Cadmium Pollution in the Tourism Environment: A Literature Review

: Cadmium is a highly-toxic metal, and, its environmental occurrence and human exposure consequently deserve close attention. The insight into the relationships between cadmium and tourism relations has deepened during the past three decades and the research into this relationship is reviewed. For this purpose, 83 relevant publications (mainly articles in international journals) were analyzed. It was found that investigation of Cd in the tourism environment took place in all continents (except Antarctica) and has intensiﬁed since the mid-2000s; Chinese researchers are the most active contributors. The Cd occurrence in air, living organisms, sediments, soil, suspended particular matter, water, and of the human environment has been studied. It has become clear that tourism contributes to Cd pollution (particularly, by hotel wastewater and increased tra ﬃ c), and, vice versa, Cd pollution of beaches, coastal waters, food, urban parks, etc. creates risks for tourists and increases human exposure to this toxic metal. Both mechanisms have received equal attention. Examples concern many places worldwide, with the Mediterranean and Central and Eastern Europe as apparently critical regions. Our signiﬁcantly incomplete knowledge of the relationships between cadmium and tourism must be ascribed to the common oversimpliﬁcation of these relationships and to the scarcity or even absence of information supplied by the most important tourist destinations. The present review demonstrates that more studies of heavy metals and, particularly, Cd in the tourism environment are needed.


Introduction
Tourism affects large areas in many parts of the world, including ocean and sea shores (beach resorts), tropical forests and wetlands (national parks for ecotourists), and mountainous areas (ski resorts). These areas experience not only crowding by numerous visitors, but also physical modification (artificial beaches, recreation zones, transport infrastructure, etc.). Tourism already requires more than 50,000 km 2 worldwide, and the areal demand from this industry might almost duplicate in the future [1]. It thus seems that a new type of environment, namely some published in languages other than English (Chinese, French, Japanese, Polish, Russian, etc.). The database was accessed in March 2020 to collect as many studies relevant for Cd in tourism environments (with the relevant terms in titles, abstracts, keywords) as possible (all years and all types of papers were addressed). The search yielded 68 studies on Cd and tourism, 87 studies on Cd and recreation, 11 studies on Cd and hotels, and 4 studies on Cd and hospitality. These studies were filtered to avoid duplications and to exclude non-relevant data. As a result, 83 studies dealing with the cadmium-tourism relationships were selected (Table 1). Undoubtedly, this list is incomplete (as no bibliographical database can be 100% complete), but it might safely be presumed that it includes the majority of the principal works.   The content of each work considered for the purposes of the present review was analyzed on the basis of two major categories, namely research parameters and research findings. Criteria of the first category allow judging whether a given work is focused on Cd or tourism (this could be determined provisionally from the work's title), when it was published, and which countries were represented by its authors. Criteria of the second category enabled analyzing the geographical distribution of the reviewed research by countries and environmental components, the mechanism discussed in the works (see below), and the main research topics (these can be defined intuitively as a result of a synthesis of the available literature). As hypothesized above, tourism can trigger Cd pollution, and Cd pollution can affect tourism activities. Additionally, some works can deal with both tourism and Cd, but without direct links. Consequently, three cadmium-tourism relationships (mechanisms) might be the research focus of the literature under study (Figure 1). Preliminary analysis of the collected studies confirmed this idea. Moreover, it was found that some works provided 'positive' evidence. This meant that either Cd pollution was studied, but not found, or that the Cd content was below dangerous levels, or that it was not the main pollutant. Moreover, some studies could be essentially methodological or focused on prevention of negative processes. All these peculiarities of the research focus were also documented. The analysis of the works on the basis of the above-mentioned criteria allowed some generalizations and subsequent interpretations that represent the available knowledge about the relationships between cadmium and tourism in a systematic way.
Geosciences 2020, 10, 242 5 of 18 by its authors. Criteria of the second category enabled analyzing the geographical distribution of the reviewed research by countries and environmental components, the mechanism discussed in the works (see below), and the main research topics (these can be defined intuitively as a result of a synthesis of the available literature). As hypothesized above, tourism can trigger Cd pollution, and Cd pollution can affect tourism activities. Additionally, some works can deal with both tourism and Cd, but without direct links. Consequently, three cadmium-tourism relationships (mechanisms) might be the research focus of the literature under study (Figure 1). Preliminary analysis of the collected studies confirmed this idea. Moreover, it was found that some works provided 'positive' evidence. This meant that either Cd pollution was studied, but not found, or that the Cd content was below dangerous levels, or that it was not the main pollutant. Moreover, some studies could be essentially methodological or focused on prevention of negative processes. All these peculiarities of the research focus were also documented. The analysis of the works on the basis of the abovementioned criteria allowed some generalizations and subsequent interpretations that represent the available knowledge about the relationships between cadmium and tourism in a systematic way.

