1. Introduction
Solar energy is the overriding factor determining all aspects of life on Earth, starting with the photosynthesis process defined by J. Ingen-Housz in 1779 [
1] and ending with the technology used in the production of clean energy [
2]. The wavelength of solar radiation reaching the upper border of the Earth’s atmosphere ranges from about 0.2 to 4 μm. This range includes ultraviolet, visible, and infrared radiation [
3]. Solar radiation reaches the surface of our planet in the form of a stream of direct and diffuse radiation, which together constitute global solar radiation [
4,
5]. The intensity of global radiation is influenced by several factors, mainly including latitude, time of year and day, terrain, and altitude, as well as the amount and type of cloud cover [
6,
7,
8]. Apart from natural factors, human activity also influences the inflow of radiation to the Earth’s surface, e.g., shading from tall buildings or air pollution [
9,
10]. The sum of the time for which the direct solar radiation exceeds 120 W·m
−2 is called sunshine duration [
11]. The sum of actual sunshine duration depends mainly on the length of the day, the amount and type of cloud cover, and the horizon covering [
12,
13,
14]. Also, the aerosol and water vapor concentration influence the sunshine duration, especially at low solar altitude.
The human body is influenced by every solar radiation range, but ultraviolet radiation is the most biologically active. The far-ultraviolet light (UVC, 0.20–0.29 µm) is entirely absorbed by the ozone layer, which also retains 90% of the mid-ultraviolet (UVB, 0.29–0.32 µm) [
15]. Thus, the near-ultraviolet (UVA, 0.32–0.40 μm) and part of the mid-ultraviolet reach the Earth’s surface [
16]. The share of ultraviolet radiation in the entire solar spectrum is small and amounts to an average of 4% at the Earth’s surface [
17].
Ultraviolet radiation has a positive and negative effect on the human body. It stimulates the circulatory system and the activity of endocrine glands, destroys bacteria and microbes in the environment of people [
18], initiates the synthesis of vitamin D in the human skin [
19,
20], which prevents rickets and osteoporosis, and plays a massive role in the human immune system [
15]. However, too high doses of solar radiation can cause erythema, sunburn [
21], the development of skin cancer, and eye diseases [
15,
22]. Sunbathing is essential to stay healthy, but the time of safe exposure to direct sunlight is essential, depending on skin pigmentation. Fitzpatrick [
23] distinguished six types of skin, considering its sensitivity to the inflow of ultraviolet radiation. This classification is used worldwide to assess the risk of burns and skin cancer from excessive UV doses.
One of the forms of health treatment is heliotherapy, which is defined as treatment with natural sunlight exposure [
24]. During sunbathing, the entire spectrum of solar radiation reaches the human body. As previously stated, the most biologically active is ultraviolet radiation, but the influence of visible and infrared radiation on the human body should not be overlooked. Visible radiation stimulates the nervous system and affects the endocrine system [
25]. The effect of visible radiation on the functioning of the pineal gland, the gland that produces melatonin, has an impact on sleep patterns [
13]. Appropriate dosing of visible radiation promotes the regulation of the body’s daily cycle and is also used in the treatment of depression [
26]. Infrared radiation provides the body with thermal energy [
13,
25]. This range of solar radiation can penetrate deep into the body (up to 3 cm) and significantly improves the blood supply and tissue nutrition, as well as reduces muscle tension [
26]. Infrared radiation can be used in the treatment of rheumatic diseases, limb injuries, chronic inflammation of the nasal cavity and paranasal sinuses, post-frostbite conditions, as well as overload syndromes of joints, tendons, and muscles [
25,
26]. During the First World War, sunlight therapy became known, and several hospitals and clinics were built with exposure to solar radiation in England and Germany [
27,
28]. Currently, sunbathing is used in the treatment of, among others: psoriasis [
24,
29,
30,
31,
32,
33,
34,
35,
36,
37], atopic dermatitis [
35,
38,
39,
40], acne vulgaris [
38,
41], as well as rickets, and other diseases associated with vitamin D deficiency [
20,
21,
42,
43]. In heliotherapy, the most important thing is the influence on the human body of the mid-ultraviolet light, whose biological activity is at a solar altitude >30° [
13,
44].
