An air-conditioned coach is an important form of transportation for business and private activities in our modern motorized society; as a result, there are increasing concerns of vehicle-related pollution [1
]. In addition to the exterior gasoline-fueled engine exhaust [4
], mounting evidence indicates that interior organic pollutants are emitted from interior materials, such as plastic, paint, leather, synthetic fiber, adhesive, foam cushion, and so on [5
]. A study conducted to investigate the pollution of aromatic volatile organic compounds (VOCs) of different means of transportation suggested that the levels of toluene, ethylbenzene, and xylene were significantly higher in air-conditioned buses than those in other means of roadway transportation [6
]. VOCs are one of the most important categories of in-vehicle air contaminants; the most frequent VOCs include benzene, toluene, ethylbenzene, xylene, and styrene. Indeed, a recent research study in Thailand reported that the in-vehicle concentrations of benzene, toluene, ethylbenzene, and xylene were 11.7, 103.0, 11.7, and 42.8 μg/m3
, respectively, in air-conditioned buses, which were higher than the concentrations in electric sky trains and passenger boats [7
]. In addition to VOCs, several published studies suggested that carbonyl compounds (CCs), such as formaldehyde, acetaldehyde, acetone, propanal, and hexanal, also showed significant contributions to in-vehicle air pollution. For example, the results of a recent study assessed the concentrations of specific VOCs and CCs inside vehicle cabins under different vehicle driving conditions in China and indicated that the mean concentrations of formaldehyde, acetaldehyde, acetone, and acrolein were 16.43, 12.47, and 20.65 mg/m3
(the sum of acetone and acrolein). Moreover, this study indicated that the concentrations of these compounds inside new vehicles were higher than those inside old vehicles [8
]. Notably, however, previous studies primarily focused on the measurements in small passenger cars [4
], and there are not enough data concerning the mass concentrations of CCs in the cabins of medium- or large-size coaches.
Vehicle cabins are recognized as an important and special type of indoor environment. Currently, people spend increasing amounts of time in vehicles because of lengthy commutes, long-distance travelling, and frequent traffic jams, leading to ubiquitous exposure to in-vehicle air pollutants. Moreover, the vehicle cabin is a relatively confined space with a high amount of interior materials; as a result, the in-vehicle concentrations of some organic compounds are significantly higher than those in other indoor environments or in the corresponding ambient air [9
]. Exposure to in-vehicle air contaminants may exacerbate allergy and asthma symptoms, or may cause nose, throat or skin irritation, coughing, headaches, or general flu-like illnesses, and can even cause cancer and neurological effects [13
]. For example, a study conducted to evaluate the health risk of in-car benzene homologues reported that in-car benzene exposure could lead to an increased risk of cancer in taxi drivers, with the average value of cancer indices being 1.21 times higher than the unacceptable value of carcinogenic risk recommended by the United States Environmental Protection Agency (USEPA) [17
In China, in-vehicle air pollution has received extensive attention over the past decade. A study determining the types and quantities of VOCs in both new and old cars suggested that toluene, xylene, aromatic compounds, and various C7-C12 alkanes were the primary pollutants, and the highest concentration of total VOCs was demonstrated to be up to 4.94 mg/m3
inside new cars [18
]. A similar investigation of large numbers (802) of cars also indicated that the highest concentrations of toluene, xylene, and benzene inside cars were 81, 18, and 16 times higher, respectively, than their corresponding limit levels according to the Chinese National Indoor Air Quality Standard (GB/T18883-2002); moreover, the study suggested that newer vehicles had higher concentrations of tested pollutants than older vehicles [11
]. Based on the basic understanding of in-vehicle air pollution provided by these studies, the Chinese government had issued the “Vehicle Cabin VOCs and CCs Testing Methods (HJ/T 400-2007)” as guidelines for conducting the necessary measurements of in-vehicle organic pollution. Subsequently, the Guideline of Air Quality Assessment of Passenger Cars (GB/T27630-2011) was established in 2012 to help control the levels of VOCs and CCs in the cabins of small passenger cars. However, guidelines and regulations that restrict the levels of hazardous air pollutants in medium- or/and large-size coaches have not yet been issued. A complete and accurate estimation of the concentrations of air contaminants in the cabins of coaches is a key step in the development of in-coach air quality assessment and control strategies. Accordingly, the present study is aimed at evaluating the levels of both VOCs and CCs in the cabins of newly produced, medium- and large-size, air-conditioned coaches. In addition, the effects of the time after assembly line completion, the in-vehicle temperature, and the interior relative humidity of the coaches on the levels of VOCs and CCs are also discussed in this study.
