Introduction

People have been living in a human-made electromagnetic environment for more than a century. In recent years, the development and introduction of new communication technologies, and especially Wi-Fi connection demand, generate the need to create more and better free Wi-Fi hotspots. In some countries, these services are offered for free, for example, in Mexico City. In 2019, the Government of Mexico City implemented actions that allowed citizens to get near connectivity free Wi-Fi hotspots, with the purpose of encouraging the progressive Access to Information and Communication Technologies, like access to the Internet (ADIP, 2022).

These actions include the installation of 13,000 free Wi-Fi hotspots throughout the city, and among the places and public squares where they have been located, we can point out the Zócalo. In addition to these actions, questions and concern in society also increase due to the possible health effects of Radiofrequency Electromagnetic Fields (RF-EMF), both the one originating from mobile phone antennas and from the Wi-Fi band (Arribas et al. 2018; Birks et al. 2018; Di Ciaula, 2018; Foster and Moulder, 2019; Gallastegi et al. 2018; Pall, 2018; Röösli et al. 2010). That is why it is necessary to measure the personal exposure levels originating from this Wi-Fi frequency band and verify their compliance with the international guidelines published by the International Commission on Non-Ionizing Radiation Protection (ICNIRP, 2020), reference levels adopted in Mexico by Federal Institute of Telecommunications, Instituto Federal de Telecomunicaciones, in Spanish (IFT, 2020). For that reason, we decided to conduct the present study.

Carrying out studies on RF-EMF with exposimeters allows us to know personal exposure levels in different microenvironments, for example, inside public transport, outdoor urban areas, and inside home (Jalilian et al. 2019; Ramirez-Vazquez et al. 2023b; Sagar et al. 2018). In this context, a microenvironment is defined as a small area distinguished from its immediate surrounding by its function (Sagar et al. 2018).

There are also several research projects that have allowed the development of models to estimate exposure by means of sporadic measurements (Aerts et al. 2013a; Aguirre et al. 2015; Beekhuizen et al. 2014, 2013; Bürgi et al. 2008; Frei et al. 2010, 2009a; Martens et al. 2016). Several authors use different assessment methods, including the characterization of personal exposure (Cucurachi et al. 2013; Gajsek et al. 2016; Markov, 2017; Röösli et al. 2010; Sagar et al. 2018), comparing the personal exposure level between different zones and different periods of the day (Aerts et al. 2018; Fernandez et al. 2020; Manassas et al. 2012; Ramirez-Vazquez et al. 2023a), or describing the exposure levels and the contribution from different sources to RF-EMF measurements (Frei et al. 2009b; Gallastegi et al. 2018; Ramirez-Vazquez et al. 2019; Röösli, 2008; Röösli et al. 2010, 2008, 2007).

Most studies are carried out with the participation of volunteers, measuring and discriminating the results by frequency bands, and subsequently, classifying the measurements in different microenvironments (Ramirez-Vazquez et al. 2023b). Most studies have been carried out in European countries and to the best of our knowledge, a first study on RF-EMF exposure measurements in Mexico (Ramirez-Vazquez et al. 2021) but in different city, this is the first study in this place and conditions.

It is interesting, both for the scientific community and the general population, to replicate these studies in other countries, in similar microenvironments but with different conditions, for example, in Mexico City where more than 13,000 free Wi-Fi hotspots have been installed throughout the city, including the Plaza de la Constitución, known as the Zócalo, located in the center of Mexico City, and travelling on the subway in order to know the levels of RF-EMF exposure from Wi-Fi.

The different free Wi-Fi hotspot connections have been installed in different strategic places in Mexico City. This service has a transference rate of 100 Mbps (ADIP, 2022). The Wi-Fi service offered inside the subway is managed by the public transport companies in charge of subway transport. The wiring is installed in the passageways, inside the tunnels, and in the stations themselves.