The research conducted in the field of remote sensing techniques has mainly included biometeorological and urban climate models [
45,
46,
47], simple modeling of bioclimatic conditions using GIS software [
48], estimation of radiation fluxes in complex-city structures [
49,
50] also taking into account the issues of urban heat islands (UHI) [
51,
52,
53]. Long-term trends of solar radiation used satellite datasets were investigated for the Iberian Peninsula by Montero-Martin et al. [
54], for the Baltic region by Lindfors et al. [
55], and for the territory of Poland by Kulesza [
56]. However, none of them analyzed in detail the relationship between the solar features of the climate and the morphometric conditions of the area with the possibility of using them in heliotherapy, which indicates the originality of the presented results.
The concept of health resorts is not understood in the same way in all countries—in many countries, such as Germany, Italy, Slovakia, and Austria, the operation of health resorts is based on the use of natural healing resources, primarily healing mineral waters, and is legally regulated [
57,
58]. In Germany, Austria, and Switzerland, apart from natural medicinal raw materials, health resorts’ assessment is also influenced by the climate [
59]. In Poland, a health resort is defined as an area where health treatment is carried out and has been granted health resort status. According to the Act on health resort treatment, health resorts, and health resort protection areas and on health resort municipalities of 28 July 2005 [
60], the status of a health resort may be granted to an area which meets all the following conditions: (i) has deposits of natural medicinal raw materials with proven medicinal properties; (ii) has a climate with healing properties confirmed on the terms specified in the Act; (iii) in its area there are health treatment facilities and health treatment equipment, prepared for the provision of health treatment; (iv) fulfill the environmental requirements specified in the environmental protection regulations; (v) has a technical infrastructure in the field of water and sewage management, energy, collective transport, and also conducts waste management. When assessing the climate of the area which is to be given the health resort status, the number of sunny hours, days with precipitation and fog are taken into account, as well as the frequency of occurrence of atmospheric stimuli that adversely affect the human body related to air temperature and humidity, wind, cloudiness, also the frequency of storms and weather changes [
61]. For the conduct of heliotherapy in the health resorts, the standard for the annual total of hours with the sun is essential, which cannot be lower than 1500 [
61].
The research aimed to demonstrate the relationship between the solar climate characteristics and the morphometry of the area on the example of two health resorts in terms of suitability for heliotherapy. The following was performed: (i) the solar radiation inflow in the annual and daily course for the period 1996–2015 was assessed based on actual heliographic and actinometric measurements; (ii) using SAGA software, the influence of morphometric conditions (coverage) on the inflow of solar radiation was determined; (iii) based on solar climate features, morphometric parameters significant for heliotherapy were demonstrated using the maximum entropy model; (iv) areas predisposed to heliotherapy were indicated.
The conducted research is important not only from the point of view of the organization of heliotherapy in already functioning health resorts but also opens a discussion on the need to search for new interdisciplinary and implementation research methods to identify future locations for heliotherapy.
4. Discussion
The effectiveness of heliotherapy is conditioned by many complex factors, which can be classified as: direct (e.g., latitude, cloud cover, etc.) and indirect—widely related to land development). Human activity has an influence primarily on factors included in the second group.
The average annual sum of actual sunshine duration in the Cieplice and Kołobrzeg health resorts for the years 1996–2015 exceeds the minimum norm of 1500 h for health resort operations in Poland (
Figure 2). In the case of both health resorts, a growing trend is observed in the annual sum of hours with the sun, which improves the conditions for heliotherapy (
Figure 2). Other authors [
14,
84,
85] also draw attention to the growing annual sums of actual sunshine duration in Poland and other regions of Europe in the 1990s and at the beginning of the 21st century.
The distribution of actual sunshine duration throughout the year is primarily influenced by latitude [
44,
86], which determines the length of the day in the annual course—in summer, as the latitude increases, the day becomes longer, and in winter, the other way round. Therefore, the most favorable conditions for heliotherapy are from April to September, when the average daily sums of actual sunshine duration in the analyzed health resorts vary from about 6 to 8 h (
Figure 4). It is also influenced by the amount of cloud cover in Poland that changes throughout the year, where lower values are observed in the warm half-year [
12,
87,
88]. In these months, the use of heliotherapy is also facilitated by the daily sums of global radiation, which is several times greater than in the autumn and winter months (
Table A2), which means that to achieve the desired therapeutic effect, the exposure time to direct solar radiation may be shorter. The histogram of the daily sums of total insolation (
Figure 9) shows that the probability of daily values exceeding 4.5 kWh·m
−2 in June exceeds 65% in both health resorts.
