Air quality in transportation cabins--Part II: air quality in a truck cabin.

ABSTRACT

A portable air quality monitoring system was developed to monitor the indoor air quality (IAQ) in a truck cabin that was used for in-city transportation. Temperature, relative humidity, and the levels of various airborne contaminants (CO, C[O.sub.2], N[O.sub.2], and particles) were measured in the truck cabin between August 16 and September 20, 2005. Temperature and relative humidity were found to be outside the comfort level recommended by ASHRAE. The concentrations of CO, C[O.sub.2], and N[O.sub.2] were acceptable, although the levels of CO and N[O.sub.2] were occasionally greater than the recommended standards. The particle number was higher than those in a typical office. Further analyses showed that the temperature and relative humidity in the truck cabin changed with the time of the day, while other IAQ parameters did not show obvious variation. Comparison with the field data in motorcars and airplanes showed that the concentrations of CO and N[O.sub.2] were higher in the truck cabin than in other transportation cabins.

INTRODUCTION

Trucking has become one of the most important industries in Canada (Transport Canada 2004). Trucking is the dominant mode of transport for both exports (53%) and imports (78%). Canada's exports to the US represented 85% of Canada's total exports to the world, and 62% of Canada/US trade was transported by trucks. In 2004, truck activity across the Canada/US border increased to 13.4 million two-way trips from 13.2 million in 2003.

The growth of the trucking industry has resulted in an increase in the risk of crashes involving trucks. Crashes involving commercial vehicles accounted for approximately 20% of all traffic fatalities and 10% of all serious injuries each year in Canada (Transport Canada 2002). Although the factors contributing to these crashes are complicated, driving while fatigued is widely accepted as one of the main contributing factors. Furthermore, since most long-distance drivers spend a large proportion of their time in truck cabins, it is hypothesized that the in-cabin air quality has a significant effect on their general health and well-being because the in-cabin air quality affects fatigue levels and quality of sleep. However, this hypothesis needs to be validated by a systematic study.

More importantly, concerns about the occupational health and safety of truck drivers are rising with more and more professional drivers joining the trucking industry. In Canada, there were over 260,000 truck drivers, according to the latest census (Transportation Canada 2004). Since many drivers spend most of their time working and sleeping in the truck cabins, the in-cabin air quality could have a significant impact on their health, well-being, and productivity.

The health effects of typical indoor air quality (IAQ) parameters in transportation cabins are reviewed in Part I of this paper. The effects of the relevant IAQ on truck drivers are briefly summarized. The temperature and the relative humidity (RH) level affect the comfort, health, and welfare of the truck occupants, and they also affect the dryness of their skin, eyes, and noses, as well as lung functions. Low humidity is also associated with fatigue, headaches, and nosebleeds. For the airborne contaminants, there is no threshold limit for the exposure to particulate matter. Coarse and fine particles are mainly responsible for respiratory diseases, while ultra-fine particles (<250 nm) and nanoparticles can penetrate through the lungs and enter the circulatory system, causing brain and heart diseases. Long-time exposure to carbon dioxide (C[O.sub.2]) levels of 1000 ppm or higher can cause drivers to feel dizzy and become less vigilant. CO poisoning has its most acute toxic effects on organs with high oxygen requirements, such as the heart and brain. As for nitrogen dioxide (N[O.sub.2]), it may damage the lungs at concentrations of 0.3 ppm or above. N[O.sub.2] also impairs vision and reduces brain function.

Despite its importance, data for the air quality in truck cabins are still very limited. As concluded in Part I of this paper, no relevant data have been found in the literature that are accessible to the public. It is necessary to quantify the IAQ in truck cabins to improve the understanding of a major occupational health and safety issue confronting truck drivers. It could also ensure a healthy and productive work force in the commercial transport sector and lower health care costs.

This paper reports the development of a portable IAQ monitoring system for real-time in-truck air quality monitoring. IAQ field measurement was carried out in the cabin of a truck traveling within the city of Calgary. The data reported here were collected in the summer of 2005 between August 16 and September 20. Data obtained have been compared with the existing standards or recommendations. They have also been compared with data for other types of transportation cabins that are reported in literature.