Air pollution is a well-known public health risk and is considered an important contributor to the global disease burden [19
]. Although the potential effects of air pollution have generally been assigned to exposure in residential locations, more studies have demonstrated that in-vehicle exposure to several air pollutants (such as VOCs and CCs) have some adverse impacts on drivers and passengers during their daily commutes. Yoshida [10
] detected a total of 162 organic compounds (aliphatic hydrocarbons and aromatic hydrocarbons), and the data showed that higher concentrations of C9–C13 alkanes, ethylbenzene, and xylene were detected in the interior air of a new car. In addition, this study suggested that CCs, except for formaldehyde, appeared to be unimportant as interior contaminants. Similarly, You [18
] found that toluene, xylene, some aromatic compounds, and various C7–C12 alkanes were the predominant VOCs in new vehicles, and Geiss [9
] reported that aromatic compounds (e.g., benzene, toluene, ethylbenzene, and isomers of xylene), dodecane, and low molecular weight CCs (e.g., formaldehyde, acetaldehyde, and acetone) were the most abundant organic compounds present in the cabin air of used private cars. Considering that there are not enough data concerning the concentration and composition of CCs in air-conditioned coaches, our study identified and quantified fifteen CCs and seven typical VOCs potentially present in the cabins of medium- and large-size air-conditioned coaches. In accordance with previous results, we confirmed that toluene, ethylbenzene, xylene, formaldehyde, acetaldehyde, acrolein/acetone, and isovaleraldehyde were relatively abundant in the cabins of the tested coaches. In contrast, the emissions of benzene were found to be relatively lower in the tested coaches throughout the monitoring period, ranging from 19.7 to 28.0 µg/m3
. Among the tested VOCs, benzene has been classified as a well-known carcinogen (Group 1) by the International Agency for Research on Cancer (IARC) [20
], and no safe level of exposure has been recommended [21
]. Therefore, adequate attention should be paid to in-vehicle benzene pollution, and further study is required to assess the relationship between the exposure to air pollutants in the air-conditioned coaches and the related health risks for drivers and passengers.
In-vehicle air contaminant levels can be influenced by the mode of transportation. Lau first reported that toluene, ethylbenzene, and xylene levels were significantly higher in air-conditioned buses than those in other forms of roadway transportation, e.g., trams, taxis, non-air-conditioned buses [6
]. Mounting research results indicate that the airborne pollution in the cabins of coaches is an important issue. A recent study that surveyed the interior air pollutants in the public buses of Changsha, China, showed that the maximum concentrations of benzene, toluene, ethylbenzene, and xylenes were 106.4, 266.0, 95.9, and 234.8 μg/m3
, respectively, in the cabins of buses [22
]. Moreover, according to the analysis of multiple linear regression equations, this study suggested that bus age and interior temperature were the two most important factors influencing the in-vehicle concentrations of VOCs. Another air pollution survey of ten selected monoaromatic hydrocarbons conducted in Hangzhou, China, reported that the mean concentration of these organic compounds inside buses was 95.9 μg/m3
, and suggested that the dominant source was vehicle emissions. Importantly, this study indicated that the mean lifetime carcinogenic risks of monoaromatic hydrocarbons for passengers and drivers were above the limit set by the USEPA [23
]. Similarly, a study examining the concentration of VOCs in the public buses of Pamplona, Northern Spain, also confirmed traffic (such as driving routes and commuting periods) as the main emission source for some VOCs, such as benzene, toluene, ethylbenzene, and xylene [12
]. In addition, the in-vehicle carbonyls concentrations were found to be closely associated with the vehicular service years and the fuel used in public buses [24
]. Although the above-described studies mainly concentrated on the chemicals originating from the vehicular exhaust emission and the infiltration of outdoor air, interior decorations were one of the dominant sources of VOCs and CCs in the cabins of buses, especially in the newly produced buses. However, it is suggested that not much work has been performed regarding new large vehicles. To address this gap in knowledge, our study identified and quantified the levels of VOCs and CCs in air samples collected from the cabins of newly produced, medium- and large-size coaches.
