6. Climate Change and UV Radiation – Prevention of Skin Cancer

Climatic changes, such as the current anthropogenic global warming caused by greenhouse gases, are accompanied by a change in meteorological parameters such as precipitation, air temperature, sunshine duration, and UV radiation.

Influence of Climate Change on Air Temperature

Due to the release of greenhouse gases, especially carbon dioxide (CO2), air temperature is rising. According to the Intergovernmental Panel on Climate Change (IPCC) [515], the concentration of CO2 in the atmosphere is now higher than at any time in the past 800,000 years. If the rate of emissions does not change, by the middle of this century there would already be so much CO2 in the atmosphere that the global mean temperature would rise 2°C above pre-industrial levels.

The global average annual near-surface (land and ocean) temperature was 0.83 to 0.89°C warmer than the pre-industrial average from 2006 to 2015, making it the warmest decade on record. Of the 16 warmest years on record, 15 have occurred since 2000. Climate models project a further increase in global average temperature in the 21st century (for the period 2081-2100 compared to 1986-2005) of between 0.3 and 1.7°C for the lowest greenhouse emissions scenario (RCP2.6) and between 2.6 and 4.8°C for the highest emissions scenario (RCP8.5) [516]. RCP stands for "Representative Concentration Pathways." A total of four RCPs are distinguished: RCP2.6, RCP4.5, RCP6.0, and RCP8.5, with RCP2.6 assuming the lowest emission scenarios for greenhouse gases and RCP8.5 the highest.

The average annual temperature for the European land area from 2006 to 2015 was about 1.5°C above pre-industrial levels. The annual average land temperature in Europe is expected to increase in the range of 1 to 4.5°C (RCP4.5) and 2.5 to 5.5°C (RCP8.5) by the end of this century (2071-2100 compared to 1971-2000), which is more than the global average increase in projected temperature. The greatest warming is projected for winter in northeastern Europe and Scandinavia and for summer in southern Europe [516].

With an increase in average temperatures, the probability of so-called heat waves also increases. Internationally, there is no uniform definition of the term "heat wave." According to a definition used for Central Europe by Jan Kysely [517], a heat wave is said to occur as soon as the maximum temperature exceeds 30°C on at least three days in a row, the average maximum temperature remains above 30°C over the entire period, and the maximum temperature does not fall below 25°C on any day.

Studies [518]; [519]; [520] show that the frequency of heat waves has increased in Germany in recent years. The years 1994, 2003, 2006, 2010, 2013, or 2018 can serve as examples. The Intergovernmental Panel on Climate Change [521], [522], [515] points out that an increase in heat days and heat waves must also be expected in the future, according to Coumou (2013) and Robinson [518]; [523] and Coumou et al. (2013) [518]; [523], possibly a fourfold increase by 2040. This is also evidenced by numerous regional studies (see, among others, [524]).

Influence of Climate Change on UV Radiation Exposure

In addition to the state of the stratospheric ozone layer, important determinants of UV radiation at the earth's surface and thus of the UV radiation exposure of humans are indirectly the causes and consequences of climate change (global warming): (i) cloud cover situation, (ii) greenhouse gases, (iii) aerosols, (iv) surface reflectivity (albedo), and (v) human exposure behaviour.

(i) Cloud Situation:

Climate change-induced changes in UV radiation over Northern Hemisphere mid-latitudes caused by global warming appear to be caused by changes in cloud cover scenarios [525].

Consistent with simulations from climate models, several independent satellite data suggest that changes in large-scale cloud patterns have already occurred between the 1980s and 2000s [526]. Between 60°S and 60°N latitudes, observed and simulated changes in cloud patterns are consistent with polar retreat of mid-latitude storm tracks, widespread reductions in aerosol air turbidity between latitudes 30° and 50° of both hemispheres (presumably leading to increases in surface UV radiation), and expansion of subtropical dry zones [527].