EXPERIMENTAL SETUP

A diagram of the IAQ monitoring system is shown in Figure 1. All the sensors with built-in computer interfaces were connected to the computer via a USB hub. In this research, commercial sensors were employed to measure temperature (T), RH, and the concentrations of CO, C[O.sub.2], N[O.sub.2], oxygen ([O.sub.2]), and airborne particulate matter (PM). The temperature and RH were measured using a platinum tip sensor and a capacitive sensor, respectively. The concentrations of CO, N[O.sub.2], and [O.sub.2] were measured using electrochemical sensors, and C[O.sub.2] concentrations were measured using an infrared sensor. The numbers of airborne particles were measured using a portable laser particle counter (LPC). It can count particles in the ranges of 0.3-0.5, 0.5-1, 1-5, 5-10, 10-25, and > 25 micrometers in geometric diameter.

[FIGURE 1 OMITTED]

Commercial software was used for coding, and all sensors were programmed to allow the data to be collected in real time. The sampling time and interval for each sensor could be adjusted by the user. The date and time were recorded using the computer clock. In this study, the samples were taken every ten minutes for particles and every two minutes for other IAQ parameters.

System Calibration 

All sensors were calibrated before application. The maximum errors of all the sensors were within the range of [+ or -]3.0%. The sensors were also maintained and calibrated regularly. Before deployment in the field, the system was set up and tried in the laboratory. After error probing and problem solving, the system was deemed to be running steadily and reliably. Data were then collected for four days in an office in a mechanically ventilated building (with all the windows sealed) for comparison purposes. It showed all the data were reasonable. The room temperature was well controlled but not the RH. The maximum concentration of C[O.sub.2] was 367 ppm, and there were negligible amounts of CO and N[O.sub.2]. The [O.sub.2] level was about 20.6%. The data obtained in the office were compared with those obtained in the truck cabin (see "Results and Discussion" section).

Field Conditions

During August 16 to September 20, 2005, the IAQ in the same truck was monitored using this system. The truck was used for deliveries within the city of Calgary in Alberta, Canada. Normally, only one person occupied the truck. The latitude, longitude, and elevation of Calgary are 51[degrees] 6' N, 114[degrees] 1' W, and 1084.10 m, respectively. In general, the air in Calgary is relatively cool and dry in summer. The ambient temperature and RH reported by the weather stations were in the ranges of -3.1[degrees]C ~ 32.3[degrees]C and 24% ~ 100%, respectively.

The IAQ monitoring system was housed in a cage and placed beside the front passenger seat. The sampling heads were above the knee level of the drivers (Figure 2). The operating power was supplied by the engine, and the built-in batteries were used as a backup supply. After the engine started, the system was turned on manually by the truck driver on duty. This system can only run continuously for about two weeks due to a limitation imposed by the memory of the laser particle counter (LPC). It should be noted that the behaviors of truck drivers were not monitored in order to collect the real-time data on IAQ without worrying about possible interference from the drivers operating the truck.

The HVAC system of the truck is illustrated in Figure 3. There is an eight-speed fan to control the speed of intake and recirculated air in the cabin and to force air through the outlets. The ventilation modes preferred by the truck drivers were of a natural manner (open windows) or the combination of a natural mode and an outside air intake mode. The airflow in the cabin is adjustable by moving louvers right and left and up and down. The air outlets include face outlets, floor outlets, defrost outlets, and outlets toward the doors.

Calm Air Sampling Criteria

Considering that the airflow in a typical truck cabin fluctuates and changes its directions, the calm air sampling criteria must be satisfied for accurate measurement of the numbers of the airborne particles (Zhang 2004). To be considered as calm air sampling, the free air velocity is recommended to be

[U.sub.0] < 0.1 [U.sub.S] [1/[St[k.sup.2/3]]], (1)

[FIGURE 2 OMITTED]

where [U.sub.0] is the free air velocity, [U.sub.s] is the sampling velocity, and Stk is the Stokes number of the particle of concern calculated using the condition at the inlet of the sampling nozzle. [U.sub.s] is determined by both the sampling flow rate and the inlet diameter of the sampling head. In order to minimize the sampling error introduced by the gravitational settling effect, the inlet diameter of the sampling nozzle has to be within the range defined by