In a used vehicle, the concentrations of some organic compounds may increase due to the combustion of fuel and tail pipe emissions; however, for a newly produced vehicle, the air contaminants primarily originated from the outgassing of interior materials, including plastic, paint, leather, synthetic fiber, adhesive, and foam cushion. Although many materials are subjected to emission tests by the producers before use and the results are often acceptable, the number and variety of interior materials used together in the cabin can still cause significant and undesirable impacts on the in-vehicle air quality. Faber [1
] examined the air composition in the cabins of new cars of the same model, but comprised of different interior materials, and found that the total VOCs concentration ranged from 1.5 to 2.1 mg/m3
in the tested vehicles and that approximately 200 different organic compounds were detected in each vehicle cabin. Brodzik et al.
] also investigated the air composition in nine new cars of the same model but with different interior materials; their study also indicated that the different materials used to equip a vehicle’s cabin obviously affected the type of the organic compounds emissions. Our study focused on the interior air quality of newly produced vehicles in a static condition. During the 0th-day monitoring period, we found that the mean concentrations of toluene, ethylbenzene, and xylene were 0.32, 1.49, and 1.22 mg/m3
, respectively, in the medium-size coaches and were 1.63, 2.45, and 1.58 mg/m3
, respectively, in the large-size coaches; these results were significantly higher than the data of small passenger cars measured within one week after manufacture [1
]. The main reason for these differences might be the significantly larger numbers of materials used for the interior decorations and furniture in the coaches. The sizes of the medium- and large-coaches were 90 × 25 × 34 m3
and 120 × 25 × 37 m3
, respectively. A larger interior surface area corresponds to an increasing number of interior materials, possibly resulting in the emission of more pollutants. In addition, the differences in interior equipment also affected the concentrations of the organic compounds. For example, the large numbers of materials found in the seats of coaches, such as poly-urethane (PU) foam, poly-propylene (PP) plastic, poly-vinyl chloride (PVC) plastic, acrylonitrile butadiene styrene (ABS) resin, and synthetic leather, emit high levels of toluene, ethylbenzene, and xylene [27
]. A research report indicated that cars with a fabric trim generally had higher VOCs levels than those with a leather interior [28
]. Hence, a large number of fabric seats, curtains, and plastic floor boards equipped in the tested large-size coaches might account (at least in part) for the relatively higher levels of VOCs emissions in the cabins of coaches than those in the cabins of small cars. Moreover, all of these factors might also help to explain the higher levels of most VOCs detected in the large-size coaches with fabric seats than those in the medium-size coaches with synthetic leather seats. The abundant usage of interior trims, such as roof linings, surface coatings, TV moldings, paints, adhesives, sealants, and lubricants (for the seat mechanism), may also result in the emission of VOCs. The adhesives, which were used to attach various interior parts, were found to be one of the major sources of VOCs; our findings suggest that the extensive use of adhesives, also partially explains why the levels of VOCs found in the cabin of a vehicle were higher but were undetected in the emission tests of interior parts before use. Regarding CCs, our findings suggest that particle board or plywood furniture containing formaldehyde-based resins, paints, or leather and adhesives with formaldehyde used for plastic surfaces, were the most important interior sources. Furthermore, considering that leather is known for its particularly high levels of CCs emissions, we speculate that the higher levels of CCs detected in the medium-size coaches might be merely due to the use of passenger seats covered with synthetic leather. Alternatively, we speculate that the usage of interior parts with different manufacturing dates could also result in different in-vehicle organic compounds concentrations. Further research studies should be performed to confirm the exact sources of the various organic pollutants present in the cabins of coaches.
It is generally accepted that the in-vehicle concentrations of organic compounds decrease significantly as time goes on. A used vehicle will have lower levels of air contaminants than a new one because of the ventilation of the interior after manufacturing. Yoshida [10
] reported that the total concentrations of in-vehicle VOCs were approximately 14 mg/m3
on the day of delivery and that the levels of most compounds decreased with time (over 3 years after delivery). Recently, Faber [26
] also reported that the concentrations of benzene, toluene, o
-xylene, and m
-xylene in the new cars tested were 11.8, 82.7, 21.2, and 89.5 μg/m3
, respectively, and these values were approximately ten times higher than those in the used vehicles tested. Moreover, this study indicated that the concentrations of toluene and isomers of xylene increased with the increasing mileage, but they did not exceed the initial concentrations. In our study, the same coaches were tested at different times after manufacture; we found that the in-vehicle concentrations of most organic compounds were significantly higher during the 15th-day monitoring period than those detected for both the 0th- and 30th-day monitoring periods. These results suggested that the emission of organic pollutants from the interior materials into the air of the cabins of the tested coaches would take a certain amount of time and that these contaminants would accumulated in the cabins for a period of time. In addition, the results indicate that the discharge of organic pollutants from the interior materials in the tested coaches undergoes two major phases: a rapid release period and a relatively steady stage. From the 0th- to the 15th-day, a large amount of organic contaminants were quickly emitted from a wide range of interior materials, thereby significantly increasing the in-vehicle concentrations. Subsequently, a “steady state” might be achieved. Moreover, 6 h of air exchange before sampling could further reduce the in-vehicle organic compounds to lower levels. All of these factors explain the approximately inverted U-shaped emission pattern for most VOCs and CCs (toluene, ethylbenzene, xylene, styrene, formaldehyde, acetaldehyde, acrolein/acetone, and isovaleraldehyde) observed in the tested coaches. Thus, we recommend that consumers should purchase air-conditioned coaches from manufacturers after the coaches have been off the assembly line for a certain time to ensure that the air pollutants of the interior materials have been volatilized as much as possible. Notably, however, despite the ability of such measures to reduce the concentrations of several in-vehicle organic compounds to lower levels, complete elimination of the emissions of the organic compounds might be impossible. Alternatively, technologies used to purify the air, such as adsorption and catalytic elimination of air contaminants, could be used in the vehicular interior environment to decrease or prevent adverse health risks.
