Radon Dose Determination and Radiological Risk in Some Mexican Caves with CR-39 Detectors

Received: September 24, 2020 Accepted: January 25, 2021 Published Online: February 10, 2021 Radon (222Rn) is a radioactive gas, from the 238U decay chain, that contributes in large part of the natural radiation dose to which humans are exposed. Radon is the second cause of lung cancer after tobacco. The US-EPA considers a concentration of 148 Bq/m3 for homes and 400 Bq/m3 for workplaces as the reference level. Caves are closed spaces where 222Rn, which emanates from the surrounding minerals and rocks, can accumulate and reaches large concentrations that can represent a health risk for the guides, speleologists and visitors who spend time in these spaces. This work uses the previously recorded radon concentrations in 8 caves in Mexico and calculates the average dose range and the average annual dose for each of them with the “Wise” public domain program (http:// www.wise-uranium.org/rdcrn.html) to determine the level of radiological risk with 2,000 1,000 and 500 working hours per year. Karmidas cave had the highest average 222Rn concentration with 27,633.3 Bq/m3 and for 2,000 working hours per year an average annual dose rate of 347.1 mSv/y. Los Riscos cave had the lowest average concentration with 384.7 Bq/m3 and for 2,000 working hours per year an average annual dose rate of 4.832 mSv/y. These results show that all the caves studied present values, with 2,000 working hours per year, that exceed 3 mSv/y for workplaces and must be considered by the people who work in these places. A casual tourist visiting the caves does not present any radiological risk, while guides and speleologists should consider it.


Introduction
Radon ( 222 Rn) is a naturally occurring gas product of the decay of radium ( 226 Ra), it has a half-life of 3.8 days, is part of the uranium ( 238 U) decay chain and it is the main source of natural radiation to which the human being is exposed. According to the United States Environmental Protection Agency (US-EPA) and the World Health Organization (OMS) 222 Rn is the second cause of lung cancer after the Tabaco. In nature, 222 Rn emanates constantly from the soil and reaches concentrations of less than 50 Bq/m 3 [12] because it's constantly being diluted on atmosphere; however in closed spaces like caves or mines it can accumulate to reach concentrations of KBq/m 3 [1][2][3][4][5][6]. Caves are cavities that are formed in the interior of the earth due to different geological, chemical, volcanic, wave and wind processes and can extend up to several kilometers and reach great depths. These geological formations have been exploited by man throughout his history in many different ways, for example, they were used as housing by our ancestors approximately 5,00,000 years ago [8]. Inside these places the 222 Rn concentrations may vary due to internal factors such as ventilation, types of surrounding minerals, length of the cave; also to external factors such as atmospheric conditions and geographical location [7].
Mexico is a country with around 7,000 caves in its territory [10] and many of them are a natural attraction for national and foreign visitors like Cacahuamilpa Caves, while others are frequented by speleologists and researchers, many of whom spend a lot of time inside them. Several studies have measured the concentration of 222 Rn inside caves in different countries such as Ireland [4], Slovenia [11], Hungary [13], Jamaica [9], Brazil [1], Ecuador [14] among others. In México Borau et al. (1993)  Rn concentration in two caves in the state of Querétaro and two in the state of Puebla, where they also observed an interesting coincidence between 222 Rn concentrations and p.170 bat distribution; these animals are distributed in areas with medium and low gas concentrations.
Due to its effect on health, it is important to monitor the 222 Rn present in caves in order to know the radiological risk to which people who frequent these places may be exposed [1]. The main of this work is to calculate the dose of 222 Rn, based on data previously collected and published in eight caves in the Mexican Republic to determine the radiological risk to which guides, speleologists who frequent these spaces may be exposed.

Global Regulations and Action Levels
Action levels and regulations for 222 Rn vary from country to country, e.g. in the United States indoor action levels are 148 Bq/m 3 in homes and 400 Bq/m 3 in workplaces [16]; in the United Kingdom workplace action levels are 400 Bq/ m 3 and dose limits are 5 mSv/year. The European Union has action level ranges from 500 Bq/m 3 to 1,500 Bq/m 3 for workplaces. Mexico does not have regulations regarding radon levels in enclosed public and private spaces; for this reason, the regulations established by the US-EPA for intramural radon concentrations are taken into account.