Lower cloud cover over the year has an influence on the annual sunshine duration. According to observations of the German Weather Service (DWD), the annual sunshine duration in Germany has increased by about 96 hours (linear trend) over the observation period from 1951 to 2018. An increased annual sunshine duration can in turn result in an increased annual sum of erythema-effective UV irradiance. Initial analyses of data from 1999 up to and including 2018 from one of the measuring stations of the UV monitoring network of the Federal Office for Radiation Protection show that, for example, the summed daily totals for 2018 were significantly higher than the totals for all other years, just as they were for 2003, which was also strikingly sunny and hot [524] – [528].

Clouds have a different impact on UV radiation depending on their type and degree of coverage. For example, according to Seckmeyer [529], thunderclouds (e.g. cumulusnimbus congestus) can attenuate incident UV radiation by more than 99% compared to the cloudless case. However, the incident radiation may also be increased compared to cloudless fall due to increased scattering from the clouds [529]; [530]. The attenuation of UV radiation in general increases with increasing cloud cover and is about 10% to 50% at 7/8 cloud cover depending on the cloud type.

(ii) Greenhouse Gases:

According to current research, greenhouse gases, which are the cause of current global warming, also have an impact on stratospheric ozone destruction and regeneration. A reduction of the stratospheric ozone layer caused by chemicals containing chlorine and bromine (chlorofluorocarbons [CFCs], halons) causes an increase in the erythema-effective UV-B irradiance on the Earth's surface. Due to the ban on halogen-containing substances (Montreal Protocol with amendments [531], a recovery of the stratospheric ozone layer seems to be occurring. Current forecasts indicate that:

• the concentration of stratospheric ozone could return to pre-1980 levels by mid-century. But due to greenhouse gases, the regeneration of the ozone layer could slow down. The interrelationships are complex, however, and are currently the subject of intensive research.
• by the end of the 21st century (2090-2100) compared to the present (2010-2020), the recovery of the ozone layer (due to the decrease of ozone-depleting substances and the interaction with increasing greenhouse gas concentrations) could lead to a reduction of UV radiation, which could be highest over Antarctica (up to 40%). Outside the southern polar region, the reduction appears to be small, less than 10% [532].
• due to the recovery of the ozone layer, the fraction of UV-B radiation could be lower again in 2075- 2095, compared to the period 1955-1975, in all latitudes except the tropics, with unabated emissions of CO2, CH4 and N2O (emission scenario RCP 8.5). Reductions are estimated to be between about 5% and 15% in northern mid-latitudes, with the largest reductions projected for the winter months [527].

In addition, global warming appears to be associated with the occurrence of so-called low ozone events ("Low Ozone Events"), also called "miniozon holes," and thus unexpectedly high UV irradiance levels in the Northern Hemisphere. Detailed explanations are given in Section 6.1.3.

(iii) Aerosols

Another factor is air pollution from aerosols. The expected improvement in air quality due to air pollution control measures and thus reduction of aerosols in the most densely populated areas of the Northern Hemisphere could lead to an increase of 10-20% in erythema-effective UV irradiance. Aerosols may be the most important contributor to future UV irradiance in densely populated areas, but their projected impacts are the most uncertain [527].

(iv) Surface Reflectivity (Albedo)

The albedo (measure of reflectivity) of, for example, the oceans, ice sheets, deserts, and vegetation zones is another factor that could generate a global and regional change in UV radiation exposure as a result of global warming. Due to the melting of the large ice sheets at the poles, as well as glaciers in high mountain areas, the albedo in these areas decreases, which should reduce the reflectance and thus the UV radiation exposure in these regions. Projections indicate that a reduction in reflectivity due to melting sea ice in the Arctic would result in a decrease in UV radiation of up to 10%, while the decrease would be smaller (2-3%) at the edge of the Antarctic [532].

(v) Human Exposure Behaviour

Humans base their behaviour, among other things, on the weather, especially in terms of the time they spend outdoors. It is to be expected that weather changes due to global warming will also influence human exposure behaviour, with sunshine duration and thermal sensation being decisive factors (see 6.2.1). In addition, there are individual expectations and preferences, which also depend on the climate zone in which people live. How human behaviour and thus UV exposure will change as a result of climate change can, however, only be predicted with great uncertainty given the current state of knowledge.

