1. Introduction
In the last decade, the number of vehicles circulating on the roads exhibited a quick year-by-year increase. In addition to the many advantages, the increase in vehicles on the roads comes with several drawbacks, such as increases in deaths on roads and traffic congestion, all of which imply an increase in CO
2 emissions [
1]. For this reason, nowadays, the concept of on-road driving is forcing the development of innovative vehicle connectivity technologies to improve road safety and manage traffic congestion through real-time traffic information shared among the cars and the road infrastructure. Car connectivity is also the backbone of the forthcoming fully autonomous driving. Much research has been conducted over the past years for the development of a new mobility concept named Intelligent Transport Systems (ITSs) [
2,
3]. ITSs deeply rely on ad hoc wireless communication technologies called “vehicle-to-everything-communication” (V2X) and on technologies for sensing the environment outside the vehicle (through Advanced Driver Assistance Systems (ADAS)) and inside the vehicle (through in-vehicle occupant detection (VOD) systems). V2X communication enables the communication of vehicles with other entities, such as vehicles (in vehicle-to-vehicle communication, i.e., V2V), pedestrians (in vehicle-to-pedestrian communication, i.e., V2P), infrastructures (in vehicle-to-infrastructures communication, i.e., V2I), and networks (in vehicle-to-network communication, i.e., V2N) [
4,
5,
6]. These V2X communications are based on two main wireless access technologies, i.e., the well-known and consolidated IEEE 802.11p that operates at the ITS-5.9 GHz band [
7] and the Cellular-V2X (C-V2X) [
8,
9,
10,
11,
12] which is a more recent technology in which V2X functionalities are widened by using 5G technology.
With the advent of these new vehicular communication technologies, people inside and nearby the connected vehicles are exposed to radiofrequency electromagnetic fields (RF-EMFs) emitted by these technologies. As evidenced in a recent survey on wireless technologies used in connected vehicles [
13], research on the assessment of exposure due to V2X and automotive sensing is scarce. There exist only a few articles that addressed this topic [
14,
15,
16]. Specifically, Tognola et al. [
14] assessed the RF-EMF dose absorbed by an adult passenger inside a car equipped with V2V antennas operating at the ITS-5.9 GHz band, whereas Benini et al. [
15] investigated the exposure outside the car in an adult pedestrian standing near a car equipped with the same V2V technology. In both studies [
14,
15], it was evidenced that the dose absorbed by the body was always below the basic restriction limits of 0.08 W/kg over the whole body, 2 W/kg in 10 g of tissues in the head and torso region, and 4 W/kg in 10 g of tissues in the limb region as recommended by the ICNIRP [
17] and IEEE [
18] guidelines for exposure in the 100 kHz–6 GHz range. However, to the best of the authors’ knowledge to date, nothing is known in terms of children’s exposure.
Thus, the objective of this study is to evaluate the RF-EMF exposure in children standing near a car equipped with V2V communication technology operating at the ITS-5.9 GHz band through an electromagnetic computational technique. In our previous study [
15], we focused on the variability of exposure levels as a function of the distance and position of an adult pedestrian near the car. In the current study, we focused on another aspect, i.e., on the assessment of the exposure as a function of body size and age. This study builds upon the knowledge acquired in [
15] for what concerns the identification of the distance and position near the car that corresponded to the worst-case exposure.
To discover whether and how the RF-EMF exposure in children varied in age and body size, four different numerical anatomical models of children of different sizes, ages, and both genders were used. Furthermore, the exposure level calculated here for the child models was then compared to that one found in similar exposure conditions in the adult model analyzed in [
15].
Finally, the exposure levels obtained in children were also compared to the basic restriction limits set by the current ICNIRP [
17] and IEEE [
18] recommendations on EMF exposure.
3. Results
We remind the reader that all the SAR values reported in this section were calculated by feeding both antennas simultaneously with 1 W (i.e., 30 dBm). This is the worst-case exposure condition because, in real scenarios, the V2V antenna system typically employs adaptive power control, so the typical real forward power might be less than 1 W.