In this work, when it comes to the 2.4- and the 5.85-GHz Wi-Fi band, we refer to the 2.4- to 2.5-GHz frequency band and 5.15- to 5.85-GHz frequency band, i.e., 2400- to 2500- and 5150- to 5850-MHz frequency bands (ISM/UNII bands, free license bands), respectively. The aim of this research was to measure personal exposure to Radiofrequency Electromagnetic Fields from the 2.4- to 2.5-GHz Wi-Fi band (referred to this document as 2.4-GHz Wi-Fi) and 5.15- to 5.85-GHz Wi-Fi band (referred to this document as 5.85-GHz Wi-Fi) in the Zócalo, located in the center of Mexico City, where free Wi-Fi hotspots have been installed, and to present the results through interpolated intensity maps. In addition to the Zócalo, we measured inside the subway, both places through which great numbers of people pass on a daily basis. In this way, we will verify if the reference levels established by international guidelines are respected. And finally, measurements were also taken while walking on the street and inside a building (at home).

Methodology

Measurement devices

Measurements of personal exposure to RF-EMF were carried out with two Satimo EME SPY 140 personal exposimeters. The exposimeter measures in 14 frequency bands between 88 MHz and 5.85 GHz; the maximum sensitivity level is 0.05 V/m (6.6 μW/m2), corresponding to the FM band. However, in the bands we measure the sensitivity is 0.0663 μW/m2 in the 2.4- to 2.5-GHz Wi-Fi band (Wireless Fidelity) and 1.06 μW/m2 in the 5.15- to 5.85-GHz Wi-Fi band (MVG, 2022).

Two exposimeters Satimo EME SPY 140 were used, configured to measure every 10 s, and we calculate the average recorded by both exposure meters. The exposimeters were carried out waist high one on each side of the researcher to reduce body shielding (Nájera López et al. 2015). The measurements were taken from the moment the researcher left her home (on the street), while using public transport (inside the subway), walking around the Zócalo (spot measurements) and nearby streets, and at home.

The researcher always had her phone off so as to avoid any interference with the personal exposimeter or so that the phone could not connect to any Wi-Fi antennas and in this way affect the measurements. The researcher also carried a GPS, a plastic watch, a map, and a personal diary in which she recorded the time and place she was at, at any given moment.

Study microenvironments

Two types of measurements were carried out, measurements walking around the Zócalo (nearby streets) and spot measurements at the Zócalo plaza (16 points with a duration of 6 min at each point, following ICNIRP guidelines (2020); Fig. 1a). The 16 points were selected strategically and previous to the measuring process, and said points were also georeferenced to make intensity maps. Apart from measuring at these 16 points, as we have specified previously, measurements were carried out inside the subway, in the street (walking from home to the subway station, and vice versa), and finally, inside a building (the researcher’s own home, using the Wi-Fi connecton and not using the Wi-Fi connection).

Fig. 1
figure 1

a Sixteen measuring points in the Zócalo and surrounding areas and b Zócalo area and location of free Wi-Fi hotspots and nearby buildings

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In Fig. 1b, we can point out the Metropolitan Cathedral to the North, the old Town Hall, and the Office of the Secretary of Finances to the South; to the East we can find the National Palace and to the West we find a Museum and several shopping centers. We would like to point out that the day the measurements were carried out there were great numbers of people. Most of them were connected to Wi-Fi with their mobile phones using internet and social media (we confirmed it by asking some people). All this was recorded in her diary by our researcher.

We carried out spot measurements in the center of the Zócalo (16 points) to elaborate the interpolated intensity maps and compare the values measured with the exposimeters and with the values calculated by the Kriging interpolation method, as presented in the “Results” section.

