The presented study describes a newly developed experimental approach to quantify PM emissions of second-hand smoke in a vehicle. The data demonstrate that in-vehicle smoking generates harmful particle emissions of varying diameters even with the vehicle ventilation system turned on. If the experimental setting mimicked a scenario with no ventilation initiated by the car occupants, the particle load was significantly higher when the ventilation was started, albeit the differences in particle load exposure between the ventilation conditions were minimal.

Prior studies have investigated tobacco smoke pollution in cars under in vivo conditions exposing human smokers [20, 21, 26,27,28,29,30,31]. Thus, acquired data of PM2.5 by Sendzik et al. under similar ventilation conditions resulted in comparable PM concentration levels as in this study [21]. There are important ethical concerns regarding the exposure of humans to toxic tobacco smoke for research purposes. Therefore, the strength of this study is to introduce a new mechanical system that allows research on secondhand tobacco smoke to be conducted without exposing the researchers or any other person to tobacco smoke. The highly standardized smoking procedure ensures optimal data acquisition and comparison. During conditions C2—C4, the on-board ventilation system of the car was kept on power level 2/4 as we considered it a realistic setting for drivers to use. Although temperature and relative humidity may impact PM concentrations, they were intentionally left uncontrolled, which might seem as a limitation of this study. This experimental approach was chosen because it imitates real-life driving conditions in a car without air-conditioning or heat [32, 33]. As mentioned in previous ToPIQ studies, the AETSE cannot exactly imitate a real smoker [22, 23, 34, 35]. Therefore, it was surprising that the PM data could be compared with data originating from research using human smokers. In contrast to ToPIQ I, the ToPIQ II study design used a larger smoking chamber (2.88 m3), thus being more comparable to the interior car volume of the Mitsubishi Space Runner (3.709m3) [23, 24]. Due to the larger interior volume of the car, the reference cigarettes PM2.5 exposure measured in the ToPIQ II study by Gerber et al. is about 9% higher than the corresponding PM values presented in this study [23]. The influence of varying interior volumes on PM burden should be the focus of future studies.

The findings of extremely high PM values after 10 min of cigarette smoking without any ventilation were alarming. Further, the 3R4F reference cigarettes smoked under diverse ventilation conditions displayed similarly high PM levels (no significance, p > 0.05). The mean concentrations of PM were 3—4 times less compared to cigarettes smoked without ventilation. Under C3, PM10, PM2.5, and PM1 decreased by 74.5%, 74.3%, and 68%, respectively, after 10 min compared to C1, representing the highest reduction of PM for the investigated ventilation conditions. On the contrary, the lowest decrease of PM after 10 min was measured under C4 (Table 1).

Even after 10 min, the measured Cmean of PM10 (> 400 µg/m3) had exceeded the WHO 24 h threshold by more than factor 9. PM10 concentrations of cigarettes smoked without ventilation were 35 times higher than the recommended WHO threshold for PM10 [8]. Moreover, these high PM concentrations drastically exceeded the measurements conducted by Dröge et al. (traffic PM measurements inside a driving vehicle cabin, among others, with closed windows) showing PM2.5 values of 5.2 – 23.2 µg/m3 and PM1 values that ranged from 4.9 – 22.6 µg/m3 [36]. The higher Cmean values after 10 min compared to Cmean after 4.5 min under C1 is due to the sustained high plateau concentration after the cigarette has been extinguished. The reduction of PM concentrations from 4.5 min to 10 min by 13.8 – 22.2% under C2 – C4 demonstrates the effect of in-vehicle ventilation during a smoking session. Ventilation dilutes the in-cabin air with ambient air, thus increasing the air exchange rate and decreasing the PM concentration [31, 37]. It is not yet known whether the ventilation produces an airstream that pushes PM into the back of the vehicle, thereby increasing the PM concentration where children are usually seated. Experiments have already indicated that smoking a cigarette with an opened window does not decrease the PM exposure in the back seat [38]. Although smoking with one or more opened windows increases the air exchange rate, the SHS exposure is still highly elevated [20, 38,39,40]. Schober et al. compared the PM emissions of IQOS, E-Cigarettes and tobacco cigarettes under six different ventilation conditions in various cars with varying interior volumes [41]. Combustion of tobacco cigarettes reached higher emissions of PM2.5 (64—1988 μg/m3) than IQOS or E-Cigarettes. Compared to our investigation (PM2.5: 407—1583 μg/m3), their PM2.5 emission range is wider due to different experimental conditions [41]. Sohn et al. measured the PM emissions of cigarettes under three different ventilation conditions [20]. Similar to our study, they divided the measurements into three phases: The pre-smoking phase, smoking phase, and post-smoking phase. While they concluded that the PM2.5 concentration exceeded the US National Ambient Air Quality Standard of 35 µg/m3, their investigation lacks differentiation of PM10 and PM1 emissions. Neither of the two aforementioned study designs investigated the effect of the on-board ventilation system on PM emissions as presented in our research model [20, 41].

