of the 316 taxi drivers who completed the questionnaire, 121 non-smokers were eligible to participate in a field exposure assessment project about air pollutants and air exchange rates inside taxi cabs. To ensure the sampled taxi drivers/cabs are representative, stratified random sampling was conducted based on car models and drivers’ age. A total number of 22 non-smoking taxi drivers out of 121 eligible drivers were selected to participate in the field exposure assessment project [11]. The study design and protocol (#12-000845) were approved by the Institutional Review Board (IRB) of University of California Los Angeles. All of the taxi drivers who participated in the field exposure assessment project provided written informed consent.

13 male taxi drivers for this study

Among the above 22 taxi drivers, we excluded 9 taxi drivers from this study whose ambulatory heart rate on the first experimental day (for details, see below) was not be able to be collected (N = 5; cases T03, T07, T08, T19 and T20, including 1 female driver) or was collected, but only for a limited time period (<1 h) (N = 4; cases T05, T12, T18, and T21). Thus, we included 13 male taxi drivers in the current study. There were some differences between the excluded and included drivers: the excluded 9 drivers were older (53.0 years vs. 42.9 years), worked more years as a taxi driver (11.6 years vs. 7.4 years), and had higher prevalence of hypertension (33 % vs. 15 %) than the included 13 drivers.

6-hour experimental on-road driving

The 13 male drivers drove 6 h on each of four consecutive experimental days in the Greater Los Angeles area between April 2013 and November 2013 as he would typically do. One field technician rode along in the taxi cab operating and maintaining all of the air sampling instruments (see below). The starting time of each day was based on the driver’s availability. No actual fares were collected during the tests and the drivers’ time and effort were compensated by the research fund [11]. The driving routes were not specifically planned for each driver. Instead, each driver was asked to drive from the start location, University of California Los Angeles, to the area where he usually works. Each driver was allowed to take breaks as he would during a typical work day. The time and location of each break were recorded by hand by one field technician and confirmed by a GPS unit (Qstarz GPS BT-1000XT, Taipei, Taiwan). The experimental conditions with regard to taxi-cab air flow varied day to day during the four experimental days [11]. The experimental conditions on the first day were least intervening (most realistic) because the drivers had a full control over all the vehicle operations such as opening/closing windows, turning air conditioning on or off, setting ventilation to recirculation or outdoor air mode as they usually did on their typical work days. Thus we restricted our analyses for this study to the first experimental day.

Assessment of ambulatory heart rate: HR_driving, HR_rest, and PMHR_driving

One trained field technician instructed the drivers to wear and use an ambulatory heart rate monitor (RS800CX, Polar Electro, Finland, with a sampling rate of 1,000 Hz) [29, 30] just before the experimental 6-hr on-road driving. Heart rate (HR) of the drivers was continuously measured based on the peak to peak interval of two consecutive QRS complex signals on electrocardiograms (called hereafter RR interval, milliseconds) with the monitor during the driving. The raw RR interval data of 13 drivers downloaded from the monitor were processed using the data analysis software (Kubios Version 2.2) [31] with the medium option for artifact correction. Then, we split the artifact-corrected RR interval data into 5-min segments (674 segments in total) from the starting time of the 6-hr driving and calculated the average HR of each 5-min segment (called hereafter 5-min HR). Afterwards, we manually examined each 5-min segment whether all RR intervals in the 5-min segment are within the normal range (300 to 2,000 milliseconds) and whether most (>95 %) of the ratios of two consecutive RR intervals in the 5-min segment are within the normal range (0.8 to 1.2) [30, 32]. Of the total 5-min 674 segments, 160 (23.70 %) segments were excluded by the last procedure from analyses in the current study.

After the above data cleaning process, each 5-min segment was classified into the following two groups: 5-min HR while driving (5-min HR_driving) and 5-min HR while taking breaks (5-min HR_breaking) based on the record by one field technician on the on-road driving. In the current study, we used only 5-min HR_driving values (of 344 5-min segments in total) for analyses as consistent with the previous environmental science studies in taxi drivers [10, 11]. The driving times noted by one field technician were confirmed by matching the driving times with the vehicle speed information (i.e., ≥ 1 km∙hr⁻¹) during the same time periods. The breaking times recorded by the field technician included the times when the drivers took a rest inside taxi cabs, or went outside taxi cabs for a stretch, walk, or meal.

Resting HR (HR_rest) of each taxi driver was estimated conservatively based on the following two-step process: (1) identifying the lowest 5-min HR_driving value of each driver [28]; and then (2) choosing the age- and gender-specific 25th, 50th, or 75th HR_rest percentile value in the United States (US) adult population reference data from the 1999–2008 National Health and Nutrition Examination Surveys [33] that was lower than, but closest to the identified lowest 5-min HR_driving value.