Statistical and geostatistical analysis

The statistical analyses were carried out using the exposimeter software, EME SPY Analysis V3, Google Earth Pro, Google Map, Microsoft Excel of office, IBM Statistical Package for the Social Sciences (SPSS) V24, Origin 2022 Software, ArcMap 10.8.1, and ArcGIS Pro. Descriptive and geostatistical analyses were carried out using the Kriging method. The levels of intensity of the electromagnetic wave (called power density by other authors) are presented in μW/m2 and with three significant figures (Ramirez-Vazquez et al. 2022).

Through the interpolation maps using Kriging method we intended to visualize the spatial distribution of the intensity of RF-EMF, where results of real measurement rather than estimated or modeled data are used to produce the GIS map, using ArcGIS Pro software.

Kriging is an advanced geostatistical and stochastic method of spatial interpolation that predicts the value of a natural phenomenon at unsampled sites, that is, generates an estimated surface from a scattered set of points with different values of a physical magnitude. This method uses a weighted mean of the available data, with the weights depending not solely on the distance but also on the geometry of the samples’ location (Aerts et al. 2013b; Cressie, 1993; Oliver and Webster, 1990; Ramirez-Vazquez et al. 2023a). Ordinary Kriging is a common variant of the Kriging technique used in spatial statistics, which was used to estimate values for the variables in locations that were not sampled, using data in the neighborhood of the estimation location.

To carry out this type of geostatistical analysis, it is very important to identify the variable to be measured (in this case, the intensity levels of an electromagnetic wave), define the study region, and collect the available observed data in the form of georeferenced points.

It is important to highlight that Ordinary Kriging is only one variant of the Kriging technique and that there are other more advanced variants, such as universal Kriging that allow the incorporation of additional information, such as covariates or spatial trends, to be incorporated into the estimation process. However, in our case, we only work with one study variable, and we believe that Ordinary Kriging is the most suitable approach.

The parameters considered in the interpolation of the intensity maps were the location of the measured points (georeferenced), the average values during 6 min of the measurement process, and the location of the free Wi-Fi hotspots (georeferenced).

Before statistical analysis, all data collected were revised to confirm if the recorded measurements were consistent with the notes that the researcher wrote in the personal diary (time and location) and correct any mistake that could have been detected (for example, incorrect time or location) but that was not the case. Recorded values below the Detection Limit (DL) from the exposimeters are called “nondetects.” Nondetect data (ND) percentage range was 75% in the two Wi-Fi bands; we were replaced by the Detection Limit divided by 2 (Najera et al. 2020).

Exposimeters are small devices that take high-resolution measurements, over long time periods and in real environments, but a limitation of all exposimeters is that they cannot measure signals below their DL, even this situation occurs in most environmental studies. It is important to highlight that when we characterize the RF-EMF exposure, we must assume that, even with small values, a minimum signal will always be present. The sensitivity of our exposimeter is 0.0663 μW/m2 for 2.4-GHz Wi-Fi and 1.061 μW/m2 for 5.85-GHz Wi-Fi band, which is the value recorded when no signal is detected (nondetects); therefore, we must assume that the real existing value at that moment must come close to zero, but it must be between zero and this DL.

Some studies have been carried out comparing different methods of analysis of nondetect data, such as ROS (Robust Regression on Order Statistic) and Detection Limit divided by 2, using the Detection Limit as a valid measure or ignoring nondetects (DL). We must not forget to review and treat ND, depending on the percentage. Another aspect to take into account is that if the percentage of nondetects is high (above 80%), we must provide the correct description of the situation by calculating mean values, median values, and percentile (90 or 95) or when the percentage of nondetects is small (below 20%) we can ignore those nondetect data (Najera et al. 2020).

Results

The intensity levels varied from one microenvironment to another. The results show the highest intensity levels; median values were recorded in the Zócalo. These are events higher than inside the subway, a transport used by great numbers of people who use the Wi-Fi service (Table 1).

Table 1 Descriptive analysis of each one of the studied microenvironments and by frequency band
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If we compare the results, we can see that the mean highest value was 2370 μW/m2 for the 5.85-GHz Wi-Fi band when the researcher was in the Zócalo. And the lowest value was 31.8 μW/m2 for the 5.85-GHz Wi-Fi when the researcher was walking on the street.