Synthesis of Research Parameters
A total of only 83 articles on the cadmium-tourism relationships published during 32 years (~2.6 articles per year) suggest relatively little research into this subject (Table 1). However, taking into account how 'narrow' and specific the subject is, the number of articles must be considered as relatively large. This implies that the cadmium-tourism relationships were of some interest to the international research community. From all considered studies, 8% focused on Cd and 20% focused on tourism ( Table 1). None of the publications focused on both Cd and tourism, and the majority of the articles dealt with environmental issues that were less specific than the cadmium-tourism relationships. These findings indicate that despite its relative importance, the subject under review

Synthesis of Research Parameters
A total of only 83 articles on the cadmium-tourism relationships published during 32 years (~2.6 articles per year) suggest relatively little research into this subject (Table 1). However, taking into account how 'narrow' and specific the subject is, the number of articles must be considered as relatively large. This implies that the cadmium-tourism relationships were of some interest to the international research community. From all considered studies, 8% focused on Cd and 20% focused on tourism ( Table 1). None of the publications focused on both Cd and tourism, and the majority of the articles dealt with environmental issues that were less specific than the cadmium-tourism relationships. These findings indicate that despite its relative importance, the subject under review remained rather 'marginal', and that the cadmium-tourism relationships were mainly considered 'incidentally', e.g., in the context of local environmental assessment studies.
The frequency of publications on the subject changed over time ( Figure 2). The earliest publication included in the present review dated from 1989; the 1990s were characterized by low research interest into the subject. The situation changed drastically in the mid-2000s, when the number of studies started to grow. This tendency persisted until now, although the dynamics remained unstable (Figure 2). The largest number of studies were published in the second half of the 2010s. Although one could hypothesize that the described distribution over time of the considered studies was affected by incomplete bibliographical data (also due to the lower coverage of pre-2000 works), the increase since the mid-2000s was clear, whereas these 'recent' works were well-covered by 'Scopus'. The acceleration of the research in the cadmium-tourism relationships thus seemed to be a real tendency, although with ups and downs.  Table 1 for data. The first three months were considered for 2020.
The importance of the cadmium-tourism relationships was demonstrated well by the geographical spreading of the authors of the studies under review (Figure 3). Researchers from Europe, the Middle East, southeastern Asia, the Americas, and Australia have all paid attention to this subject, i.e., the research was truly international. The majority of studies were published by researchers from China (16% of the studies), Spain and the USA (7% each), and India and Poland (6% each). The absence of studies by Japanese researchers was surprising because the influence of Cd pollution on human health is best known from this country [45]. One should also note the low degree of international collaboration. Only 11% of the works were written by the authors from two or more countries (only one study was written by authors from three countries). Chinese and, to a lesser degree, Mexican specialists seemed to be the most active international collaborators. Generally, the geographical spread of the authors indicates the absence of a 'mature' international research network.  Table 1 for data. The first three months were considered for 2020.
The importance of the cadmium-tourism relationships was demonstrated well by the geographical spreading of the authors of the studies under review (Figure 3). Researchers from Europe, the Middle East, southeastern Asia, the Americas, and Australia have all paid attention to this subject, i.e., the research was truly international. The majority of studies were published by researchers from China (16% of the studies), Spain and the USA (7% each), and India and Poland (6% each). The absence of studies by Japanese researchers was surprising because the influence of Cd pollution on human health is best known from this country [45]. One should also note the low degree of international collaboration. Only 11% of the works were written by the authors from two or more countries (only one study was written by authors from three countries). Chinese and, to a lesser degree, Mexican specialists seemed to be the most active international collaborators. Generally, the geographical spread of the authors indicates the absence of a 'mature' international research network. researchers from China (16% of the studies), Spain and the USA (7% each), and India and Poland (6% each). The absence of studies by Japanese researchers was surprising because the influence of Cd pollution on human health is best known from this country [45]. One should also note the low degree of international collaboration. Only 11% of the works were written by the authors from two or more countries (only one study was written by authors from three countries). Chinese and, to a lesser degree, Mexican specialists seemed to be the most active international collaborators. Generally, the geographical spread of the authors indicates the absence of a 'mature' international research network.  Table 1 for data.  Table 1 for data.

