In-Vehicle Exposure to Ultrafine Particles While Driving through a Tunnel System and Associated Lung Deposition Calculations

Tunnels serve as enclosed particle loaded environments where mostly stagnant and stable abiotic conditions prevail throughout the year. In-vehicle exposure to ultra-fine particles (UFP) is therefore significant while driving through long tunnels. The objective of this study was to assess exposure to UFP while driving a car through a 3.4 km long highway tunnel system in Austria and to calculate the associated lung deposition. On board a mobile Scanning Mobility Particle Sizer (SMPS, Grimm model 5403) was used to monitor the particle number concentration in the size range of 5.5 nm to 350 nm. The influence of various air ventilation settings inside the passenger cabin of the car was investigated in terms of UFP concentrations. These ventilation settings and hence the associated particle concentrations inside the cabin were used in the lung deposition model IDEAL to determine their fate in the lung. To examine a real life situation, this heavily frequented highway tunnel system was chosen to serve as a particle laden environment. The data were obtained over several days throughout the year to analyze the effect of traffic mix on the resulting particle regime. Our study revealed a decrease of in-cabin air pollution by up to 88% while using recirculating air (RA) settings in combination with the air-conditioning system. This mode of operation in comparison to plain tunnel air (TA) reduced the corresponding lung deposition by up to 95%. Our observations thus suggest making use of recirculating air setting while driving through poorly ventilated areas such as tunnels. Doing so considerably reduces the resultant particle deposition in the lung.


INTRODUCTION
The increase in urbanization either by migration or natural growth has resulted in tremendous increase in traffic volume and hence in air pollution.The increasing demand for mobility due to rapid economic growth has compelled transport planners to build road tunnels as means to increase traffic mobility.At present, the overall length for operational transportation tunnels throughout Europe is well in excess of 15,000 km (CEMBUREAU, 2004).The high traffic densities on European roads result in elevated Ultrafine Particle (UFP) concentrations particularly in semi-enclosed environment such as tunnels.Hence, invehicle exposure to UFP becomes significant in the stagnant and stable abiotic conditions of the tunnels having limited modes of ventilation (Zhu et al., 2007;Fruin et al., 2008; While circulating in the body, they become a burden to the immune system as it tries to remove or neutralize these xenobiotic components.It has been proposed that alveolar deposition is associated with increased cardio-circulatory problems, as the immune response is the primary system to remove entrapped UFPs in that region of the lung (Donaldson et al., 1998).A study by Rudell et al. (1999) showed that exposure to diesel exhaust induces neutrophil and alveolar macrophage recruitment into the airways and suppress alveolar macrophage function.In addition, exposure studies in healthy humans have shown a number of inflammatory changes in the airways and even before changes in pulmonary function can be detected (Sydbom et al., 2001).Especially vulnerable are those individuals suffering from respiratory-related complications.Patients with Chronic Pulmonary Diseases (COPD) for example, tend to show exacerbated health conditions upon exposure to elevated short-term UFP-concentrations (Arbex et al., 2009).This is consistent with epidemiological observations of Hunt et al. (2003) and Stenfors (2004) who showed that even a small increase in UFP concentrations may cause significant health effects.
Different ventilation settings inside an operating vehicle have been found to influence the mechanisms of particle mixing and hence lung deposition (Huda et al., 2011).Although particle filters mounted into the ventilation system of vehicles, depending on the particle size, offer an in-cabin protection up to 50%.However, depending on filtration efficiency, large variations in exposure inside vehicles are expected to occur because of the enormous inside-vs-outside UFP gradient (Hudda et al., 2011).
Since UFP can efficiently deposit in the respiratory tract and exposure to these particles may result in far-reaching adverse health effects, investigations to quantify actual particle penetration, retention and translocation across the body is of up-most priority.Lung deposition calculations is thus a key tool to investigate how deep particles can penetrate into the lung and what is their ultimate fate.Owing to inhomogeneous deposition of inhaled particles in the lung, UFP in contrast to fine particles seem to follow different routes in the organism (Kreyling et al., 2006).The very accurate IDEAL-code used here enables precise modelling of pulmonary deposition in both number concentrations as well as in mass (Hofmann et al., 2005(Hofmann et al., , 2006)).Hence the current study was carried out with the intent to determine UFP concentrations inside the microenvironment of a car while driving through tunnels under various ventilation settings.In addition, corresponding lung deposition calculations were performed for the measured UFP concentrations under various ventilation settings.