Figure 2 shows that the intensity levels decrease on leaving the subway, while walking on the street and inside the home without using Wi-Fi connection.

Fig. 2
figure 2

Personal exposure to RF-EMF recorded in the various microenvironments measured. Intensity of electromagnetic wave in μW/m2. Average over 6 min

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If we analyze the data from the 2.4-GHz Wi-Fi band, throughout the whole measuring period, taking into account an average time of 6 min for each measurement (Fig. 2), the highest value was 2800 μW/m2 in the Zócalo and inside the subway, and the lowest level was 14.0 μW/m2 in the subway.

In Fig. 3 we can see the exposure levels for each Wi-Fi frequency band (2.4- and 5.85-GHz Wi-Fi). Figures 4 and 5 present the distribution of the data for each of the measured microenvironments, in each of the measured Wi-Fi bands (2.4- and 5.85-GHz Wi-Fi, respectively).

Fig. 3
figure 3

Box plot of personal exposure to RF-EMF by Wi-Fi band. Intensity of electromagnetic wave in μW/m2

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Fig. 4
figure 4

Violin plot of personal exposure to RF-EMF by microenvironments in the 2.4-GHz Wi-Fi band. Intensity of electromagnetic wave in μW/m2

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Fig. 5
figure 5

Violin plot of personal exposure to RF-EMF by microenvironments in the 5.85-GHz Wi-Fi band. Intensity of electromagnetic wave in μW/m2

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In Fig. 3 we show two whisker plots, one for each of the Wi-Fi bands. The intensity of the 5.85-GHz band has an upper whisker that is considerably larger than the whisker of the other band, 2.4 GHz, which means that there are higher values in the 5.85-GHz band. The average of the second band is three times the mean of the first band; this is due to the fact that high values were measured in some points in the 5.85-GHz band, as will be observed later in the interpolation map of this band (Fig. 7).

Figure 4 shows four violin diagrams, on the street (walking), in the subway, in the Zócalo plaza, and at home, in the 2.4-GHz Wi-Fi band. The two violins in the subway and in the Zócalo are similar in height, while the others are much smaller, less than a third. That is, the highest values that were measured were in the Zócalo and in the subway, data that we can see in Table 1.

Figure 5 shows four violin diagrams, on the street (walking), in the subway, in the Zócalo plaza, and at home, but this corresponds to the 5.85-GHz Wi-Fi band. The Zócalo violin is much higher than another three, which means that this band is the dominant one in the Zócalo; we can see it in Fig. 3, and we will also see it in the interpolation map of this band (Fig. 7).

In Figs. 6 and 7, we can see the levels of intensity of the RF-EMF recorded at strategic points in the Zócalo, spot measurements (at each point indicated by a letter A to P) and interpolated with the Kriging method, both in the 2.4- and the 5.85-GHz Wi-Fi band, respectively.

Fig. 6
figure 6

Electromagnetic wave intensity map through Kriging interpolation, 2.4-GHz Wi-Fi band. Intensity of electromagnetic wave in μW/m2

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Fig. 7
figure 7

Electromagnetic wave intensity map through Kriging interpolation, 5.85-GHz Wi-Fi band. Intensity of electromagnetic wave in μW/m2

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The Kriging interpolation method (Figs. 6 and 7; interpolated intensity maps) shows the points at which the measurements were the highest (shown in red) and the lowest (shown in blue). This revealed that higher average intensity levels are mainly concentrated near to located free Wi-Fi hotspots; it is in the Zócalo center and near to the shopping center.

In the map of intensity of Fig. 6, we see the results from the 2.4-GHz Wi-Fi band. The highest values are in the center of the square, points M, N, O, and P. We highlight that the highest values calculated using the Kriging method are close to 915–1350 μW/m2, mainly at point O, and it spreads to the square. The lowest levels are around 79.2–128 μW/m2 at points C, D, E, H, I, K, and L, all this in the 2.4-GHz Wi-Fi band.