Table 4 shows the wbSAR values across the children’s models and the two positions near the car. The child models in
Table 4 are listed from the shortest (Roberta) to the tallest one (Dizzy). To facilitate the comparison, the bottom row of
Table 4 also displays the wbSAR of the adult model Ella investigated in [
15]. In all the child models, the wbSAR at the Back position was always lower than that at the Front position. The wbSAR at the Front position was almost the same across the children. At the Front position, the children and the adult showed almost similar values of wbSAR, ranging from 0.15 to 0.18 mW/kg. Vice versa, at the Back position, the difference in the wbSAR values across the human models was very pronounced and ranged from 0.02 mW/kg in the shortest child model (Roberta) up to 0.19 mW/kg in the tallest model (Ella). Indeed, when the human model was near the back antenna, the increase in the model height resulted in a decrease of the relative height Δh
1 from the back antenna, going from +319 mm in Roberta (the shortest child) to −212.3 mm in the adult Ella (
Table 2). The decrease of Δh
1 resulted in an increase in the models’ upper body area exposed to the radiation of the back antenna, reaching the maximum wbSAR in the tallest model. On the contrary, at the Front position, thanks to the tilted configuration of the frontal antenna, the children and adult models were always entirely exposed to the same degree of radiation, regardless of their height. Differently from the children, the wbSAR of the adult model (Ella) did not significantly change between the two positions near the car because the model is tall enough to be radiated also when it is at the back of the car.
Figure 4 shows the maximum peak SAR values averaged over 10 g (pSAR
10g) of the skin of the whole body, the head, and the genitals area, while
Figure 5 displays the maximum peak SAR values averaged over 1 g (pSAR
1g) of the eyes. As in
Table 4, to facilitate the comparison,
Figure 4 and
Figure 5 also show the pSAR values obtained with the adult model [
15].
Consistently with the wbSAR values of
Table 4, at the Front position, the pSAR
10g values in the skin (
Figure 4) did not significantly change across the children; also, in this position, the children and the adult had almost similar pSAR
10g values. Vice versa, at the Back position, the pSAR
10g values of the skin of the children were significantly lower than in the adult, especially for the shortest children, i.e., Roberta and Thelonious, for which pSAR
10g was nearly negligible. At the Back position, the exposure of the whole body and the head region of children increased with the child’s height because the relative height Δh
1 between the back antenna and the head region decreased with the increase of the model height (
Table 2). Considering both the Front and Back positions, the pSAR
10g value in the skin of the whole body and in the head region ranged from 1.04 mW/kg to 9 mW/kg of the children, while in the adult, the pSAR
10g value obtained in the whole body and head region reached the maximum of 34.70 mW/kg (
Figure 4). Instead, the pSAR
10g value in the skin of the genitals area was very small for both the children and the adults, being almost negligible at the Back position where the lower body region is scarcely exposed to the radiation of the back antenna.
Figure 4.
pSAR10g in the different body regions of the skin for the children and adult models at the Front (left panel) and Back (right panel) positions near the car.
Figure 4.
pSAR10g in the different body regions of the skin for the children and adult models at the Front (left panel) and Back (right panel) positions near the car.
Figure 5.
pSAR1g of the eyes in the Front and Back positions near the car for the children and adult models.
Figure 5.
pSAR1g of the eyes in the Front and Back positions near the car for the children and adult models.
As for the eyes (
Figure 5), it is noteworthy to observe that the pSAR
1g values in the children were generally higher at the Front position than at the Back position, ranging from 0.40 mW/kg in the shortest child (Roberta) to 10.80 mW/kg in the tallest one (Dizzy). This was because, at the Front position, the eyes of the children were always well exposed to the radiation of the frontal antenna; in this position, the higher the child, the higher the exposure because the antenna-to-eyes distance decreased with the child’s height. In contrast, the eyes of the adult had a lower exposure at the Front (5.61 mW/kg) than at the Back position (54 mW/kg) because the distance Δh
1 between the eyes and the back antenna is smaller than the distance Δh
2 between the eyes and the front antenna (|Δh
1| = 212.3 mm and |Δh
2| = 287.4 mm (
Table 2)).