The consequences of climatic changes on health are still subject to considerable uncertainty. The reason for this is the multicausal relationships associated with climatic changes, which make it difficult to differentiate which consequences are due to climate change and which are due to other factors (e.g. lifestyle).

Morbidity and Mortality with Rising Air Temperature

Rising air temperatures and heat waves (see Chapter 6.1.1) imply a thermal load. One can speak of thermal stress when the individual thermal comfort zone is left. According to Parsons [533] and Menne and Matthies [534], 

the individual thermal comfort zone varies according to geographical location, season, and acclimatisation (physiological adaptability of the body to environmental conditions) or thermophysiological adaptability of the population. When considering health effects of thermal stress, it is important to keep in mind that thermal stress is controlled not only by ambient temperature, but also by additional meteorological parameters such as humidity, wind speed, and radiation patterns.

Numerous studies now show that thermal stress is very likely to have significant effects on human health, well-being, and performance [535]. The effects of thermal stress vary in nature and severity depending on the region, as the effects depend on the conditions prevailing locally, the population composition and its adaptive capacity, or even the existing health infrastructure [536].

Studies show that high thermal stress in the form of heat waves can be associated with increased mortality (see [537]). For various regions of the world, there are average temperatures that are optimal for health and at which the mortality rate is lowest (e.g. 16.5°C for Amsterdam, 20°C for New York [538]).

During heat waves, increased morbidity occurs, especially for pulmonary and cardiovascular diseases [539]; [540]; [541], as well as increased mortality [542]. A 2008 study suggests that higher temperatures increase the induction of non-melanocytic skin cancers (NMCs, PECs) by UV radiation [543]. However, this evidence could not be confirmed in a more recent study [544].

Infants, young children, the elderly, and the sick are particularly at risk from thermal stress, as the thermoregulatory system in them has limited function [545]; [D'Ippoliti, D. et al. 2010]; [547].

An increased mortality due to thermal stress in summer was also demonstrated by Koppe [548], Heudorf and Meyer [549], Schneider et al. [550], and Muthers et al. [551] for Germany. This affected not only southern Germany, but also parts of western and northern Germany, where increased mortality rates were recorded during heat waves [552]. According to Gabriel and Endlicher [553], during the three-week heat wave in 1994, 10–50% more people died in predominantly rural Brandenburg, and in some districts of Berlin even 50–70% more than the average.

In this context, it should be added that the effect of thermal stress on health is intensified by increasing air pollution, especially by nitrogen oxides, ground-level ozone, and particulate matter [554]; [555]; [556]; [557]. Due to the high volume of traffic with corresponding pollutant emissions, people living in cities are particularly affected. Added to this is the fact that cities heat up or store heat more than the surrounding rural areas due to dense development and heat storage. This is significant in that cities often lack the nighttime cooling necessary for health.

Morbidity and Mortality Due to UV Radiation Exposure

Rising skin cancer incidence rates can be shown since the beginning of cancer registration. Possible reasons for this, as well as risk factors, are discussed elsewhere in this guideline (see Section 4.3).

The extent to which climate change, in conjunction with processes in the stratospheric ozone layer, has or will have an impact on the incidence and prevalence of skin cancer can only be properly quantified in the coming decades, as decades can pass between genetic damage of a general nature or caused by UV radiation and the diagnosis of skin cancer.

Accordingly, at present, only the possible consequences of altered UV irradiances or altered UV radiation exposures due to climate change and changes in the stratospheric ozone layer on human health--in particular on skin cancer--can be discussed. Lucas et al. [558] The authors conclude that in the future there will be a wide variety of interactions of factors related to stratospheric ozone and global climate change. The consequences of global climate change will affect entire populations, including, for example, forced migration that will leave skin types exposed to different levels of UV radiation than they were originally. At the individual level, climate change may alter behaviour so that individuals may be exposed to more or less UV radiation, depending on acclimatization to warmer temperatures. At this stage, we can only conclude that changes may occur. At the moment, we can only speculate about the possible or probable risks to human health [558].