In the months from April to September, there is also an appropriate height of the sun for heliotherapy (
Figure 12), which affects not only the intensity and sum of global radiation but also the intensity and range of ultraviolet radiation reaching the ground. At a time when the height of the sun is low, the path length of the solar radiation through the atmosphere increases, and the radiation with the shortest wavelengths are absorbed and scattered more strongly. For heliotherapy, the key factor is the inflow of UVB, the biological impact of which is visible starting from >30° of solar altitude [
13,
44]. In the period from April to September, it is possible in Poland to conduct heliotherapy in terms of increasing the production of vitamin D3 in the human body. In the late spring and summer, it is also possible to conduct effective anti-psoriasis heliotherapy [
36]. For safe and effective heliotherapy, it is necessary to take specific daily solar radiation doses [
20]. Before starting therapy, it is also important to exclude possible contraindications for this form of treatment [
24].
Comparing the research results obtained for Cieplice and Kołobrzeg, it can be concluded that in the warm half-year, especially in May-July, the conditions for sunbathing are more favorable in Kołobrzeg, located in the north of Poland, where the day is longer and the sunny days are more frequent (
Figure 6), which results in a greater number of hours with sun and greater sums of global radiation. Czerwińska and Krzyścin [
20] indicate the favorable solar features of the climate for anti-psoriasis heliotherapy in the summer season on the Baltic coast. According to Krzyścin et al. [
89], the contact of human skin with seawater enhances the effectiveness of anti-psoriasis heliotherapy, which also indicates the great potential of seaside health resorts in Poland to conduct this form of health treatment.
In the rest of the year, the climatic conditions are less favorable for heliotherapy in the analyzed health resorts. A particularly unfavorable situation occurs in Kołobrzeg in the months from November to February, when the daily sums of actual sunshine duration range from 1 to 2 h (
Figure 4), and approximately half of the days in each of these months are sunless days (
Figure 6). During this time, there are also multi-day sunless periods, the length of which may be up to 15 days (
Table 3). Longer periods of weather with complete cloudiness adversely affect the human body’s psyche and biological functions [
13]. In Kołobrzeg, from the beginning of October to the first days of March, the height of the sun at noon does not exceed 30°, which prevents the synthesis of vitamin D3 in the human skin. For the proper functioning of the body, vitamin D supplementation is recommended at this time because the consumption of foods containing large amounts of it is usually insufficient [
19,
42]. On the other hand, from the beginning of November to the first days of February, the height of the sun at noon does not exceed 20°, which limits the possibility of biological effects of UVA [
44]. In Cieplice, the duration of these unfavorable periods is 2–3 weeks shorter.
The remote sensing results determined based on the DEM were correlated with the actual actinometric characteristics of the studied health resorts (
Figure 13b). Generally, they indicate more favorable conditions for heliotherapy in the summer months, reflecting the direct factors described above. The summer period on the Baltic Sea was also indicated in their research by Krzyścin et al. [
89] as a way to support UV cabinet exposures during the treatment of psoriasis.
Terrain morphometry, less and less untransformed into urbanized areas, affects the solar inflow. There is an increase in the area of built-up and urbanized land in the city [
90], which confirms the thesis about the urbanization process [
91], leading to the intensification of the urban heat island phenomenon [
92].
In the microscale, areas with appropriate exposure (S at the expense of N) are predisposed. Research of Ali-Toudert and Mayer [
93] pointed out that the E-W orientation of canyons has high stressful thermal comfort, indirectly dependent on solar radiation conditions. The element that strongly differentiates the inflow of solar radiation, and thus the effectiveness of heliotherapy, is the land cover. The presence of buildings (especially tall or compact) shades the area. Buildings intercept the direct and reduce the diffuse solar radiation, yet they may also enhance irradiance by reflecting sunlight [
94,
95]. An equally important parameter is the building’s location concerning the world’s directions, the building’s shape, or the number of floors (height). These differences generate contrast in energy budgets that lead to interactions in radiative exchange [
95]. Urban structures modify the visible horizon and influence the incoming radiation fluxes [
50]. Advantageous solar features are characteristic for areas with high terrain openness, and therefore high SVF values. As confirmed in the research of Barring et al. [
96], street geometry influences the surface temperature (indirect solar radiance) using SVF analyses.