0.4 ([Q.sub.s]/[pi][tau]g)[.sup.1/2] [greater than or equal to] [D.sub.S] [greater than or equal to] 10([[Q.sub.s][tau]]/4[pi])[.sup.1/3], (2)

[FIGURE 3 OMITTED]

where [Q.sub.s] is the sampling flow rate, [D.sub.s] is the inlet diameter of the sampling nozzle, and [tau] is the relaxation time of the particles of concern. The relaxation time is defined as

[tau] = [[[rho].sub.p][d.sub.p.sup.2]]/18[eta], (3)

where [[rho].sub.p], [d.sub.p], and [eta] are the density of the particles, the diameter of the particles, and the viscosity of the air, respectively. The density of the particles is assumed to be the standard value of 1000 kg/[m.sup.3].

Using the equations above, the calm air sampling criteria can be checked. For the LPC used in this study, the sampling flow rate is 28.3 L/min, the inlet diameter is about 25 mm, and the corresponding sampling velocity, [U.sub.s], is 0.667 m/s. The maximum free air velocity estimated using Equation 1 is 1.25 m/s. The free air velocity in the truck cabin was measured with the ventilation system operating at the maximum level, and the velocities at all the locations, including the outlet of the vent, measured in the truck cabin were less than 1.20 m/s. In addition, assuming normal conditions, Equation 2 is also satisfied for the particles with a diameter of 10 micrometers or smaller.

RESULTS AND DISCUSSION

Depending on the operating schedule of the truck, samples were taken continuously for 24 hours on some days but were only taken for several hours on other days due to truck inactivity and service and maintenance in the workshop. Since the external power supplied to the system was switched on and off as the truck started and stopped, extreme data were recorded right after the system was started. For example, occasionally the measured C[O.sub.2] concentration was greater than 18,000 ppm, but in other cases, the C[O.sub.2] concentrations were recorded as 0. Therefore, these extreme data were not considered valid for the analyses below. In total, about 1200 sets of valid data for particles and 5880 sets for each of the other IAQ parameters were used for the analyses herein.

The IAQ at Different Times of the Day

The values of the IAQ parameters against the time of the day are shown in Figure 4. It shows that the recorded temperatures were in the range of 10[degrees]C-40[degrees]C, with the majority of the readings lying between 15[degrees]C and 35[degrees]C (which were higher than the outdoor temperatures provided by Alberta Environment). The RH values were in the range of 15%-60%, with occasional high values of 60%-75% at night. The concentrations of CO were mostly below 4 ppm; however, occasionally it did reach as high as 14 ppm. The C[O.sub.2] concentration reached a peak of 1600 ppm, but the majority of the readings were below 800 ppm. Most of the measured concentrations of N[O.sub.2] were below 0.1 ppm, although there were noticeable times when N[O.sub.2] levels were above 0.2 ppm and, occasionally, as high as 0.8 ppm. As an indicator of the indoor air consumption, the concentration of [O.sub.2] was consistently recorded in the range of 20.2%-21%, which showed no depletion of [O.sub.2]. Therefore, it could be assumed that during most of the times when the samples were taken, the drivers did not continuously keep their doors, windows, and ventilation system closed. The numbers of the particles smaller than 10 microns and those greater than 0.5 microns are presented in Figures 4g and 4h, respectively. The numbers for particles smaller than 10 micrometers were in the range of 1.5-198 particles/cc. The particles greater than 0.5 micrometers were in the range of 0.22-114.4 particles/cc. For the particle number, there was no obvious variation over time.

The IAQ in the truck cabin was anticipated to change depending on the time of the day because the outdoor air quality along the road was expected to change with the changing traffic conditions. However, among all of the parameters, only the temperature and RH showed obvious variation along the time dimension. High temperatures and low RH were observed between 2:00 p.m. and 5:00 p.m. On the other hand, there were no strong variations for airborne contaminants inside the truck cabin. This could be explained by the fact that the temperature and RH outdoors changed with time everyday, but the outdoor air was relatively clean and fresh in Calgary. During the cool summer months in Calgary, many drivers open their windows, allowing the outside air in, even when the heating system was on at night. However, the situation could be different in winter when it is extremely cold in Canada, and drivers would keep their windows closed most of the time. Therefore, continuous measurement of IAQ in winter is recommended for a better understanding of the air quality in a truck cabin.