MATERIALS AND METHODS
This study investigates the effects of various ventilation setting on in-vehicle UFP concentrations and associated lung deposition using a portable Scanning Mobility Particle Sizer (SMPS Model 5403) while traveling through a tunnel system.The following sections describe the methodology used in data collection and associated lung deposition calculations.

Site Description
The measurements were taken while driving through a 3.4 km long highway tunnel consisting of two separate sections (Ofenauer Tunnel 1.4 km, Hiefler Tunnel 2.0 km) near Salzburg, Austria (Fig. 1).These tunnels are one of the busiest tunnels in Austria with daily traffic flux of approx.36,000 vehicles (for both north-and south-bound directions).The samples were taken in both directions of the tunnels.Since urban areas are subject to elevated aerosol production and dispersal (Dorsey et al., 2002), this type of environment was chosen to serve as an enclosed particle loaded environment, where mostly stagnant and stable abiotic conditions exist throughout the year.

Sampling Strategy
The measurements were taken on several days by including both weekdays and weekends during the months of June, August and December, 2011.All samples were taken in the afternoon, starting between 1400 and 1500 hrs.with each sampling cycle lasting for 3 to 4 hours.On weekend, circulation of Heavy Duty Vehicles (HDV) is limited by law on Austrian highways.Hence it was of interest whether this would significantly reduce the particle load for passengers inside a car or whether any reduction in particles could be associated to a difference in traffic mix.Elaborated statistical traffic data for each sampling cycle have been obtained from the Austrian Traffic Authorities (ASFINAG), showing exact traffic volume, which is based on weighting factors for various vehicle types.ASFINAG uses a sophisticated network of remote sensing devices that measure traffic density, vehicle type, average speed, and weather conditions.Induction loops implemented into the substratum enable discrimination between heavy (HDV) and Light Duty Vehicles (LDV), passenger cars, trailers and motorcycles.
The sampling campaign was carried out using a biodieselfuelled BMW 330 D Touring (2001 model).On-road tests were carried out using the in-cabin ventilation system operated in three different modes: (1) air from outside routed through the filter into the cabin [ventilation air, VA], (2) re-circulating air within the cabin [re-circulated air, RCA] and (3) recirculating mode coupled with the car's own air-conditioning system [RCA/AC].All ventilation settings have been applied using a medium-strength fan setting to create a slightly elevated cabin pressure.In addition, and as a reference, the tunnel air [TA] was sampled separately.For each setting, at least four repetitions were made and all settings were used during each sampling day, beginning with TA, VA, RCA and RCA/AC.In-between transition from one ventilation setting to the next and to avoid any accumulating effects, the cabin was flushed with fresh air at an isolated place, several minutes' drive away off the highway.TA samples were taken by parking the car in one of the emergency park bays available within the tunnel.
During measurements, an average speed of 60 km/h was maintained throughout the tunnel against the speed limit of 80 km/h.Sampling mode of the SMPS was set to "fast scanning" mode, enabling a single measurement cycle over all size classes while driving through the tunnel system.