On a macroscale, the land cover type characteristics determine the possibility of an inflow of solar radiation. Urban areas, in comparison with rural areas, have distinctive biophysical features [
97]. Based on the conducted studies, the most favorable areas from the point of view of heliotherapy were: non-irrigated arable land (type 2.1.1), pastures (type 2.3.1), and sea (type 5.2.3)—only for Kołobrzeg (
Figure 14). Here, the range of areas predisposed for heliotherapy should be noted—designated by health resorts’ boundaries. Arable lands (type 2.1.1) are areas with a different function, so their suitability for heliotherapy is negligible. Only pastures (type 2.3.1) as walking areas meet the functional assumptions. Attention should be paid to the zonal type of land cover—sea area (type 5.2.3), which is associated with the beach. Due to its environmental features (flat, with large openness, no buildings), it is extremely beneficial for heliotherapy. On the other side of the scale, there are forest areas (types 3.1.1–3.1.3), the least favorable from the analyzed angle. This is a kind of paradox because these areas are very often used for health activities (e.g., walking areas in spa parks, hiking trails in forests, etc.), however, in the case of heliotherapy, they are not of significant importance. They often fall within the range of health resorts. Sunny glades, which were shown by the research of Thorsson et al. [
98], are characterized by good thermal outdoor environment conditions for human behavior, including the usage of heliotherapy. Equally weak solar characteristics are in type 1.4.2 (non-agricultural vegetated areas), where health resorts (spa houses) are usually located and where the patients spend most of their time. These areas are also (obligatorily) included in the health resorts. Similar results in their research on bioclimatic comfort were shown by Zeren-Cetin and Sevik [
99], preferring agricultural land and natural and semi-natural areas over settlement areas.
The usage of the MaxEnt model made it possible to determine the influence of morphometric parameters on the effectiveness of heliotherapy. From several parameters tested, the most important turned out to be AHM and SLP (
Figure 16). Their values are directly related to the configuration of actinometric factors, morphometry, and land cover. The inversely proportional correlation noted in the results, stronger in the summer period (
Figure 15), shows that the increase in the intensity of the relief or coverage and the season of the year reduces the inflow of solar radiation—and thus the effectiveness of heliotherapy.
Analyzing in detail the results of the MaxEnt model for Cieplice and Kołobrzeg, it can be concluded that AHM limits the effectiveness of heliotherapy for urbanized areas (including type 1.1.2). On the other hand, the SLP parameter determines heliotherapy in semi-natural areas (including type 2.4.3) and natural areas (including type 3.1.1 and 3.1.3, which subjects the less use of this cover type due to the shading of the dense surface of trees for heliotherapy). Additionally, in the area of Kołobrzeg, the ASP was an essential parameter for some types of land cover, which indicates that in relatively flat areas (Plain of Poland), the exposure of the area is more critical than in mountain areas (Cieplice).
The analyzed health resorts are predisposed for health resort activities focused on heliotherapy (the total area with AUC > 0.75 was 2.7% for Cieplice and 0.6% for Kołobrzeg). Taking AUC > 0.50 into account, the area increases to 49.8% and 33.8%, respectively. Much more advantageous for the analyzed form of health activity are high mountain areas [
100] or marine areas located in subtropical latitudes, e.g., Black Sea Coast [
99]. The types of land cover shown in the model, environmentally predisposed for heliotherapy (AUC > 0.50), characterized by the highest percentage (2.1.1 for Cieplice and 1.1.2 for Kołobrzeg), due to the way of use, are not indicated for heliotherapy. On the other hand, the types preferred both in terms of the environment (AUC > 0.50) and utility (possible real health resorts use), i.e., pastures (type 2.3.1—about 25% for Cieplice and about 35% for Kołobrzeg) and sea-beach (type 5.2.3—about 5% for Kołobrzeg) (
Figure 16) do not occupy large areas of the health resort.