In the case of 5.85-GHz Wi-Fi band, in the map of Fig. 7, we can find that the highest values are found at points E and I, with an approximate value of 915–7380 μW/m2, also calculated with the Kriging method. These points are located near Wi-Fi antennas. The lowest values are found at points A, L, N, and P, with an approximate value of 376–905 μW/m2.

The values calculated and interpolated with the Kriging method (considering the recorded measurements at each indicated points, A to P) were verified with the measurements to confirm that the approximate calculations produced with the said method are correct. In fact, they were correct as we can see now. For example, at the highest points where a value of 1350 μW/m2 was obtained close to the point O (2.4-GHz Wi-Fi band; Fig. 6), the exposimeter measured a value of 1390 μW/m2. While in the case of 5.85-GHz Wi-Fi band (Fig. 7), where we obtained a value of 7380 μW/m2 near points E and I, the exposimeter registered a value of 7350 μW/m2.

The levels of intensity are far from the maximum allowed in the international reference levels established by the ICNIRP (2020), which is 10 W/m2, reference adopted and applied in Mexico by Federal Institute of Telecommunications (IFT, 2020).

Comparing the results of the 2.4- and 5.85-GHz Wi-Fi bands, we observe that the band with highest contribution is the 5.85-GHz Wi-Fi band (Figs. 6 and 7). The mean highest was obtained in the 5.85-GHz Wi-Fi frequency band with a value of 7380 μW/m2.

In reviewing results of previous studies from different countries, we can see that our data were similar, as in the case of the overview by Chiaramello et al. (2019) concludes with a value of 2500 μW/m2 (0.97 V/m) in public transport, or with the results published by Joseph et al. (2010), in which recorded a value of 13 μW/m2 at home in the W-LAN band (in Belgium). Our results were also comparable with another study conducted in Spain, for example, an average value of 211 μW/m2 in the 5.85-GHz band, outdoor around the school building (Ramirez-Vazquez et al. 2023a), or other studies carried out in Europe and other countries (Ramirez-Vazquez et al. 2023b), for which they are also values well below the ICNIRP limits (ICNIRP, 2020).

Discussion

The study has been carried out without incidents, in the same way it was planned, although with much care since the researcher had to carry the exposimeters tied to her waist during the measuring process. The Zócalo has an area of 21,600 m2, approximately, and more than a hundred thousand people can be found there at any given time.

When we analyze the results of Figs. 6 and 7, we can observe results which may seem contradictory. The day on which the measurement at the Zócalo took place, the highest values occurred when there were a lot of people there, mainly tourists using internet and a protest of the union of workers called Coordinación Nacional de Trabajadores de la Educación in Spanish (CNTE). As an estimate, we could say there were at least 10,000 people inside the square.

As a result, there were many mobile phones being used, and that fact allows us to explain why there was an intensity of 1350 μW/m2 (1390 μW/m2 measured) in the 2.4-GHz frequency band in the center of the Zócalo. This value was mainly due to the general public’s own terminals connected to the free Wi-Fi hotspots. The antennas are connected to the phones, but since there are several antennas, the hotspots are shared between all of them.

In the case of the intensity registered in the 5.85-GHz frequency band, in Fig. 7, we can see that the highest levels of intensity are found at points E and I, around Zócalo, with a value close to 7380 μW/m2 (7350 μW/m2 measured). These points are very close to the location of the free Wi-Fi hotspots (Figs. 6 and 7). And as we can see, the lowest values are at points A, L, N, and P, mainly at point L, with an approximate value of 905 μW/m2. If we look at Fig. 1, we realize that near points A and L there are no hotspots nearby; therefore, these values come from the use of the Wi-Fi by people and shops that are near those points.