Table 5 reports for each human model the sites on the body skin characterized by the maximum exposure (i.e., characterized by the pSAR
10g value shown in
Figure 4). When the models were in the Back position, the maximum exposure in the whole body was always located at the head, namely at the nose and forehead. Vice versa, at the Front position, the maximum exposure in the whole body was not always at the head level but, except for Dizzy, tended to move from the top (the shoulders) to the bottom (the hands) as the model height increased. Specifically, for the head, the sites of the maximum exposure across the Front and Back positions were the chin, the forehead, and the nose. As an example,
Figure 6 shows the SAR
10g distribution on the skin of the whole body in the shortest (Roberta) and the tallest (Eartha) child models. From
Figure 6, it is possible to observe how the location of the peak SAR value changes as the height of the model increases. Finally, for the genital area (
Table 5), only for two models (Thelonious and Dizzy) in the Back position, the maximum exposure was located right at the genital organ (i.e., in the penis); in all the other models, the maximum was located elsewhere, such as at the upper thighs or the waist.
Table 5.
Location of the maximum exposure in the skin of the whole body and at the head and the genital area for the Front and Back positions of the models near the car.
Table 5.
Location of the maximum exposure in the skin of the whole body and at the head and the genital area for the Front and Back positions of the models near the car.
Human Model | Body Region | pSAR10g Site |
---|
Model Position: Front | Model Position: Back |
---|
Roberta | Whole-body | shoulder | nose |
Head | chin | nose |
Genitals area | upper thigh | upper thigh |
Thelonious | Whole-body | arm | forehead |
Head | forehead | forehead |
Genitals area | upper thigh | penis |
Eartha | Whole-body | forearm | forehead |
Head | forehead | forehead |
Genitals area | waist | waist |
Dizzy | Whole-body | nose | forehead |
Head | nose | forehead |
Genitals area | waist | penis |
Ella | Whole-body | hand | nose |
Head | nose | nose |
Genitals area | waist | waist |
Figure 6.
SAR10g distribution on the skin of the whole body in Roberta (the shortest child) and Eartha (one of the tallest children) at the Front and Back positions. For both models and positions, the SAR10g distribution was normalized to the maximum SAR found among the children, i.e., 4.68 mW/kg at the Front position and 9 mW/kg at the Back position.
Figure 6.
SAR10g distribution on the skin of the whole body in Roberta (the shortest child) and Eartha (one of the tallest children) at the Front and Back positions. For both models and positions, the SAR10g distribution was normalized to the maximum SAR found among the children, i.e., 4.68 mW/kg at the Front position and 9 mW/kg at the Back position.
To further delve into the study of the exposure,
Figure 7 reports some examples of SAR distributions in terms of Cumulative Distribution Function (CDF) for each tissue region investigated at the Front position in Roberta and Eartha (the same children illustrated in
Figure 6). In order to conduct a more comprehensive and quantitative analysis of the exposure,
Table 6 reports the maximum (also illustrated in
Figure 4 and
Figure 5 as pSAR
10g for the skin and pSAR
1g for the eyes, respectively), the media, and the skewness of the distribution of the SAR
10g and SAR
1g in the skin and eyes of the children and the adult model Ella. Generally, it can be noticed that the median values were two orders of magnitude lower than the maximum values for all the human models and both positions near the car. Also, for both the children and the adult, the SAR
10g and SAR
1g distributions had a positive skewness, meaning that there was a greater density of SAR values towards the lower range of exposure levels.
The median value of SAR
10g of the skin of the whole body ranged from 0.01 to 0.06 mW/kg across the two positions and all models. As already observed for the maximum exposure (see
Figure 4), the head was the body region with the greatest exposure, with a median value ranging from 0.05 mW/kg in the adult in the Front position to 0.60 mW/kg in the tallest child (Dizzy) in the Back position; the dose absorbed at the genital region was always negligible with very low median values near to zero. As already observed with the maximum, the dose absorbed by the smallest children (i.e., Roberta and Thelonious) at the whole body was higher at the Front position than at the Back, while for the tallest child and the adult, the dose absorbed was higher at the Back position. This was because in the Back position, the upper body regions were more exposed to the radiation in the tallest child and adult (because of their lowest Δh
1) rather than in the shortest child, which instead had a greater exposure at the Front.
