Introduction

Solar radiation reaching Earth's surface and passing through the atmosphere is divided into two components: direct and diffuse. Direct radiation comes from the Sun without being scattered by Earth's atmosphere and diffuse radiation is defined as the radiation received from the Sun after its direction has been changed by atmospheric phenomena of absorption, scattering, refraction, and reflection of light as it is scattered by dust, aerosols, and atmospheric particles. The sum of direct and diffuse components constitutes the total global solar radiation. UV radiation, microwaves, infrared radiation (heat), visible light, X-rays, and gamma radiation, together form the entire spectrum of solar electromagnetic radiation (^{Duffie & Beckman, 2013}). Solar radiation is beneficial to humans, as it is essential in the production of vitamin D; however, prolonged exposure causes alterations associated with aging, skin modifications, skin cancer, skin allergies, and other existing skin conditions (^{Benedetti, 2019}).

Overexposure to UV radiation can result in acute and chronic diseases with harmful effects on the skin, eyes, and immune system; in the long term, radiation-induced degenerative changes in cells, fibrous tissue, and blood vessels lead to premature skin aging. Ultraviolet radiation can also cause inflammatory reactions in the eye, such as photokeratitis. Chronic effects include two major health problems: skin cancer and cataracts (^{Samaniego Rascón et al., 2017}).

UV radiation represents only a small region of the electromagnetic spectrum, from 400 to 100 nm, and is divided into three ranges, UV-A 400-315 nm, UV-B 315-280 nm, and UV-C 280-100 nm. As sunlight passes through Earth's atmosphere, the UV-C range and approximately 90 % of the UV-B range are absorbed by ozone, water vapor, oxygen, and carbon dioxide present in the atmosphere. Only UV-A radiation is less affected as it passes through the atmosphere, so UV radiation reaching the Earth's surface is largely composed of UV-A and a small UV-B component (^{World Health Organization [WHO], 2002}). UV-B radiation can penetrate ecological, aquatic, and biomass production support systems affecting the entire ecosystem food chain. The concentration and chemical composition of biological organisms are altered by climate variability and UV-B radiation, and cause alterations in the upper trophic levels of ecosystem structure (^{Häder et al., 2007}). UV radiation is variable, depending on site latitude, day hour, season, and sky conditions (^{Basset & Korany, 2007}). UV radiation fluctuates with time, especially in the 27-day solar rotation and the time scales of the 11-year solar cycles. Even high UV radiation levels have an impact on climate change (^{Floyd et al., 2002}), such as the Haigh-Shindell model, which suggests that UV radiation variability has an impact on global temperature. Terrestrial measurements of UV radiation with high quality and scientifically reliable instruments are limited, and although there are non-scientific instruments that are freely marketed to evaluate the UV index (UVI), the results are not reliable; comparative studies between commercial and scientific instruments indicate differences between the UVI greater than 50 %, so they should be inspected by government health agencies (^{Corrêa et al., 2010}). Another option to evaluate the UV index is atmospheric monitoring satellite tools, which are used as data sources to create a climatology of the UVI at local noon. A study conducted by Valdés-Barrón pointed out the importance of measuring surface UV-B radiation, due to its implications for public health ^{(Valdés-Barrón et al., 2013}). In this work, the author states that the methods for estimating UV-B radiation with satellite measurements are useful, but their certainty is reduced due to the low temporal resolution of the data, the procedure takes a daily measurement, during the same hour; therefore, the uncertainty to know the availability and variability of the UV index, is high. Furthermore, satellite measurement per se is not intended to measure throughout the day; its objective is to spatially cover as many areas on Earth as possible. The satellite-derived UV index usually corresponds to extrapolation to solar noon, from the measurement made at the hour passing through a city/region.

The UV index was internationally adopted to raise awareness and encourage the public to protect their skin, especially from melanoma. The UVI, a measure of UV radiation levels at the Earth's surface, was published in 1995 as a result of a collaboration between the WMO, WHO, the United Nations Environment Program, the International Commission on Non-Ionizing Radiation Protection, and the German Federal Radiation Protection Office Agency. According to another author, the UV index was created to establish an information criterion regarding the potential damage of overexposure and prevent its consequences (^{Chambi, 2018}).