The correlations between the different IAQ parameters were also analyzed. There was a negative and strong correlation ([rho] = -0.8) between temperature and RH, but only moderate to low correlations (values below 0.3) existed between the other parameters. Therefore, it is recommended that the levels of multiple pollutants be monitored rather than using one of them as the IAQ indicator for the truck cabin.

Driver performance was not monitored during this period. Qualitative analyses could not be conducted to identify if the air quality in the cabin had an adverse effect on the drivers. Quantitative analyses, based on the levels of contaminants, were performed. Driver performance supervision is recommended in future studies.

Comparison with ASHRAE Standards and IAQ in Other Transportation Cabins

Table 1 summarizes a comparison of the measured data with existing standards and data measured in an office, passenger cars, and airplanes. ASHRAE standards are used for comparison. The office chosen in this study is located in a mechanically ventilated office building in the city of Calgary. Due to the limited availability of data on IAQ in transportation cabins, only the few studies that are publicly available in literature have been used in the comparison.

Comparison with ASHRAE Standards. ASHRAE (2001) recommends that the RH levels be maintained at 30%-60%. Temperature is recommended to be controlled within the range of 19.5[degrees]C-23[degrees]C in winter and 22.8[degrees]C-26[degrees]C in summer. The temperatures and RH levels measured in the truck cabin were outside of these recommended comfort levels. The excessively low or high temperature and humidity may result in low thermal comfort for the truck drivers.

[FIGURE 4 OMITTED]

There is no recommended standard with which our particle number can be compared, but as a reference, the number of particles larger than 0.5 microns is 10 particles/cc in a typical office room (Zhang 2004). The numbers of the identical particles measured in the truck cabin were much higher than this number (see Figure 4h), although this does not necessarily imply that the truck cabin was below standard.

Most of the concentrations of CO and N[O.sub.2] were found to be below the threshold values. Their concentrations were expected to be low during the summer in Calgary. Most truck drivers usually open the windows to use natural ventilation or use the vehicle's ventilation system to bring outdoor air into the truck cabin. The incoming outside air diluted the airborne pollutants inside the truck cabin. On the other hand, it is worth-while to mention that during certain times, these concentrations exceeded the standards recommended. This outcome may occur more often in winter when it is extremely cold outside. However, a quantitative conclusion cannot be made until more field data are available in the future.

Comparison with Data Obtained in the Office. The IAQ in the truck cabin was poorer than that in the office. The temperatures and RH values in the office were closer to the standards or guidelines. The concentrations of all the airborne pollutants monitored in the office (CO, C[O.sub.2], N[O.sub.2]) were lower than those in the truck cabin. This may be explained by the fact that the truck spent most of its time on or beside the street, whereas the inlet for the ventilation system of the office building was farther away from traffic. Airborne pollutants from the exhausts of the other automobiles on the roads had a greater chance to enter the truck cabin, whereas they were likely to be well diluted before entering the office building.

Comparison with Data in other Transportation Cabins. Data obtained in three cars and three airplanes were compared to those measured in the truck cabin. The temperatures in the cars and airplanes were mostly within the comfort level, which was different from those in the truck cabin. The air temperatures in the airplanes are usually well controlled via ventilation systems. But our short-haul truck drivers have to open and close their doors quite often when they are delivering to different places in the city, bringing the outdoor air into the truck cabin and resulting in variations in the temperature.

Regarding relative humidity, the RH values in the car measured by Riediker et al. (2003) were within the comfort range, generally in the range of 25.7% to 46%. However, the RH values in the truck and the airplanes were of a much wider range, lower than the recommended comfort level of 30%-60%. The outdoor air was cool but dry in Calgary during the period of our measurements, and this would be even more so for the intake air of the airplanes due to higher elevations.

The majority of the CO concentrations were in the range of 0-7.0 ppm for all of the data measured in the truck, cars, and airplanes. However, a peak CO concentration of 14.0 ppm was measured in the truck cabin. This reading is reasonable considering that CO is produced mainly from the fuel combustion process. The concentration of CO at the ground level should be much higher than in the air around an airplane, except when parking at the airport. It is likely, therefore, that truck drivers are exposed to higher CO concentration than occupants of airplanes.