Instrumentation
For all measurements, a mobile Scanning Mobility Particle Sizer (SMPS, Grimm, model 5403) was used to monitor the particle size inventory ranging from 5.5 nm to 350 nm.The SMPS is a continuous nano-particle counter, which combines a CPC (Condensation Particle Counter) attached to a mid-sized DMA (Differential Mobility Analyzer).It enables determination of particle size distributions within the scanned spectrum over a wide concentration range of up to 10 7 particles/cm 3 .This system is suitable for both stationary as well as mobile sampling.The SMPS was placed on the back seat of the car on a solid shock absorbing base.For convenience, the ventilation system was set on medium fanning strength to obtain the desired ventilation effect.The SMPS' sampling hose was fixed above the armrest between the driver's and the passenger's seat, allowing it to collect air from the middle of the cabin at neck height.In vehicle temperature and relative humidity were measured simultaneously with a portable Testoterm 6010 sensor kit to document the temperature drop of 3-5°C during application of the AC-mode.
The built-in particle filter-class of the vehicle used (BMWsedan 330D Touring) corresponds to allergy standards (i.e., pollen filtering efficiency category), however, the vehicle's user manual did not provide any detailed filtering efficiency data for a given particle size range below 300 nm.Nonetheless, dust loading test conditions according to Cabin Air Filter Test Rig DIN 71460-1 (ISO11155-1,) denotes a particle filtration efficiency for passenger cars of at least 90% or higher for particle size ≥ 300 nm and at least 60% or higher for particle size ≥ 100 nm respectively (VDA, 2007;ISO, 2001).Lab test verification was performed to determine the actual efficiency of the built-in particle filter.Over a period of 120 minutes, the particle distribution of an exhaust air sample was measured.For SMPS measurements, such a setup enables dual operation in which the UFPladen air was either routed through the filter-capsule or in bypass mode.The filter efficiency for each particle size range was determined by using the following equation: (1)

Lung Deposition Calculations
To model the fate of inhaled UFP, the stochastic lung deposition code "Inhalation, Deposition and Exhalation of Aerosols in the Lung (IDEAL)" originally developed by Koblinger and Hofmann (1990) and Hofmann and Koblinger (1990) was used in its latest version.In this model, the geometry of the airways along the path of an inhaled particle is selected randomly by Monte Carlo methods, whereas deposition probabilities are computed by deterministic formulae.This deposition model enables the computation of total, regional (bronchial and alveolar), and differential (individual airway generations) deposition in the adult human lung.The bronchial region corresponds to generations 1-16, whereas the alveolar region corresponds to generations 17-27.For the calculations, data resulting from the SMPS were grouped into 44 size channels, each being determined as a specific fraction of the particle load.This resulted in fractional probability distributions of deposited particles, which was then applied to the actual particle count to obtain the probability of particle deposition.The model was set to run for a human adult under sitting breathing conditions with a tidal volume of 750 cm 3 and a total lung capacity of 3.3 L considering a breathing frequency of 5 seconds (inhale/exhale) was considered.Apart from lung deposition fractions over the various lung generations, the code also calculates nasal deposition.

RESULTS
During this investigation a significant reduction in UFPs routed into the passenger cabin was found as compared to the ambient particle load for various ventilation settings.Even though traffic mix varied throughout the measurement periods, no significant difference was observed in the total particle concentration measurements between weekdays and weekends or between summer and winter days.Fig. 2 highlights the traffic densities during the measurement timeslots of various sampling days.The seasonal variations become indistinct, as vehicle densities are subject to fluctuations related to prevalent holiday-periods during the months of August and December.While HDV-densities peaked with 48.8% of the overall traffic volume (see Table 1), the observed drop during the other days was related to the contributions of automobiles travelling to and from holiday destinations during weekends.UFP concentrations positively correlated with traffic flow with relatively high contribution from HDV.On weekends, HDVs seem to be more frequent than Passenger Cars (PC) but this attributable due to the higher portion of vans, trailers and busses (Fig. 2), thereby compensating the decline in overall UFP concentrations.Similarly, no significant difference was observed in aerosol concentration in both directions of the tunnel.
The ventilation system set to route outside air through the built-in filter into the passenger cabin (VA setting), revealed a filtering effect up to 60% compared to the overall particle concentrations in ambient tunnel air (TA).This could be observed for all scans performed while driving through the tunnel systems.An even better particle reduction inside the passenger cabin was obtained by applying the circulated air (RCA) setting.Activating the air conditioning system in addition to the RCA-setting (RCA/AC) resulted in a further reduction of nano-particle inside the cabin (Fig. 3).