The interior temperature appears to be an especially important factor influencing VOCs emission [29
], because the increased temperature inside a car cabin may promote increased vaporization and outgassing of various volatile compounds from the interior materials. Indeed, we found that the concentrations of some organic compounds were increased between 20 °C and 35 °C, but the main compositions of the air pollutants present in the cabins of the tested coaches did not change at each temperature considered. The most significant concentration growth with increasing interior temperature was observed for toluene, ethylbenzene, and xylene. Total VOCs concentrations in the air sampled from the cabins of the tested coaches were also increased with an increasing interior temperature. During summer days, the temperature in the vehicle cabin may exceed 65 °C [31
]; such a high temperature would encourage a substantial emission of VOCs from the interior materials. The use of air conditioning could decrease the interior temperature and reduce the emission of some organic compounds from the interior materials; however, interference from ambient air intrusion is also possible. Indeed, a previous study comparing the levels of benzene, toluene, ethylbenzene, and xylene in air-conditioned and non-air-conditioned buses showed that the levels of these tested compounds in air-conditioned buses were 59.3%, 59.1%, 60.1%, and 60% greater than those in non-air-conditioned ones, respectively [22
]. In contrast, another study examined the exposure levels of traffic-related VOCs in four popular public commuting modes and showed that there was no significant difference between air-conditioned buses (13.5 μg/m3
) and non-air-conditioned buses (11.3 μg/m3
]. Accordingly, the effect of air conditioning on interior air pollution warrants further investigation; we recommend that the natural ventilation of air-conditioned coaches must be further increased for an appropriate period while the coaches are running. Interior relative humidity (RH) is another important factor that influences in-vehicle air quality. Several CCs, such as formaldehyde, acetaldehyde, acrolein/acetone, and isovaleraldehyde, were found to exhibit higher concentrations when the interior RH was greater than 60%. However, unlike the temperature effect, a significant positive correlation was observed between RH and the concentrations of toluene in the cabins of the tested air-conditioned coaches. As noted above, we recommend that the interior temperature and RH should be controlled at appropriate levels, especially when a new air-conditioned coach is running.
In 2012, the Guideline of Air Quality Assessment of Passenger cars (GB/T 27630-2011) was issued in China, which limited the concentrations of benzene, toluene, ethylbenzene, xylene, styrene, formaldehyde, acetaldehyde, and propionaldehyde in the cabins of newly produced cars. However, our study showed that the sum of target VOCs was observed at an average concentration of 6.32 mg/m3, with a minimum of 1.50 mg/m3 and a maximum of 14.03 mg/m3; moreover, the emissions of all tested VOCs, except benzene, during the 15th-day monitoring period, exceeded the established values of the GB/T 27630-2011 Guidelines. Although the levels of the identified CCs were significantly lower than those of the VOCs, the emissions of formaldehyde, acetaldehyde, and acrolein/acetone exceeded the GB/T 27630-2011 Guideline values during both the 15th- and 30th-day monitoring periods. All of these results implied that the GB/T 27630-2011 Guidelines could not be applied to evaluate the air quality in air-conditioned coaches. The data from our study will be helpful to further understand the air quality in the cabins of coaches and could contribute to the development of both specific and general rules to apply to medium- and large-size coaches; such rules have not been enforced to date, despite an evident need for them.