In contrast to the CO results, the concentrations of C[O.sub.2] in the truck cabin were lower than in airplanes. The air pollutants within airplanes are mainly from the respiration of the occupants and penetration through the ventilation system. In most cases, the occupants in airplanes are crowded together and the ventilation may not be sufficient. Therefore, there was more C[O.sub.2] accumulated inside the airplane cabin, resulting in a higher C[O.sub.2] concentration.

The N[O.sub.2] concentrations in the truck cabin were higher than those in the cars (Chan and Chung 2003; Riediker et al. 2003) and airplanes. Like CO, N[O.sub.2] in a truck cabin is mainly from the incomplete fuel combustion, especially when the engines are idling and the recirculation mode ventilation is active. CO and N[O.sub.2] can accumulate inside the cabin when the truck is idling and the driver is using the ventilation mode of recirculation. The relatively higher concentrations of N[O.sub.2] in the truck cabin may also have come from the N[O.sub.2] penetration through the filtration system. On-road N[O.sub.2] concentration is very high and can reach 0.212 ppm (Riediker et al. 2003). This happens more likely during rush hour traffic.

The number of the particles (smaller than 10 micrometers) in the truck cabin was below 200 particles per cc. This magnitude seems to be much lower than that measured in the airplanes (428-75,369) (BRE 2003). However, these readings are not directly comparable due to the difference in the instrumentation used. The LPC used in this research can only detect particles greater than 0.3 micrometer (300 nm) and above, the number of which is negligible compared to particles that are smaller (Whitby 1978). More data in the numbers are needed for a better comparison.

CONCLUSIONS

A real-time portable IAQ monitoring system was developed, calibrated, and deployed in the field for the study of IAQ in a truck cabin. Data were collected from August 16 to September 20, 2005.

Overall, the air quality in the truck cabin was acceptable in the summertime. The numbers of the particles greater than 0.5 microns were in the range of 0.22 to 114.4 particles/cc. The concentrations of CO, C[O.sub.2], and N[O.sub.2] were within acceptable levels; however, the temperature and RH were outside the recommended comfort levels. The temperatures measured were in the range of 10[degrees]C-40[degrees]C, with the majority of the readings between 15[degrees]C and 35[degrees]C. The RH values were mainly in the range of 15%-60%, with occasional high values of 60%-75% at night. The concentrations of CO were mostly below 4 ppm; however, it did reach as high as 14 ppm. The C[O.sub.2] concentration reached a peak of 1600 ppm, but the majority of the readings were below 800 ppm. The majority of the measured concentrations of N[O.sub.2] were below 0.1 ppm, although occasionally it did reach as high as 0.8 ppm.

There was a significant negative correlation (a value of -0.8) between the measured temperature and RH, but only moderate to weak correlations (values below 0.3) existed between the other parameters. As expected, the temperature and RH varied with the time of day. High temperatures and low RH were observed between 2:00 p.m. and 5:00 p.m. However, there were no strong variations for airborne contaminants inside the truck cabin.

Comparison with the limited field data available for cars and airplanes showed that the temperatures in the cars and airplanes were mostly within the comfort level, but they were less so for those in the truck cabin. The particle number in the truck cabin was higher than in a typical office. The RH values obtained in the truck and the airplanes had a much wider range compared to the data obtained in cars. C[O.sub.2] concentrations were not dramatically different, but the concentrations of CO and N[O.sub.2] in the truck cabin were higher than those in airplanes.

Continuous measurement in truck cabins in the winter-time is recommended for future studies to provide a better understanding of the IAQ in truck cabins. During winter, the IAQ measurements could be vastly different due to the cold weather conditions in Canada and the resulting change in ventilation mode for the truck cabins.

ACKNOWLEDGMENTS

Support from the Centre for Transportation Engineering and Planning, Economy Carrier Limited, the Alberta Motor Association, and Alberta Environment is gratefully acknowledged. However, the views expressed by the authors do not necessarily reflect those of the organizations.