The dose absorbed by the eyes was generally higher than that of the whole body for both the children and the adult, with a median value ranging from 0.03 mW/kg to 3 mW/kg across the two positions and all models. This is because the eyes were generally entirely well exposed to the radiation, especially in the Front position. For the children, the median value of the dose of exposure at the eyes was higher when the model was at the Front position of the car. Vice versa, in the adult, the median value of the exposure dose in the eyes was higher at the Back position.
It can also be observed from
Table 6 that the maximum and the median value of the exposure in the eyes progressively increased with the child’s height because the taller the child, the closer the antenna was to the eyes. The greatest exposure in the eyes was observed in Dizzy, which was the tallest among the child models. Although the Dizzy model includes only two (the vitreous humor and lens) of the four eye tissues of the other models, its SAR values in the eyes were in line and even greater than those calculated in the other children’s models. It is noted that the Dizzy model includes the vitreous humor, which is the eye tissue with the greatest weight and the highest dielectric properties due to its 99.7% content in water [
34]. These characteristics make the vitreous humor the most relevant tissue for SAR characterization in the eyes. As such, we can safely conclude that the SAR values we obtained in the eyes of Dizzy were not underestimated.
4. Discussion
This study investigated RF-EMF exposure in children in the ITS-5.9 GHz vehicular connectivity exposure scenario. This study calculated and compared the exposure levels in anatomical models of children with the ones of an adult female model previously investigated in [
15] in the same exposure setup. Exposure was evaluated through numerical dosimetry in four anatomical models of children of different body sizes, ages, and both genders, namely in the Roberta (female, 5 years old), Thelonious (male, 6 years old), Eartha (female, 8 years old), and Dizzy models (male, 8 years old). The exposure of each child model was simulated and evaluated at two different positions—Front and Back—near a car equipped with two V2V antennas.
In all the models and both positions, it was found that the RF-EMF dose was absorbed mainly in the most superficial tissues and organs, i.e., in the skin and eyes. The body region with the highest RF-EMF absorption was found to be in the upper body, specifically, in the shoulder and arm for the shortest children (i.e., Roberta and Thelonious) at the Front position of the car and in the head region, i.e., the nose and forehead, for the tallest children (i.e., Eartha, Dizzy) and the adult model when they were at the Back of the car.
The dose absorbed by the children was lower than that of the adult. The difference between the children and the adult was more pronounced for the exposures at the Back of the car. Overall, considering the exposures across the Front and Back positions together, the dose absorbed by the whole body (wbSAR) ranged from 0.17 to 0.19 mW/kg for the adult and from 0.02 to 0.18 mW/kg for the children. For the tissue-specific SAR for both the children and the adult, the greatest exposure was observed in the skin. Among the children, the maximum absorption of 9 mW/kg was found at the skin of the head region of the tallest child (i.e., Dizzy). This latter value was much lower than the maximum exposure of 34.70 mW/kg found in the adult (Ella). Similarly, also for the eyes, the children exhibited lower exposure levels than the adult. The highest value of 10.80 mW/kg was found again in the tallest child, whereas the maximum exposure in the adult was 54 mW/kg. The exposure of the lower body region, i.e., at the genital area, was negligible for both the children and the adults, especially at the Back position.
For all the children and positions near the car, the dose absorbed by the whole body and by the different body regions was well below the basic restriction limits recommended by the ICNIRP and IEEE guidelines for the exposure of the general public in the 100 kHz–6 GHz range, which are 0.08 W/kg for the whole body, 2 W/kg in 10 g of tissue of the head and torso regions, and 4 W/kg in 10 g of tissues of the limb region [
17,
18]. As described above, the highest dose of exposure in the children was in the eyes (equal to 10.80 mW/kg). It is worthwhile to remind that we calculated the dose in the eyes over a 1 g mass of tissues instead of using a 10 g mass as we did for the skin. For the eyes, we preferred to use the 1 g mass instead of the 10 g mass as indicated in the current ICNIRP and IEEE guidelines because of the small mass of the eyes. As such, the compliance of the dose calculated by us in the eyes of the ICNIRP and IEEE basic restriction limits can be done only at a qualitative level. Indeed, since the calculation of the dose absorbed by a 1 g tissue usually is greater than the value that would be obtained with a 10 g mass, we can conclude that the dose of 10.80 mW/kg estimated in our study on a 1 g mass would be in any case well below the ICNIRP and IEEE safety limit of exposure of 2 W/kg in a 10 g mass.