To measure the biological effectiveness of UV-B radiation, the expression "Minimum Erythemogenic Dose" (MED) is used and is defined as the dose capable of originating skin erythema of noticeable limits at 24 h of exposure. The UV radiation dose in MED/hour is calculated as 1 MED/h = 5.83 ∗ 10-6 [W/cm²] effective power which relates time and effective power of exposure to UV-B radiation ^{(Solar Light Co. Inc., 2006).} The harmful effects of UV-B radiation on living beings are evaluated using instantaneous radiation magnitude with units of power per unit area. The energy level contained in a MED is equal to 210 J/m². A study conducted in Mexico showed that white-skinned people, when exposed to the sun during the middle of the day during the summer, get burned in 21-33 minutes, while dark brown-skinned people take 39-73 minutes to suffer a burn, however, the latter group has the risk of chronic exposure without warning consequences ^{(Castanedo Cázares et al., 2012)}. The representation of effective power of UV-B radiation in MED/h is little used, due to the confusions that arise from its interpretation, since it is taken as a reference the reddening of the skin of people with low skin pigmentation or light tone. Therefore, to effectively represent and communicate the magnitude of UV-B radiation, the UV index is used, which is equivalent to the MED/h. Due to the lack of public understanding of the MED, a table was created with general values from 0 to 12, with 0 being the lowest value and with the lowest risk and 12 as the highest value, indicating extreme risk, as shown in Table 1 below.

The WHO calls on governments of countries with high levels of UV radiation to encourage the use of UV index information as part of public awareness programs, to support the establishment of shaded areas in schools, parks, public places, and swimming pools, and to train broadcasters, meteorologists and reporters to use and provide information on UV index variability to the public (Rehfuess & Organization, 2002). Hence the importance of presenting this type of information in a creative way that allows people to enjoy the sun safely, but at the same time raises awareness to avoid overexposure.

A study was carried out on a group of Mexican population over 15 years of age, on their knowledge and attitudes regarding the time of sun exposure to identify their phenotypic characteristics and their reactivity to solar radiation, to know their exposure habits and preventive measures, and to evaluate their general knowledge regarding the harmful effects of the sun on the skin, indicates that, although most of the population recognizes that the sun causes aging and skin cancer, their exposure is prolonged. The authors suggest increasing efforts to raise awareness among the population about the benefits of appropriate photoprotection through clear and concise communication and the need to focus essentially on vulnerable and high-risk groups and/or those who are exposed to solar radiation for prolonged periods for work or recreational reasons (^{Castanedo-Cazares et al., 2006}).International studies pointed out, that even among individuals with knowledge of UVI, there is a need to effectively communicate information, for adequate behavior, that promotes protective measures (^{Heckman et al., 2019}). Many people see tanning as a way to improve appearance, and in the face of this, UV index information has been misused by some to accelerate tanning and promote unprotected exposure, justifying appearance rather than modifying behavior towards protection. Given this, some research groups propose targeting subsets of the population, calling them to action with anti-tanning legislation, age- or gender-specific screening campaigns, anti-tanning beauty campaigns, UV photography, and other targeted strategies. Photoprotection messages should move from education to behavior-based targets, focusing on interventions that inspire change rather than simply promote knowledge (^{Goulart & Wang, 2010}).

Mexico ranks sixth worldwide in tourism with more than 20 million tourists per year since the beginning of the 21st century. Data from the World Tourism Organization, indicate that tourism in Mexico generated 8.2 % of the national GDP in 2018, of which 45 % was produced by coastal areas (^{Hoil et al., 2020}). Some parameters that determine a good tourist beach area, according to the hierarchical analytical model, are divided into physical parameters, including temperature, waves, solar radiation, humidity, and wind speed; biological parameters as are watercolor, sand type, landscape, type of vegetation and microorganisms, and urban parameters; including noise, lifeguards, garbage, access and infrastructure (Hoil et al., 2020). The experience of changing the routine and the way they live is what tourists enjoy (^{Bachimon et al., 2016}). However, in sun and beach tourism it is necessary to take care of sun exposure, decide the time and amount of time that can be left, and be aware that prolonged exposure times, cause damage to the skin; they are immediate, uncomfortable, and painful, and then the experience is not as good as expected. Post-vacation behavior depends on the experience they have had in the tourist destination, and if the experience is satisfactory, there is a greater possibility that tourists will return or recommend that destination (^{Araslı & Baradarani, 2014}).