In addition, point I, where we have one of the highest values, is near the Old Town Hall. In this building there are many offices with municipal workers connected to internet, through cable and Wi-Fi, presumably 5.85-GHz frequency band. This allows us to explain such a high value in the Zócalo area.

The other point where a maximum value was registered was point E, a point near shopping centers and crowded streets full of tourists who in all probability carry the latest mobile phone which they connect to the 5.85-GHz frequency band.

As we have pointed out previously, measurements were carried out in the center of Zócalo, so as to compare the values obtained with the exposimeters to the values calculated with the Kriging method. In Table 2, we summarize the data measured in the point Q, which is located in the middle of points M, N, O, and P in Figs. 6 and 7.

Table 2 Average values and values interpolated using the Kriging method of wave intensity in the center of Zócalo, point Q
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Therefore, we can conclude that interpolation with the Kriging method is reasonably good. We obtained similar data in the measurements carried out in a Jordan University (Ramirez-Vazquez et al. 2020). The levels of intensity are far from the maximum allowed in the international reference levels established by the ICNIRP (2020) and IFT (IFT, 2020), which is 10 W/m2. If we use the logarithmic scale, the highest level is 31 dB below the maximum allowed.

We have compared the results of this work with the results of a recently published comment (Arribas et al. 2022) and other studies and the values are comparable. For example, the mean recorded in Spain by Gallastegi was 623 μW/m2 in the parks (outside) from total RF exposure and 52.1 μW/m2 from frequency band recorded at home, both based on spot measurements (Gallastegi et al. 2018).

These results are comparable with the mean value measured in a Mexican city (Tamazunchale, San Luis Potosí), with the participation of volunteers, in which a mean value was 212 μW/m2 outside and 487 μW/m2 at workplace (both in the 5.85-GHz frequency band). They are also comparable results with those obtained by Velgue (Velghe et al. 2019), measurements carried out in different places in Belgium, where the highest mean total exposure was found in Brussels with 2630 μW/m2 (0.00263 W/m2) which is also a very small value compared with the ICNIRP reference levels (and below the restrictive limit applied in Belgium, 0.0239 W/m2 and 0.0477 W/m2).

And as in all the studies carried out with exposimeters, due to the sensitivity of the personal exposimeter, nondetect values were recorded, which were treated as discussed in the “Methodology” section. Exposimeters have a high resolution in measurements over long periods of time, but a limitation is a high percentage of non-detection, which must be treated according to nondetect percentage (Najera et al. 2020). Personal exposure to RF-EMF characterization is influenced by nondetect data, and this problem makes it difficult to conclude and compare results obtained in different studies.

Conclusion

The aim of this study was to measure personal exposure to Radiofrequency Electromagnetic Fields from the 2.4- to 2.5-GHz frequency band and 5.15- to 5.85-GHz frequency band in the Zócalo inside public transport “subway,” on the street, and at home, in Mexico City.

The mean values recorded in the measured points at the Zócalo (walking and spot measurements) were 326 μW/m2 in the 2.4-GHz frequency band and 2370 μW/m2 in the 5.85-GHz frequency band. In the case of the mean values recorded on the street, they were 119 μW/m2 in the 2.4-GHz frequency band and 31.8 μW/m2 in the 5.85-GHz frequency band.

If we compare the results of the measurements and the resulting values after applying the interpolation method, we observe that the results of the interpolation with the Kriging method are reasonably consistent with the measurements carried out in situ. The Kriging interpolation gives us a good visualization of the spatial distribution of RF-EMF exposure in the study area, showing the highest and lowest intensity levels.

In the case of the measurements carried out inside the subway, the mean value recorded was 680 μW/m2 in the 2.4-GHz frequency band and 193 μW/m2 in the 5.85-GHz frequency band. The values are well below the reference levels established by the International Commission on Non-Ionizing Radiation Protection and the limits adopted by the Federal Institute of Telecommunications.