Synthesis of Research Findings
Tourism environments are globally widespread, and they include-but are not limited to-resort zones at sea coasts and in mountains, regions with a rich culture and history, and recreation areas in forests and along rivers. All countries have more or less important tourism resources [150]. The reviewed literature makes it clear in which of these countries the cadmium-tourism relationships were studied. Although this did not (and cannot) reflect the true geographical distribution of Cd in the tourism environment due to 'natural' research biases, these findings were important because they showed the global significance of the problem. Numerous countries formed the focus of the studies (Figure 4). The cadmium-tourism relationships have been studied on all continents, except Antarctica. China (14% of the studies) and Spain (6%) are the countries that have attracted most attention. One should note, however, that interest in this research topic has remained low in Western Europe and South America. This meant that although the cadmium-tourism relationships were studied worldwide, including developing countries, the scope of the considered studies was mainly local, and none of the studies focused on more than one country (except for the study by Macdonald et al. [106] which dealt with the entire Arctic).
The cadmium-tourism relationships were traced for the different components of the environment, among which the best studied were water, sediments, and soil (Table 2). Importantly, anthropogenic components were also involved in the cadmium-tourism relationships. One should also note that numerous studies examined the entire landscape (Table 2). Apparently, Cd cycles in the tourism environment are complex, and they included not only the concentration of this metal in any particular component (e.g., soil or water), but also its dispersal in the entire environment, uptake by organisms, and direct exposure to it by tourists.
Different mechanisms were analyzed in the articles under review (Table 1). Roughly the same number of studies dealt with the influences of Cd pollution on tourism (48%) and the contribution by tourism to Cd pollution (49%); in two papers, both mechanisms were considered. A few studies (5%) did not address direct relationships. Some 28% of the studies dealing with influences of Cd pollution on tourism bore 'positive' evidence, while 41% of the studies dealing with the contribution of tourism to Cd pollution bore 'positive' evidence. These results meant that tourism triggered Cd pollution and that Cd pollution commonly affected tourism activities; both relationships received adequate scientific investigation. It was also evident that the presence of Cd in the tourism environment was, in a significant number of cases, not linked to the risks, although cases with such risks were numerous. Most available literature supports the idea that the relationship between cadmium and tourism deserve attention, as they dealt with an increased tourist exposure to this toxic metal. areas in forests and along rivers. All countries have more or less important tourism resources [150]. The reviewed literature makes it clear in which of these countries the cadmium-tourism relationships were studied. Although this did not (and cannot) reflect the true geographical distribution of Cd in the tourism environment due to 'natural' research biases, these findings were important because they showed the global significance of the problem. Numerous countries formed the focus of the studies (Figure 4). The cadmium-tourism relationships have been studied on all continents, except Antarctica. China (14% of the studies) and Spain (6%) are the countries that have attracted most attention. One should note, however, that interest in this research topic has remained low in Western Europe and South America. This meant that although the cadmium-tourism relationships were studied worldwide, including developing countries, the scope of the considered studies was mainly local, and none of the studies focused on more than one country (except for the study by Macdonald et al. [106] which dealt with the entire Arctic).  Table 1 for data.
The cadmium-tourism relationships were traced for the different components of the environment, among which the best studied were water, sediments, and soil (Table 2). Importantly, anthropogenic components were also involved in the cadmium-tourism relationships. One should also note that numerous studies examined the entire landscape (Table 2). Apparently, Cd cycles in the tourism environment are complex, and they included not only the concentration of this metal in any particular component (e.g., soil or water), but also its dispersal in the entire environment, uptake by organisms, and direct exposure to it by tourists.   Table 1 for data.
Although the only internationally-reported examples of these relationships were considered in the present review, their worldwide distribution shed light on the geographical patterns of these mechanisms. The contribution of tourism to Cd pollution was found in many countries of the world ( Figure 5A). A concentration was found in the Mediterranean countries (Algeria, Croatia, Egypt, Greece, Italy, Spain, and Turkey). This is not unexpected, considering the intensity of tourism and the consequent anthropogenic pressure here [151][152][153][154][155][156]. Cd pollution creating risks for tourism has been established widely, but not in South America ( Figure 5B). Examples tend to concentrate in Central and Eastern Europe, which could be explained tentatively by post-industrial shifts [157,158], when heavily-affected areas became important for recreation.