Methodology
For this work the results of Borau et al. (1993) and Espinosa et al. (2008Espinosa et al. ( , 2013 were used. These authors used the technique of nuclear track in solids in CR-39 plastic (Lantrack®) detectors to measure the concentration of 222 Rn in eight caves in the Mexican Republic. This method is the most suitable for this type of environment since it is low cost, can be used for long periods of time, is not affected by environmental factors such as temperature and humidity, is easy to transport and handle, and has a high efficiency in the detection of alpha particles. Total exposure times for detectors inside the caves were between three and six months ( Table 1)

Caves Description
Aguacachil cave is located in the state of Guerrero ( Figure 1), at 18°35'30" north and 94°34'2" west, in a region with an altitude of 1,500 m above sea level. It has 1,618 m in its course, 169 m of maximum depth ( Figure 2). Four sampling points were established where the CR-39 detectors were placed.
Las Pozas Azules are located in Acuitlapan, also in the state of Guerrero (Figure 1  The San Jerónimo cave is located near Tenancingo, also in the state of Guerrero (Figure 1), at 18°40'11'' north and 99°30'36'' west. The San Jerónimo River runs through this place. Four sampling points were established where the CR-39 detectors were placed. It is a space visited by tourists and speleologists who can take several hours to visit this cave ( Figure 2).
Gabriel cave is located in the southern state of Oaxaca at 18°27'25'' north and 96°40'34'' west at an altitude of 110 m above sea level. The cave ( Figure 2) is 1,500 m long, with a chimney 250 m from the entrance. Fifteen sampling points were established where CR-39 detectors were placed. The temperature average is 25.2° C.
Los Riscos cave is located in the state of Querétaro (Figure 1) at 21°11'38'' north and 99°30'50'' west at an altitude of 1,122 m above sea level, with a length of 550 m and a depth of 39 m (Figure 2), an average temperature of 18°C and a relative humidity of 85%. Twelve sampling points were established where the CR-39 detectors were placed.
El Judío cave, also located in the state of Querétaro (Figure 1), at 21°11'38'' north and 99°30'50'' west, is made of limestone, is 48 m long and 25 m deep, has two entrances, an interior temperature of 21° C and a relative humidity of 50% ( Figure 2). Six sampling points were established where the CR-39 detectors were placed.
The Coyozochico cave is located in the Sierra Norte of the state of Puebla (Figure 1) at 19°57'00'' north and 97°24'36'' west. It has a length of 170 m, an average temperature of 19° C, 68% relative humidity (Figure 2). Seven sampling points were established where CR-39 detectors were placed.
Karmidas Cave is located in the state of Puebla ( Figure 1) at 19°58'10" north and 97°38'36" west. It is 260 m long (Figure 2), has an average temperature of 18° C and 65% relative humidity. Nine sampling points were established where the CR-39 detectors were placed.
For more detail description of each caves consult Borau et al. (1993) and Espinosa et al. (2008Espinosa et al. ( , 2013. The dose calculation for each sampling point of the areas studied was carried out using the public domain software "Wise" [17], to determine the radiological risk to which guides and speleologists who frequent these sites may be exposed. Three different times were considered for doses calculation: 2,000, 1,000 and 500 hours per year and an equilibrium factor of 0.4 was considered for 222 Rn and its children, since this is an average value among the values reported in the literature [15]. U and 226 Ra present in the minerals of each cave, rock porosity, the ventilation of the site which may be due to the presence of chimneys or the proximity to the cave entrance; the presence of rivers and water accumulation as well as the internal temperature of each cave and earthquakes are factors that affect the concentration of 222 Rn [3][4][5]. All these characteristics of each cave make them unpredictable and dynamic environments, which can be influenced by external factors such as rain, wind, altitude, atmospheric pressure and season. For this reason it is necessary to carry out constant measurements to better understand the dynamics of these environments.   concentration in table  1 for the eight caves studied. With 2,000 working hours per year all caves exceeds the reference value of 3 mSv/y for workplaces; Karmidas cave has the highest value of annual dose rate (347.1 mSv/y) and Los Riscos cave has the lowest (4.832 mSv/y). A tourist guide or a speleologist who spends 2,000 hours in any of these caves should consider the radiological risk to which they are exposed. With 1,000 working hours per year, the caves that present an average annual dose below the reference limit are El Judio (2.962 mSv/y) and Los Riscos (2.416 mSv/y); the Coyozochico cave presents a value of 3.215 mSv/y slightly exceeding the reference value.

Results and Discussion
With 500 working hours per year Karmidas cave still exceeds the reference valuefor workplaces recording an average annual dose value of 86.8 mSv/y, 29 times higher than the 3 mSv/y. Aguacachil and PozasAzules caves present values of slightly higher than the reference value with 3.643 mSv/y and 3.596 mSv/y respectively. Table 3 shows the number of hours required by an individual in each cave to reach the annual reference level of 3 mSv/y and the percentage of risk of cancer. These calculations were also done with the "Wise" program [17]. In Mexico speleologists and cave guides have a radiological risk associated with their profession that they should be aware of. The doses of environmental radiation they receive during the tours they take in these environments vary from cave to cave and must be measured constantly. Therefore, the use of portable monitors will be of great help in regulating the times of stay and the length of the tour.

Conclusions
All studied caves have high 222 Rn concentration. The doses calculated for each cave with 2,000 working hours per year exceed the recommended doses for workplaces. It is recommended that guides and speleologists routinely use portable radon monitor so they can know the dose of environmental radiation they receive during their tours. Tourists who occasionally visit the eight caves studied and stay a few hours in them will not exceed the recommended annual limit and therefore will not present a radiological risk.

Competing Interests
The corresponding author declares no conflicting of interests.