The state of Nayarit has 320 km of coastline with the Pacific Ocean and an infrastructure of 780 hotels. The Bahía de Banderas municipality, on the southern coast of the state, receives the largest number of national and international tourists and has 214 lodging establishments. Through tourism, it has been possible to reduce marginalization and poverty rates and combat unemployment, and has improved the local life quality, making tourism an important economic activity for the state, contributing to 73.4 % of the local GDP (^{Moy, 2021}).

In the present work, UV-B radiation levels for the city of Tepic, Nayarit are evaluated with surface measurements obtained from the Mexican Solarimetric Network (RESOLMEX). Also, the level of knowledge of tourists arriving on the southern coast of Nayarit about the damage caused by exposure to the sun and UV radiation. From this study, it is possible to design strategies and recommendations to warn tourists about the damage caused by prolonged sun exposure. Given that this knowledge helps tourists to become aware, reduce exposure time, and adapt their behavior to live a pleasant and responsible tourist experience that above all takes care of their health from the damage caused by overexposure to UV radiation present in sunlight.

Methodology

In the present work, a descriptive study of the availability and variability of UV-B radiation was carried out. The information was acquired from a reference solarimetric station that is part of the RESOLMEX in charge of the Mexican Solarimetric Service of the Geophysics Institute of the UNAM. To measure UV-B irradiance, a 501A ^{(Solar Light Company Inc, 2006)} biometer-radiometer was used, which has a spectral response of 250 to 380 nm, with an output range of 0 to 10 MED/hour, connected to a CR3000 data acquisition system from Campbell Scientific, Inc. Figure 1a shows the 501A biometer-radiometer sensor installed in the solar station. Figure 1b shows the Campbell Scientific, Inc. CR3000 data acquisition system. The data acquisition system takes a sample every 3 seconds and stores the arithmetic mean of the sample every minute. The data is stored in an NL 116 Ethernet interconnection and storage module. A network interface provides remote access for data download using LoggerLink software. The solar station began operations in March 2016, the calibration of the sensors is performed biannually and is in charge of the Institute of Geophysics of UNAM.

Figure 1 Setup of the 501 biometer-radiometer on the platform of the solar station of the Mexican Solarimetric Network located at the central campus of the Universidad Autónoma de Nayarit (a); and Data acquisition system CR3000 from Campbell Scientific Inc (b). 

Through non-participatory observation, from a social, cultural, and environmental perspective, using the field diary as a tool, the experience of tourists arriving at Nayarit's coasts was studied in relation to their sun exposure. A survey was applied, using the questionnaire as a tool to evaluate the experience lived by sun and beach tourists during their recreational activities. The previously designed questionnaire was validated by experts in social sciences, energy, and health of the academic body UAN CA 01 Natural Resources, this allowed obtaining information on the knowledge that people who perform sun and beach tourism activities have about the damages of exposure to UV radiation and obtaining data on practices related to exposure to solar radiation, and through indicators such as the hours of exposure and the level of knowledge about solar radiation and the UV index to determine the impact on the experience lived in the tourist destination.

Results and Discussion

UV index evaluation through surface measured data.

The solar station is located at geographical coordinates 21.5° N, 104.9° W at 928 masl, at the central campus of the Universidad Autónoma de Nayarit, in the city of Tepic, Nayarit. Figure 2 shows the macro-location of the study sites.

Figure 2 Macro-location of the solar station sites in Tepic City, Nayarit, and the municipality of Bahía de Banderas on the southern coast of the state. 

For solar UV-B radiation, up to 1440 measurements were recorded each day, so 1,006,848 measurements were analyzed (100 %) distributed in the period from 2019 to 2020, where all atmospheric conditions were considered, for which, by visual inspection, the historical series graphs were reviewed to select only complete daily records, in total 826,560 measurements were selected (82 %), and through data validation, out of range and negative values were discarded, giving a total of validated data by visual inspection and range filter of 821,535 measurements (81. 6 %); the equivalent of 570 days analyzed. The final results were processed by setting the same percentage value with the average of each month, so the final value represents the annual average of the behavior of the UV index, composed of the monthly average values, regardless of the different number of records for each month analyzed.