REFERENCES

ASHRAE. 2001. ANSI/ASHRAE Standard 62-2001, Ventilation for Acceptable Indoor Air Quality. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

BRE. 2003. Client report number 212034. Extending cabin air measurements to include older aircraft-for the UK Cross-Departmental Aviation Health Working Group (AHWG). Report number 212034. Building Research Establishment Ltd., Garston, Watford, UK.

Chan, A.T., and M.W. Chung. 2003. Indoor-outdoor air quality relationships in vehicle: Effect of driving environment and ventilation modes. Atmospheric Environment 37:3795-3808.

Lee, S.C., S. Lam, and F. Luk. 2000. Investigation of cabin air quality in commercial aircraft. Proceedings of Healthy Buildings 1:471-77.

Pierce, W.M., J.N. Janczewski, B. Rowthlisber, and M.G. Janczewski. 1999. Air quality on commercial aircraft. ASHRAE Journal 41(9):26-34.

Riediker, M., R. Williams, R. Devlin, T. Griggs, and P. Bromberg. 2003. Exposure to particulate matter, volatile organic compounds, and other air pollutants inside patrol cars. Environmental Science and Technology 37:2084-93.

Rodes, C., L. Sheldon, D. Whitaker, A. Clayton, K. Fitzgerald, and J. Flanagan. 1998. Measuring Concentrations of selected air pollutants inside California vehicles, Final report (950339). Sacramento, CA: California Air Resources Board.

Transport Canada. 2002. Transport Road Safety Vision 2010. Ottawa, Ontario, Canada.

Transport Canada. 2004. Transport Canada Transportation in Canada 2004. Ottawa, Ontario, Canada.

Whitby, K. 1978. The physical characteristics of sulphur aerosols. Atmospheric Environment 12:135-59.

Zhang, Y. 2004. Indoor Air Quality Engineering. Boca Raton, FL: CRC Press.

Zhongchao Tan, PhD

Associate Member ASHRAE

Xiaoying Cheng

Student Member ASHRAE

Richard Tay, PhD

Zhongchao Tan is an assistant professor and Xiaoying Cheng is a graduate student in the Department of Mechanical and Manufacturing Engineering, University of Calgary, Calgary, Alberta, Canada. Richard Tay is a professor in the Department of Civil Engineering, University of Calgary.

Table 1. Comparison of the Data Collected with Standards/Guidelines, an
Office, and Recent Field Measurements in Cars and Aircraft

                             T              RH         CO
                             ([degrees]C)   (%)        (ppm)
                             19.58-23(W)    30-60(W)   9
Standards/guidelines*        22.8-26(S)     40-60(S)   (8 hours ave.)
In truck                     11.1-40        14.9-71.8  0-14
(majority)                                  (18-5)     (0-6)
Office                       22-24          16-56      <0.1

Cars      Rodes et al.        -              -         <6
          (1988) ([dagger])
          Chan and Chung      -              -          0.9-1.4
          (2003)
          Riediker et al.    20.9-30.4      25.7-46     0.7-5.9
          (2003)
Aircraft  BRE                16.7-27.7       7.4-54.2  <0.1-1.3
          (2003)
          Lee et al.         17.8-29.8       4.9-76.8   1-6
          (2000)
          Pierce et al.      17.8-26.1       8.8-27.8  <0.1-7
          (1999)

                             C[O.sub.2]  N[O.sub.2]      Particle Count
                             (ppm)       (ppm)           (#/cc)
Standards/guidelines*        1000         0.2 hourly       -
In truck                      241-1625    0-0.8            0.2-114
(majority)                   (250-800)   (0-0.1)         (>0.5[micro]m)
Office                        267-367    <0.1              1-53

Cars      Rodes et al.          -         -                -
          (1988) ([dagger])
          Chan and Chung        -         0.08-0.23        -
          (2003)
          Riediker et al.       -         0.0016-0.212     -
          (2003)
Aircraft  BRE                 769-2219    0.02-0.07      428-75369
          (2003)
          Lee et al.          423-4752    0.0039-0.0138    -
          (2000)
          Pierce et al.       942-1959    -                -
          (1999)

* W = winter, S = summer.
([degger]) This research mainly focused on volatile organic compounds.