Thematically, the considered studies were very diverse, and they addressed both general and highly-specific issues. Three points in particular needed attention. The first one was that numerous studies discussed tourist exposure to Cd pollution, particularly on beaches and in restaurants. For instance, Nour [114] addressed Cd in recent beach sediments at Sharm El-Sheikh, a world-famous Egyptian resort, and found that the concentration of this metal was very high, also in comparison to other recreation areas in Egypt, Saudi Arabia, and Russia. Moreover, he demonstrated that tourism constituted the main source of this pollution. In another study, Torres et al. [139] demonstrated that tourism growth on the Azores made Haliotis tuberculata an important food resource, although higher-than-normal levels of Cd in this organism posed risks to human health. Second, some studies have indicated wastewater, including that of hotels, as a source of Cd pollution. This was documented in particular by Eddaoudi et al. [82] for the Agadir Bay (Morocco) and the same was forecasted for the nearest future by Liu et al. [105] for the Swat River (Pakistan). Third, it is sensible to emphasize that several studies pay attention to Cd pollution in urban recreation areas and along roads. These include studies by Ji et al. [96] on the effects of the Chinese Spring Festival on air pollution, Li et al. [102] on Cd pollution along the roads in the Beijing Olympic Park, and Wang et al. [146] on the traffic-related pollution on the Tibetan Plateau, where heavy metals pollute 'stripes' up to dozens of meters wide along highways. Table 2. Principal environmental components relevant to the cadmium-tourism relationships in the studies under review.
Thematically, the considered studies were very diverse, and they addressed both general and highly-specific issues. Three points in particular needed attention. The first one was that numerous studies discussed tourist exposure to Cd pollution, particularly on beaches and in restaurants. For  Table 1 for data.
These studies are commonly difficult to classify thematically as a consequence of the 'haphazard' character of the research, with respect to the cadmium-tourism relationships. Nonetheless, it seemed sensible to classify them on the basis of the analyzed environmental component (Table 2).

Discussion: Tourism Environment and Lacking Research Topics
As explained above, the tourism environment should be distinguished from other types of environment. The analysis of the considered studies implies that the relationships between Cd and tourism are (or can be) linked to this environment and its specific components ( Figure 6) or rather, these relationships could be understood through the 'prism' of the tourism environment. Undoubtedly, these relationships resembled those established for densely populated areas, but some specific features should be mentioned explicitly. First, tourism is a factor of people concentration and fairly deep interacts with the environment (e.g., via an increased demand for local food), which increases human exposure to Cd pollution. Second, new sources of Cd pollution are linked to tourism, namely hotels. Third, tourism requires intensification of traffic and other activities that lead to Cd pollution; moreover, it appears that tourism creates new 'pools' of Cd accumulation (e.g., soils of artificial recreation areas). The above findings were intuitive and were limited to the relatively small group of case studies (Table 1); there is a significant lack in research on this topic. Three of them are worth discussing in the light of the available knowledge. The first research gap was linked to oversimplification of the cadmium-tourism relationships. Some (if not most) studies dealing with them offered quite simple interpretations regarding either Cd pollution or tourism. It was commonly stated, for instance, that a high Cd-content in coastal water or beach sediments is dangerous because such areas constitute direct tourism resources. In fact, it required detailed investigations to find out which particular mechanisms affect human health, e.g., how Cd affects health when a tourist swims in the polluted water or lies on a polluted beach. Scientifically correct approaches of this problem are not really common in the literature. An example of this was provided by Nilsen et al. [113], who were able to relate Cd accumulation in alligators to tourist exposure to this toxic metal. The explanations of tourism contribution to Cd pollution are often restricted to stating various activities like traffic as a source of Cd. However, the knowledge of these activities needs to be detailed-for instance, is tourism seasonality important for Cd pollution? Usage of which hotel products lead to a high Cd-content in wastewater? A good example was the study by Ji et al. [96], who explained the importance of the Chinese Spring Festival for air pollution by Cd.