Particle Number Concentration
The particle size spectrum observed in the tunnel air (TA) revealed a high amount of nano-particles in the lower size regimes. .The average TA concentrations measured to be 3.0 × 10 5 particles/cm 3 were comparable with the finding of Knibbs et al. (2011).Here, the standard ventilation setting (VA) that routs outside air via the particle filter into the cabin provides a significant reduction in particle burden.Obviously, the RCA setting filters up to 80% of the UFP (within the size range of 5 to 40 nm) of the particle enriched tunnel air.This filtration effect is further enhanced (up to 3036 74.9 2641 53.7 3878 91.3 2337 51.2 7652 92.0 7753 91.8 4599 92.6 6832 92.9 HDV 2 1017 25.1 2279 46.3 369 8.7 2231 48.8 661 8.0 689 8.2 370 7.4 525 7.1 ( 1 ) PC: passenger cars consisting of motorcycles, automobiles and vans.
( 2 ) HDV: consisting of passenger cars w/trailer, trucks, light duty vehicles w/trailer and buses.Fig. 3. Cumulative total particle concentration at various ventilation settings for all sampling cycles.TA (tunnel air), VA (outside air fanned through the built-in filter into the cabin), RCA (re-circulating air within cabin), RCA/AC (re-circulating air combined with the car's built-in air conditioning system).90%) with the increased particle size range (10 nm to 60 nm).(Fig. 4).Applying the RCA/AC setting reduced the UFP burden even further i.e., up to 88% of the measured UFP as compared to TA in the same particles size range.These results are comparable to recent findings of Tartakovsky et al. (2012) in which they reported 95-99% reduction in the measured UFP number concentration using a portable air purifier inside their vehicle.This phenomenon is most likely attributed to increasing filter efficiency of the built-in filter especially for the larger particles.Since this effect was observed on all sampling days and a reduction of particle concentration at the RCA/AC setting exists for all test cycles, it is assumed that some influence of the air conditioning system, i.e., forced agglomeration due to cooled air may be present.

Filtration Efficiency
Lab-test analysis of the vehicle's built-in filter efficiency yielded an average value of 60% (Fig. 5).In-cabin protection up to 171.7 nm particles varied between 73.7-58.1%,but dropped to 17.7% at that threshold diameter.These results are comparable to the findings published by Qi et al. (2008).Zhu et al. (2007) reported filter efficiencies ranging from 20% to 50% (50% for particles in the 7-40 nm size range and 20-30% for particles in the 40 to approximately 200 nm).These data suggest that there is generally similar protection over these size ranges; however each type of filter needs to be evaluated separately to determine its real characteristics.

Lung Deposition
Corresponding to the applied ventilation setting, lung deposition was calculated by using the sampled particle inventory.For all ventilation settings, deposition calculations revealed a deposition peak past the 17 th lung generation.Regardless of the employed ventilation modes, the model revealed that nasal deposition oscillated between 18-22% of the measured UFP concentration, leaving the remaining particle concentrations to model their fate within the respiratory tract.For TA and VA settings, deposition peaks occurred around generation 17 with averages around 17285 and 3729 particles/cm 3 respectively.Deposition peaks for RCA and RCA/AC settings occurred at generation 18 with averages of 912 and 828 particles/cm 3 , respectively.Hence, an overall 95% decrease in deposition fraction may take place when combining RCA with AC ventilation settings.In each case, modelling results underline prevalent alveolar deposition-for simplicity we follow Yeh and Schum's (1980) threshold definition where bronchiolar regimes fade over to the alveolar region around 17 th generation -which is typical for predominantly hydrophobic particles such as medium aged urban traffic exhausts.Fig. 6 shows the particle  deposition pattern (starting with TA, over VA, RCA, etc.) among all lung generations.It can be observed that the simple ventilation setting (VA), routing tunnel air through the built-in filter into the passenger compartment, considerably reduces particles deposition in the lung.Yet optimal reduction is achieved by applying a combination of RCA and AC settings.
The effectiveness of the RCA setting is clearly visible in Fig. 6.A closer look at both the RCA and RCA/AC setting reveals an even lower particle deposition when the airconditioning system is activated (Fig. 7).Although the RCA/AC deposition pattern closely follows that of the RCA and seems not to differ substantially, the former tends to be more efficient in removing UFPs -a fact most likely attributable to the condensation and agglomeration effects induced by cooling air.In Fig. 8, the particle lung deposition ratios from tunnel air (TA) versus simple ventilation (VA), and recirculated air (RCA) versus VA are shown for each lung generation.Here the reduction of particles per lung generation is significant -especially among higher generations.Both ratios are around 20% from generation 1 to 16.For the alveolar region, this effect decreases in both cases.While the larger particles of the agglomeration mode efficiently deposit in the upper generations i.e., up to generation 16, the UFPs readily penetrate the alveolar region.
Deposition of UFPs during a single breath, revealed the actual particle load on passengers during exposure while driving through this tunnel system.Fig. 9 depicts the modelled deposition data for a single breath.Similar to Figs. 6 and 7, deposition of UFPs for the TA-and VA-setting peaks at generation 17 with a total particle load of 307,466 and 57,171 per breath, respectively.The corresponding RCA and RCA/AC-sets however reveal a shift towards the 18-19 th generation with 12,705 and 10,455 deposited particles.Similar deposition patterns for single breaths were observed for all sampling days.This reduction follows the observed pattern from the deposition model as displayed in Figs.6-8.Both TA and VA settings do not provide a significant reduction in particle deposition as could be obtained with RCA and RCA/AC settings.