Figure 3 shows the descriptive statistics of the behavior, availability, and variability of the surface UV index. Six curves are observed: i) The arithmetic mean of the UV index value, which reaches a maximum value of 10.06 or very high), ii) The first quartile of measured data (25 %) which is below an index of 9. 17 or very high; iii) The second quartile (Q2 or median) which represents that 50 % of the measured and sorted data is below an index of 10.64 or very high; iv) The third quartile (Q3) represents that 75 % of the measured and sorted data is below an index of 11.46 or extreme. Percentile values (v) 5 and (vi) 95 state that, for Tepic, Nayarit, the maximum value of the annual UV index ranges from 6.28 (high) to 12.4 (extreme), at least 50 % of the days of the year, the UV index ranges around 10.64 (very high), with variability from maximum indexes of 9.17 (very high), to 11.46 (extreme).

Figure 3 Descriptive statistical analysis of the UV index in Tepic, Nayarit. 

Figure 4 shows the average hourly behavior of the monthly UV index. The highest risk or extreme values occur between 12:00 and 14:00 hours during the months of March to June when solar declination is positive and there is more energy per unit area, in addition to the fact that these are the driest months of the year and there is little or no cloud cover, and these are hours when the population in general and tourists in particular, carry out a large number of outdoor activities. It is also observed that during the months of February to October between 11:00 and 15:00 hours, there are very high-risk values and they are present almost all year round at the times when people carry out the greatest amount of outdoor activities.

Figure 4 Average hourly UV index for each month in Tepic, Nayarit. 

The UV index values of low and moderate risk cover a greater part of the day throughout the year, these values occur at times when people are mainly active indoors. Finally, high values are scarce since they are transitional.

Figure 5 shows the absolute maximum values of the UV index. A bimodal trend behavior biased to the right is observed, where the mode is distributed between values from 13 to 15 or extreme. The results suggest that for more than 7 months, maximum values of UV index are extreme risk values, the rest of the year, maximum values of UV index remain in a very high-risk interval. Maximum values of UV index in the high-risk level are rare and are very unlikely to occur.

Figure 5 Annual distribution of the maximum UV index in Tepic, Nayarit during the period 2019-2020. 

Experience of sun and beach tourists on the Nayarit coast as influenced by exposure to solar radiation.

From a total of 140 randomly selected respondents of legal age who were engaged in tourism activity in all-inclusive Sun and Beach hotels on the southern coast of Nayarit during the September-October 2021 season, 11 have a master's degree, 88 have a bachelor's degree, and 41 have a high school education. Figure 6 shows the responses of the respondents on the reasons and their experiences during their stay at the destination. Fifty-nine percent of the people surveyed made their trip for rest, 20 % for fun, 6 % for work and 15 % for leisure. When asked about their experience during their stay in the destination, 49 % of respondents answered: "How was your experience during your stay in the destination? Forty-nine percent of the respondents answered that their stay was excellent, 46 % that it was good, 5 % regular and none said it was bad. On the possibility of returning to the place, 94 % gave positive answers about their return and only 6 % answered negative opinions.

Figure 6 Experience of sun and beach tourists on the Nayarit coast. 

Tourist knowledge about solar radiation and the UV index

Figure 7 shows the results of tourists' knowledge of solar UV radiation. Of 140 respondents, 78 % responded that they do know what UV radiation is and 22 % do not know what it is. When asked what they understand by UV radiation, 24 % responded that UV radiation is a type of electromagnetic radiation, 35 % associated it with the color of sunlight, 18 % mentioned that it is solar energy, and 23 % admitted not understanding what UV radiation is. They were also asked “Where do you think UV radiation comes from?”, to which 47 % said that the radiation originates in the atmosphere, 29 % mentioned that it originates in the Sun, 12 % that it originates in cities and 11 % answered that it is emitted by lamps. Regarding the respondents' knowledge of the UV index, to the question “Do you know the values of the UV index?” 49 % answered that it is expressed by the color green, yellow and red, 37 % answered that the indexes are UV-A, UV-B, and UV-C and 14 % answered that the UV index is classified from 1 to 11. Finally, they were asked, “In what season of the year do they consider sun exposure is more dangerous?” Of this, 26 % said in spring, 52 % in summer, 19% in autumn, and only 3 % in winter.

Figure 7 Tourist knowledge of solar radiation and UV index. 