As a way to control for the individual differences in age and resting and maximum HR, we estimated the 5-min percent maximum heart rate range (PMHR_driving) of each taxi driver during the driving times using the following equation [9]:

[TeX:] \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ 5\hbox{-} \min\ {\mathrm{PMHR}}_{\mathrm{driving}}\left(\%\right) = \frac{5 \min {\mathrm{HR}}_{\mathrm{driving}}-{\mathrm{HR}}_{\mathrm{rest}}}{{\mathrm{HR}}_{\max }-{\mathrm{HR}}_{\mathrm{rest}}} \times 100 $$\end{document}5‐minPMHRdriving%=5minHRdriving−HRrestHRmax−HRrest×100

The maximum heart rate (HR_max) of each driver was estimated by using the formula, 205.8 − (0.685 × age) [34, 35].

Assessment of five physical (environmental) hazards while driving inside taxi cabs

Five physical hazards (PM_2.5, CO_2, relative humidity, temperature, and noise) were continuously assessed inside taxi cabs during the experimental 6-hr on-road driving. One DustTrak (Model 8520, TSI Inc., St. Paul, MN) was used to measure the in-cabin PM_2.5 concentrations. One Q-trak monitor (Model 8554, TSI Inc., St. Paul, MN) was also used to measure the in-cabin CO₂ concentrations, relative humidity, and temperature simultaneously. The noise level in cabin was measured by a Quest 2900 Sound Level Meter (3 M, St. Paul, MN). All of these instruments were synchronized with the heart rate monitor and were set to record one reading every second, to provide data with high time resolution. All instruments were calibrated to ensure the quality of measurement. After the experimental driving, data were downloaded. Data were then observed in Excel for visualization and obvious outliers - mostly caused by instruments malfunctioning - were removed before further data analysis. Then as with the HR data, the physical hazards data were also split and averaged into 5-min segments for analyses and only the 5-min averaged data while driving (excluding the 5-min averaged data while breaking) was used in the current study.

Assessment of work hours, work stress, and other covariates

Work hours per week were calculated using the two questions in the recruitment survey questionnaire (“Typically, when do you start and end your work day?” and “How many days do you typically work as a taxi driver each week?”). Work stress experience of taxi drivers on a typical workday was also measured with one question (“How often do you find your work stressful?). Age, race/ethnicity, exercise during leisure-time (the frequency of the moderate or vigorous level of aerobic exercise: frequent (2 or more times per week) and infrequent (0–1 times per week))), health condition (“In general, would you say your health is: Excellent; very good; good; fair; and poor), body weight and height, hypertension (“Have you ever been diagnosed with hypertension?”), and the use of anti-hypertensive medication were also assessed with the recruitment survey questionnaire.

Data analysis

At first, the distributions of five physical hazards inside taxi cabins during the 5-min HR_driving periods for each driver were examined. Then we examine the distribution (maximum, mean, and minimum) of the 5-min HR_driving values for each driver with a focus on the extent of the elevation of HR and the threshold HR value for a sustainable 8-hour work (i.e., 35 beats per min above HR_rest) [9]. The above analyses were replicated with the 5-min PMHR_driving values among the 13 taxi drivers with a focus on the threshold PMHR value for a sustainable 8-hour work (i.e., 30 % of percent maximum heart rate range) [27, 28]. As a sensitivity test, we tested whether the above analyses would be affected by the status of hypertension, obesity, and exercise frequency of the taxi drivers. All data analyses and graphs were performed using PASW version 23.0 (SPSS Inc., Chicago, IL, USA) and Sigma Plot software, version 12.5 (Systat Software Inc. USA), respectively.

Sociodemographic and work environmental characteristics of the 13 male taxi drivers

The mean age of the taxi drivers was 42.9 years (range: 24–67 years). Their mean career years as a taxi driver were 7.4 years (range: 0.5–17 years). They were mostly Asian or Black (Table 1). Their self-reported health was mostly good or very good. Four out of the thirteen taxi drivers reported that they engaged in moderate or vigorous level of leisure-time aerobic exercise 2 or more times per week. The average body mass index (BMI) of taxi drivers was 26.4 kg/m² and three out of them (23 %) had greater than BMIs of 30 kg/m². The drivers worked, on average, about 12 h (range, 9 to 14 h) on a typical work day, 6 days per week (range 5 to 7 days), and 72 h (range, 54 to 98 h) per week. Seven (53.8 %) drivers reported that their job was often stressful on typical work days, while six drivers reported that their job was sometimes or hardly ever stressful on typical work days. Two (15 %) drivers had hypertension and only one driver was under treatment of anti-hypertensive medication at the time of the questionnaire survey.

Table 1

Physical hazards and ambulatory heart rate (HR_driving) and percent maximum heart rate range (PMHR_driving) while driving in the 13 male taxi drivers

Distributions of in-cabin environmental physical hazards among 13 male drivers

The means and standard deviations of the five in-cabin physical hazards while driving were as follows: temperature (26.4 ± 3.0 °C), relative humidity (40.7 ± 10.4 %), PM_2.5 (21.5 ± 7.9 μg/m³), CO₂ (1,267.1 ± 580.0 ppm) and noise (69.7 ± 3.0 dBA). The means of the physical hazards for each driver were presented in Table 1.