Figure 3 differentiates PM into PM10-2.5, PM2.5–1, and PM1, thus comparing the individual mass of different particle sizes created through cigarette combustion. After 10 min 70.5—89.6% of the total PM mass is ≤ 1 µm, while PM2.5–1 accounts for 9.7—28% and PM10-2.5 for 0.7—1.5%. Due to gravitational settling, coarse particles have faster deposition rates than fine particles [42, 43]. Nevertheless, the deposition rate of PM is highly variable and depends on many factors (e.g., humidity, temperature, air turbulence, surface roughness, thermophoresis, turbophoresis, spatial distribution, electrostatic effects) [33, 44]. In contrast to the PM fractions PM2.5–1 and PM1, PM10-2.5 is more impacted by gravity [33]. Therefore, the high concentration of fine particles after 10 min is due to slow gravitational sedimentation and high fine particle generation during cigarette combustion [7, 42, 45]. The portion of PM1 was 16.3 – 19.1% higher for C2 – C4 compared to C1 after 10 min. That is likely caused by the ventilation, creating air turbulence leading to slower gravitational sedimentation of fine particles [33].

A high concentration of small particles < 2.5 µm is particularly alarming, as they can penetrate deeply into the respiratory system causing severe health burdens [46]. Children exposed to small particles are especially vulnerable and can develop various diseases (asthma, cancer, decreased lung function, otitis, neurobehavioral problems, etc.) [47, 48]. In 2010, additional health care services for US children aged 3–14 of 62.9 million dollars were linked to preventable SHS [49]. An observational study carried out in Italy showed that children were exposed to in-cabin SHS in 0.9% of passing by vehicles [50]. Therefore, children should be protected from SHS by law, prohibiting in-door (in-vehicle) smoking.

The on-board ventilation system (C2 – C4) drastically reduced the PM peaks after 4.5 min and 10 min. Compared to C1, it decreased PM10, PM2.5, and PM1 peaks at 4.5 min by 61.7—71.5%, 61.4—71.2%, 51.3—61.8%, and at 10 min by 93.5 – 93.8%, 93.4 – 93.7%, and 91 – 91.4%, respectively (Table 1). Therefore, the majority of PM concentration peaks (> 61%) is reduced at the end of the second interval (after 4.5 min).

In 2019 Campagnolo et al. accurately showed a correlation of PM concentration inside a car cabin depending on the emission standard of the car driving ahead. New emission standard cars (Euro 6) generate 34% less PM0.3–1 for the following car than compared to its older predecessors (i.e., Euro 0–2) [51]. This study presents a great example of the benefit of strict PM emission laws for vehicles.

The new measuring platform poses multiple opportunities for future investigations. PM exposure under diverse ventilation scenarios with different degrees of window openings is of the highest interest. The two outside fans can be used at different power levels to simulate an airstream around a moving vehicle. Measurements during this simulation may add important data about PM exposure during a car drive with opened windows. The influence of air conditioning could be investigated in upcoming studies. The cigarette smoking device can aid in investigating the effects of chain-smoking on PM concentration in a vehicle. Positioning multiple LAS at different locations inside the car cabin could generate important data about the distribution of SHS inside the car cabin under various ventilation conditions simultaneously.