Evaluation of tourist practices related to solar radiation exposure

The relationship between tourist recreational practices during their stay at the destination and solar exposure was determined through non-participatory observation and the survey. Figure 8 shows the responses of the tourists when asked “At what time of the day do you do your recreational activities?”, of the 140 respondents 2 % responded that from 9:00 to 11:00 a.m. they do these activities, 62 % responded that they do their recreational activities from 11:00 a.m. to 2:00 p.m. and 36 % responded that from 2:00 p.m. to 6:00 p.m. They were also asked the question "What time do you consider that outdoor UV radiation is most dangerous?" 25 % responded that it is from 9:00 am to 11:00 am, 74 % of the respondents consider that the time when radiation is most harmful is from 11:00 am to 2:00 pm, and 1 % responded that it is from 2:00 pm to 6:00 pm.

Figure 8 Tourist practices linked to solar radiation exposure. 

Finally, they were asked if they consider important the alerts on the intensity of the UV radiation index in the atmosphere, to prevent solar burns, to which 94 % of the 140 respondents answered that they did consider it important to be informed about this while 6 % did not consider it important.

Discussion

Estimating the risk of harmful effects of solar UV radiation requires detailed knowledge of the intensity and spectral composition of the global radiation reaching the Earth's surface (^{Seidlitz et al., 2001}). In this work, it is demonstrated for the city of Tepic, that the highest risk or extreme UV index values occur between 13:00 and 14:00 hours during the months of March to June when the solar declination is positive and there is more energy per unit area, in addition to the fact that these are the driest months of the year and there is little or no cloud cover, and that these are hours when the population in general and tourists in particular, perform a large number of outdoor activities. The results coincide with those reported by Castanedo-Cazares and collaborators who found extreme exposure risk values during those months of the year, for a study conducted in the city of San Luis Potosí ^{(Castanedo-Cazares et al., 2012).} It is important to mention that not only the frequency of the presence of extreme and very high values is a risk for the population, but also the time that these values are present, therefore, the exposure times to acquire 1 MED should be made known to all people, with the aim of providing information on the health risks caused by prolonged exposure if care and adequate protection are not taken. The Organization for Economic Cooperation and Development (OECD) recommends regulatory changes for the tourism sector in Mexico, to evaluate the scientific basis based on international standards for UV radiation regulations involving content and design of warnings, and to redefine a recommended exposure program in the tourism sector, and the possible calculation of recommended annual dose limits. Thus, the results of the present study, although local in nature, are important at the national level and contribute to the design of public tourism policies, to understand of interactions between beach tourists and solar radiation and, consequently, modify their behavior toward a culture of care and protection against prolonged exposure to solar radiation. Finally, it is important to mention that UV-B radiation measurements were taken in Tepic city, the capital of the Nayarit state, and the surveys were applied 156 km away from the measurement site, in Bahía de Banderas on the southern coast of the state, so it is expected that the UV index levels at the study site will be slightly lower, due to the misty and cloudy conditions of the coastal areas. However, surface UV-B radiation measurements should be taken at the study site to generate accurate information on the UV index in the coastal zone of the state, or to estimate the UV index using mathematical models from global radiation data, which motivates us to continue research efforts with this approach in future work.

Conclusions

The construction of a UV index for the city of Tepic, Nayarit with surface measurements, from data obtained from RESOLMEX, as well as statistical analysis to determine with high certainty the variability and availability of UV-B radiation levels, are the main conclusion of the present work. The results show that the UV index in Tepic, Nayarit reaches extreme values during solar noon, during the months of March to June. However, daily maximum values indicate that from March to October extreme values of the UV index are registered. The sun and beach tourists surveyed in the present study, staying in all-inclusive hotels on the southern coast of Nayarit, are knowledgeable about solar radiation, however, they are totally unaware of specific concepts about UV radiation. Respondents have a high preference for the tourist destination, however, the lack of knowledge about the damages of prolonged exposure to UV radiation during recreational activities indicates that they are exposed to solar radiation at peak hours, for a prolonged time, so they are at risk of suffering burns, which may cause severe skin damage. Non-participatory observation in the tourist recreation sites analyzed in this study shows that tourists do have behaviors and practices focused on sun protection, such as the use of sunglasses, hats, umbrellas, and sunscreen creams; however, more research is needed to determine how to help tourists understand and use the information on the UV index for effective protection. Likewise, programs should be defined in the health sector to help the population, in general, to be aware of the risks of exposure to the sun at peak hours and act accordingly, and specifically, in the tourism sector, values of the UV index during recreational activities should be made well-known to all people, with the objective of providing information on health risks caused by prolonged exposure if care and adequate protection are not taken, especially at peak hours.