DISCUSSION
The high concentrations of UFP in tunnel air -and   subsequently in the passenger cabin -are caused by fresh vehicle exhaust that contains a large amount of particles in the lower nano-size range (Imhof et al., 2005).Although roof top fans within the tunnel-system are computer controlled and automatically adjusted according to built in nephelometric sensors, the observed UFP-load still reached values that can be compared to heavily polluted urban areas with high traffic densities.Considering health-related aspects of nanoparticles, the variations in UFP composition and hence lung deposition by the application of different vehicle ventilation settings was calculated in this study.The calculations revealed that a high amount of UFPs can penetrate deep into the lung.Although HDVs are restricted during weekdays, there was no significant variation between weekdays and weekends.This effect has already been observed in a previous study by Kwasny et al. (2008) at another section of the same highway, in which it was found that relatively higher number of diesel-powered passenger cars on Austrian roads (SA, 2011) counterbalanced the weekly fluctuating HDV frequency.The decline in particle number concentration and change in particle size distribution outside the tunnel system as stated by Rosenbohm et al. (2005) -was most likely induced by dilution with background air.This decline in particle number concentration is hardly possible to achieve within enclosed environments like tunnels.High traffic volume and their constant velocities associated with a continuous production of fresh exhaust aerosols keep most of the particles airborne and within the UFP regime.
The results show that the built-in filters corresponding to pollen-filter standards can provide up to 88% reductions in total particle deposition.The built-in filters already in its simplest application (VA-mode) proved to be effective by reducing the particle concentration up to 60%.While it is clear that the reduction in particle concentration in the VA and RCA setting is a result of filtration that allows fewer aerosols to enter the passenger cabin, whereas, the RCA/AC setting alters abiotic conditions thus facilitating particle agglomeration dynamics to achieve a further reduction in particle concentration.Although the reduction from RCA to RCA/AC is not significant, which is most likely attributed to condensation and coagulation processes that take place due to changes in temperature and humidity as a result of the air-conditioning system (Asgharian, 2004).Evidence of that can be deduced from Fig. 4 as the overall particle RCA-AC distribution in the smaller size range (< 100 nm, in comparison to the RCA-setting) is somewhat decreased, whereas at the same time there is an increase in the larger particle size range (> 100 nm), which is characteristic to temperature-induced coagulation.
The correlation factors shown in Fig. 8 reveals higher deposition of particles deeper into the lung (alveolar region), thus highlighting the fewer protective properties of the VA settings against UFP particles.Similar results of alveolar deposition of UFP have been obtained by Hussain et al. (2012).With clearance mechanism in the alveolar region being slower as compared to bronchial muco-ciliary clearance in the upper respiratory tract -i.e., half time of alveolar clearance is in the order of 714 days (Bailey et al., 2008), here deposition of UFP in the alveolar region implies the risk of their translocation from the respiratory epithelium to the circulatory system and hence subsequently to target organs, i.e., heart, liver, brain and so forth.Eventually these particles may induce adverse effects on cardiac function and blood coagulation, as well as on functions of the central nervous system (Kreyling et al., 2006;Madl and Hussain, 2011;Madl, 2012).As the overall relative surface area of deposited UFPs is higher than deposited coarse siblings of the same mass, UFPs are considered to be more hazardous in terms of faster release of adsorbed toxic compounds (Hussain et al., 2012).
Modeled deposition results for a single breath provide interesting insight on the amount of deposited particles in a given ambient environment.Unfortunately, these are hardly applicable to the public in large, as intersubject variability of total lung capacity, breathing cycle, and other physiological parameters, require exposure calculations at individual level (Yip et al., 2006).Indeed, exposure to particulate matter is an important challenge in population-based risk assessment.The European Union (EU) aims to establish particle number based limits for particulate matter (EC, 2009).Hence, this study may be of special interest to set exposure standards in work-related or otherwise unavoidable commuting environments.The effectiveness of applying the RCA setting while driving significantly reduces exposure to UFPs has been demonstrated in this study.Thus, rather than driving with open windows, the finding of this study recommends the use of the RCA setting during journeysparticularly through the long sections of tunnels.