Our SAR levels were obtained by driving both antennas with 1 W of transmit power. If we scale our SAR values to the maximum transmit power of 33 dBm (i.e., 1.99 W) for each of the two V2X communication antennas, as allowed in the EU and the US for non-government services [
7], the SAR levels would be almost doubled. Nevertheless, also in this latter circumstance, the dose absorbed would remain below the limits recommended by ICNIRP and IEEE [
17,
18].
Considering that the dielectric properties of the children and adult tissues are the same, the differences in the exposure levels within the children and between the children and the adult models were most probably due to a combination of two factors, namely the different anatomical characteristics of the models (such as the height) and to the position of the models near the car. Both factors have an impact on the degree to which each model was radiated by the nearest antenna. For the anatomical characteristics, height seems to play an important role, mainly for the exposure at the Back of the car. In this latter position, the field emitted by the rear antenna propagated mostly in a horizontal plane parallel to the ground. As such, this field could radiate only those models (the tallest ones) that were at the same or at a higher level than the antenna. At the Back position, the increase of the model height resulted in an increase in the upper body regions that could be directly exposed to the radiation. As a result, the whole body and the tissue-specific SAR at the Back position gradually increased as the model’s height increased. Vice versa, at the Front position, where the field of the antenna propagates more towards the ground (because the antenna is tilted), all models are well exposed, and the effect of the model height was less relevant.
As described in the Results, at the Back position, there was a relevant difference in the SAR of the smallest child and the adult. In particular, the whole-body SAR was 0.02 mW/kg in the smallest child (Roberta) and 0.19 mW/kg in the adult; the maximum of the SAR of the adult was almost four times higher than that of the children for the skin and 5.3 times higher for the eyes. On the contrary, at the Front position, the SAR values of the whole body and the skin of the children and the adult were more similar. For the eyes, the local SAR increased with the child’s height, reaching the maximum value of 10.80 mW/kg for the tallest child at the Front position. For the adult, the maximum SAR of the eyes (54 mW/kg) was observed at the Back position.
The statistical analysis revealed that the distributions of the SAR values in the skin and the eyes were positively skewed, meaning that most of the values were distributed in the lower range of exposure levels. The median value of the SAR in the skin of the whole body ranged from 0.01 mW/kg to 0.06 mW/kg in the children and from 0.03 mW/kg to 0.06 mW/kg for the adult.
To provide a more comprehensive picture of the RF-EMF exposure in the vehicular connectivity scenario, it is worthwhile to compare the SAR values obtained here for the child models with the ones obtained in [
14] for an adult model placed inside a vehicle equipped with the same V2V antennas used in the current study. The authors in [
14] mounted four antennas symmetrically at the front/rear roof and left/right mirrors of the car. Each antenna was fed with the maximum transmitted power allowable in the US for government services of 44.8 dBm (i.e., 30.2 W) [
7]. If we scale our SAR levels by feeding each of the two V2V antennas with the maximum input power of 30.2 W, the highest wbSAR value obtained in the tallest child at the Front position, i.e., 0.18 mW/kg, would become 5.43 mW/kg. This last value is lower than the wbSAR values obtained in [
14] of 8.33 mW/kg, most likely because of the higher number of antennas used in [
14] (four antennas) compared to our case (two antennas).
Furthermore, it is expected that the exposure scenario investigated in [
14] would induce higher SAR values than in our scenario also because of the shorter distance between the human model and the nearest antennas, namely 0.1–0.5 m in [
14] and 0.5–1.6 m in our scenario. Similarly to our study, the highest local SAR value observed in [
14] was located in the skin. However, while in [
14] the maximum peak was located in the skin of the head region, in our case, this holds true only for the tallest children (i.e., Eartha and Dizzy) because for the smallest ones (i.e., Roberta and Thelonious) the maximum SAR was located in the shoulder and arm. In particular, comparing the local SAR values, the pSAR
10g found in [
14] was 1581 mW/kg, which is much higher than the maximum pSAR
10g value found by us among the children, i.e., 271.80 mW/kg in the tallest child at the Back of the car when the antennas were fed with 30.2 W. This relevant difference is most likely due to the shorter distance between the human model and the nearest antennas [
14].