The second research gap was geographical. Insight into the Cd-tourism relationships would become essential when examples from the most important tourist destinations and the most polluted areas are considered. Good examples concern Egypt-the Cd in environments of the world-famous resorts along the Red Sea, including Sharm El-Sheikh and Hurghada, was discussed by Ahdy and Youssef [67] and Nour [114]. According to the World Tourism Organization [150], the ten most visited tourist destinations (at the country level) are France, Spain, the USA, China, Italy, Turkey, Mexico, Germany, Thailand, and the UK (totally, >0.5 bln international tourist arrivals per year). However, only China and, to a lesser degree Spain, the USA, and Turkey, could be proud of paying more or The above findings were intuitive and were limited to the relatively small group of case studies (Table 1); there is a significant lack in research on this topic. Three of them are worth discussing in the light of the available knowledge. The first research gap was linked to oversimplification of the cadmium-tourism relationships. Some (if not most) studies dealing with them offered quite simple interpretations regarding either Cd pollution or tourism. It was commonly stated, for instance, that a high Cd-content in coastal water or beach sediments is dangerous because such areas constitute direct tourism resources. In fact, it required detailed investigations to find out which particular mechanisms affect human health, e.g., how Cd affects health when a tourist swims in the polluted water or lies on a polluted beach. Scientifically correct approaches of this problem are not really common in the literature. An example of this was provided by Nilsen et al. [113], who were able to relate Cd accumulation in alligators to tourist exposure to this toxic metal. The explanations of tourism contribution to Cd pollution are often restricted to stating various activities like traffic as a source of Cd. However, the knowledge of these activities needs to be detailed-for instance, is tourism seasonality important for Cd pollution? Usage of which hotel products lead to a high Cd-content in wastewater? A good example was the study by Ji et al. [96], who explained the importance of the Chinese Spring Festival for air pollution by Cd.
The second research gap was geographical. Insight into the Cd-tourism relationships would become essential when examples from the most important tourist destinations and the most polluted areas are considered. Good examples concern Egypt-the Cd in environments of the world-famous resorts along the Red Sea, including Sharm El-Sheikh and Hurghada, was discussed by Ahdy and Youssef [67] and Nour [114]. According to the World Tourism Organization [150], the ten most visited tourist destinations (at the country level) are France, Spain, the USA, China, Italy, Turkey, Mexico, Germany, Thailand, and the UK (totally, >0.5 bln international tourist arrivals per year). However, only China and, to a lesser degree Spain, the USA, and Turkey, could be proud of paying more or less adequate attention to the cadmium-tourism relationships on their territory (Table 1, Figure 4). None of the considered studies dealt with France or the UK. Of course, this did not mean that there was no Cd in the tourism environments of these countries. In contrast, there was clear evidence of pollution increase/decrease, although examined without reference to tourism. Noack et al. [159] detected high and dangerous Cd concentrations in air, soil, and vegetables in Marseille (Southern France) and also demonstrated that subsequent factory closure led to a decrease in the level of air pollution. Strady et al. [160] discussed bioaccumulation of Cd in the Marennes-Oléron Bay, known as the largest oyster production site of France. In the UK, Goddard et al. [161] found a significant decrease in the Cd content in ambient air, since the beginning of the 2000s, and residential and industrial burning of wood and biomass had become the main source of this metal emission. Undoubtedly, all of these findings should be discussed in the light of tourism development.
The third research gap was linked to the necessity of serious investigation of such a mechanism as tourist exposure to Cd, due to its concentration in tableware and canned food. Previous research (not linked to tourism) has demonstrated the importance of this issue [162][163][164]. Presumably, hotel spaces and food consumption in remote places pose risks to tourists. However, the significance of these risks and their geographical distribution are yet to be determined. This gap should be filled by future research.

Conclusions
The review of the literature on the cadmium-tourism relationships permitted us to draw five general conclusions that reflect the present state of the relevant knowledge. First, Cd occurrence and cycling in the tourism environment was proven to be a common phenomenon, and a significant amount of knowledge of Cd in tourism environment was obtained worldwide in the course of three decades, although through somewhat 'marginal' and locally-focused research. Second, Cd was associated with both common components of the tourism environment (air, soil, water, etc.) and its specific components (beaches, hotels, parks, etc.). Third, the contribution of tourism to Cd pollution and the danger of the latter to tourists seemed to be equally important topics to researchers. The relevant studies focused largely on evidence from China and Spain. Fourth, the reviewed literature focused on how tourism contributed to the Cd pollution in the Mediterranean and how risky Cd pollution was for tourism development in Central and Eastern Europe. The other regions were addressed with less attention or not addressed at all. Fifth, two principal research gaps were related to the insufficiently developed idea of what scientific knowledge should be obtained, as well as to geographical biases. Consequently, our current understanding of the cadmium-tourism relationships is unavoidably incomplete.
This literature review has evident implications for further research. It makes clear that the cadmium-tourism relationships constitute a really distinctive phenomenon that is worth analyzing for both fundamentally scientific and managerial purposes. Therefore, future research should be shifted to a higher level, i.e., studies should focus simultaneously on Cd pollution and tourism. Evidently, such studies need to be conducted in both popular tourist destinations and areas known for their high levels of Cd in the environment. Nonetheless, the already available knowledge reviewed in the present paper forms a firm ground for correct planning of future research projects.