CONCLUSION
The objective of the present study was to investigate the influence of various ventilation settings on lung deposition while driving through a tunnel system.By using the car's built-in ventilation system to route ambient tunnel air into the passenger cabin (VA setting), a strong filtering effect compared to the overall particle load of ambient tunnel air (TA) was found.An even better particle reduction inside the passenger cabin was obtained by applying the recirculated air (RCA) setting.Activating the air conditioning system in addition to the RCA-setting (RCA/AC) resulted in a further reduction of the nano-particle load inside the cabin.Labtest analysis of the filter efficiency yielded an average of 60% which is similar to those found in other studies.For the observed sampling cycles, no significant effect was observed in the total particle concentration among various sampling periods.Under optimal conditions and by using the RCA/AC mode, the study achieved a decrease of in-cabin air particle pollution by up to 88%.
For each type of ventilation setting used, the corresponding computations for lung deposition peaked between generation 15-19, or in other words, predominantly within the alveolar region of the lung.Maximization of the filtering effect was achieved with the combined setting of RCA and AC and amounted to a reduction of up to 95%.Further studies in lung deposition calculations could be of interest in less enclosed areas such as office buildings or hospital environments, where air conditioning systems provide similar effects.This would be beneficial especially for high risk groups (the very young, elderly and sick people) as well as institutions like intensive care units.Of particular interest could be buildings or office blocks located near urban pollution hot-spots.

Fig. 1 .
Fig. 1.Aerial view of the study site -a 3.4 km long highway tunnel system consisting of the Ofenauer Tunnel, 1.4 km long and immediately followed by the 2 nd one, the Hiefler Tunnel with 2.0 km in length.The site is located 30 km south of Salzburg City, Austria.Modified and adapted from Google Earth.

Fig. 2 .
Fig. 2. Traffic volume observed during the sampling days.Vehicle frequencies are grouped into direction (north-vs.southbound).The graph reveals a combined class of averaged numbers of passenger cars (automobiles and vans) and HDV (automobiles with trailer, trucks, light duty vehicles with trailer and buses).

Fig. 4 .
Fig. 4. Particle size distribution (average) measured during various ventilation settings for the sampling cycle performed during the month of December.Sampling cycle include tunnel air and tunnel air with VA, RCA, RC/AC-settings.

Fig. 5 .
Fig. 5. Filter efficiency of built-in particle filters in cars.Results of our lab test (black trend-line) and results from two other studies Qi et al. (2008) and Zhu et al. (2007).

Fig. 6 .
Fig. 6.Total lung deposition averaged over all sampling days for tunnel air (TA) and with various ventilation settings (VA, RCA, RCA/AC).

Fig. 7 .
Fig. 7. Detail of Fig. 6, total lung deposition averaging all sampling days, highlighting the difference between RCA and RCA/AC setting.

Fig. 9 .
Fig. 9. Lung deposition during a single breath in an adult male with a tidal volume of 750 cm 3 under sitting breathing condition.Graph shows deposited particles in various lung generations for TA and other ventilation settings.

Table 1 .
Seasonal variations of traffic flow through the tunnels for the timeslot of the measurements (15-19 hrs) separated according to north-and southbound orientation.