Distributions of the 5-min HR_driving values

The number of 5-min HR values of each driver on the 6-hr experimental on-road driving ranged from 16 (90 min in total) to 46 (230 min in total): on average, 26.5 (132.5 min in total) among the drivers (Table 1). Among the 13 drivers, the mean of 5-min HR_driving values was 80.5 bpm (range, 73.7 to 86.3 bpm). It was 11.2 bpm higher (p < 0.001) than the mean of the estimated HR_rest values, 69.3 bpm. In 1 out of the 13 drivers, 4 of the 26 (15.4 %) 5-min HR_driving values were 35 bpm higher than his HR_rest value (Table 1 and Fig. 1). There were no significant differences in the mean of 5-min HR_driving values between normotensive and hypertensive drivers: 81.3 bpm and 75.9 bpm, respectively; between non-obese and obese drivers: 80.3 bpm and 81.1 bpm, respectively; and between drivers who reported frequent exercise and drivers who reported infrequent exercise: 78.1 bpm and 81.5 bpm, respectively.

Fig. 1

Distribution of the 5-min HR (HR_driving) values for each of the 13 taxi drivers (T01 to T22) while driving on his 6-hr experimental on-road driving

Distributions of the 5-min PMHR_driving values

In general, the results of the 5-min PMHR_driving values were similar to those of the 5-min HR_driving values. There was a strong correlation between the two parameters among the drivers (Spearman rho = 0.77, p < 0.01). Figure 2 shows the distribution of the 5-min PMHR_driving values of each driver on the experimental driving. In 1 out of the 13 drivers, 7 of the 26 (26.9 %) 5-min PMHR_driving values were greater than his 30 % PMHR value (Fig. 2). The mean of the 5-min PMHR_driving values was 10.7 % (range, 5.7 to 19.9 %) among the 13 drivers. There were no significant differences in the mean PMHR_driving between the normotensive and hypertensive drivers: 11.3 and 7.6 %, respectively; between non-obese and obese drivers: 10.9 and 10.1 %, respectively; and between drivers who reported frequent and drivers who reported infrequent exercise: 10.5 and 10.8 %, respectively.

Fig. 2

Distribution of the 5-min percent maximum heart rate range (PMHR_driving) values for each of the 13 taxi drivers (T01 to T22) while driving on his 6-hr experimental on-road driving. The dotted line indicates the 30 % PMHR_driving value of T09

Limitations

This study has three main limitations. First, the sample size was small. Future studies with a larger sample of taxi drivers, assessing both worksite physical and psychosocial hazards, will be needed to confirm the findings of the current study. However, our sample of taxi drivers in Los Angeles were comparable with a larger sample (N = 309) of Los Angeles taxi drivers in the study by Wang and Delp [15]. The two samples were very comparable in terms of age, years of driving, race/ethnicity, work hours per week, and work stress level. Second, we had to estimate resting HR of each driver using the two-step process in the current study. It would have been a better approach to measure resting HR of each driver before starting the experimental on-road driving. Nonetheless, we think that our estimation process is better than the previous approaches by other investigators: assigning a single resting HR (60 bpm) to all male subjects [37] and/or identifying the lowest HR during work [28]. The former ignores the individual difference in rest HR. The latter may overestimate resting HR of each subject. Our two-step approach was an effort to overcome those disadvantages using the individual lowest HR_driving value and the age-and gender-specific resting HR value of US adult population. In addition, our results with HR_driving values that had no direct relationship with the estimated resting HR were also similar to those with PMHR_driving values based on the estimated resting HR values. Thus, we think the bias due to the use of estimated HR values, if any, would be minor in the current study. Rather, this study provides a good method for estimating the PMHR of workers at work when their resting HR values are not able to be measured or not available. Third, we conducted a sensitivity test to see whether the results in the current study would be affected by the status of hypertension, obesity, and exercise frequency among the 13 taxi drivers. It appeared that the means of HR_driving and PMHR_driving were similar whether the drivers were hypertensive or not, obese or not, and exercised frequently or not. However, it should be reminded that all information on hypertension, obesity, and exercise were self-reported in the current study and its sample size of the current study was small as a pilot study. Furthermore, the current study was not designed to investigate the potential differences in the HR_driving and PMHR_driving by the status of hypertension, obesity, and health-related behaviors (e.g., exercise, smoking, and alcohol consumption) among professional taxi drivers, which is one of the understudied topics among taxi drivers [14, 38] and working populations [39]. For the purpose, future studies are warranted in a larger sample of professional taxi drivers with objective measures of blood pressure, obesity, and physical activity.