Besides skin cancer, as the most serious risk of increased UV exposure, other UV-related diseases associated with climate change and UV radiation should not be ignored, such as photodermatoses. These are inflammatory skin diseases that are induced or exacerbated by UV radiation [559] and have a significant negative impact on sufferers in terms of school, work, family, and social activities [560], as well as mental health [561], due to their clinical presentation and the fact that light must be avoided. Photodermatoses are most common in (or even limited to) the spring and summer months. For example, for polymorphous light dermatosis (a photodermatosis that still affects about 18% of the European population [562]), it has been shown that its occurrence follows the annual variations of ambient UV radiation in different European locations. Changes in UV radiation as a result of processes in the stratospheric ozone layer and global climate change may therefore alter the frequency and severity of a number of photodermatoses [558].

Although the mechanisms of ozone depletion are fairly well understood, several unexpected ozone depletion events [563]; [564] have sporadically occurred in the Arctic during the last two decades due to special meteorological conditions. hese so-called "low-ozone events" (LOEs), also called "miniozon holes" (OMs) [565] ; [566]; [567], are characterized by significantly reduced stratospheric ozone for one to three days over geographically limited regions [568].

According to current knowledge, the reduction of the ozone column (OC) in Antarctica is the result of chemical destruction by photolytic reactions in the very stable polar vortex, which creates favorable conditions for ozone depletion [569]; [570]. Compared to Antarctica, comparatively high ozone concentrations are maintained over the Arctic and the northern hemisphere by the air mass circulation processes prevailing there [571]. In contrast to the conditions over Antarctica, the polar vortex over the Arctic is less stable, which inhibits the ozone depletion by chemical processes over the Arctic [572]. However, long and cold Arctic winters favour the formation of stable polar vortices and thus chemical ozone loss over the Arctic [573]; [572]; [574]; [575]; [564]. One such event in 2011, caused by an extremely cold and stable polar vortex, reached Antarctic proportions in its intensity [563]. Accompanying the seasonal warming in spring, an instability and dissolution of the polar vortex sets in and the ozone-depleted air masses (LOEs or OMs) from the interior of the polar vortex can be transported towards the south, including Europe. LOEs occupy a rather small area compared to the "ozone hole" over Antarctica and lead to unexpectedly high levels of UV radiation in the Northern Hemisphere [525]; [528].

Increased occurrence of these events seems to be related to global warming according to previous knowledge. Due to greenhouse gases, less warming radiation reaches the stratosphere, causing it to cool more. This, in turn, favours the occurrence of LOEs [576]. This increases the likelihood that Europe will be increasingly affected by LOEs, resulting in a high level of UV radiation on the Earth's surface for a few days at the end of March/beginning of April. However, the intricate details of the interactions between the ozone layer and greenhouse gases are not yet fully understood and are therefore the subject of research (such as in the RECONCILE project [577].

In addition, LOEs may occur over Europe in spring and summer as a result of balancing processes of stratospheric ozone concentration between subtropical air masses and those in higher latitudes [578].

The frequency of low ozone events – either due to a local change in atmospheric dynamics or due to an altered transport of ozone-depleted air masses from polar vortices or lower latitudes [579]; [580] - has increased over the European-Atlantic sector in the last two decades [579]; [581].

Need for Research

With regard to the prevention of skin cancer, it is necessary to quantify the effects of climate change on UV radiation exposure. To this end, the influence of the chemical, physical, and meteorological processes of climate change on the stratospheric ozone layer, on the occurrence of low ozone events, and thus on the UV radiation load close to the ground, which is relevant for Germany/Europe, must be described quantitatively. In addition, it is necessary to analyse the current UV radiation exposure in Germany and the region-specific UV radiation exposure recorded in recent decades. Furthermore, as a prerequisite for health policy decisions, projections of the near-surface UV radiation exposure for Germany/Europe are to be prepared in relation to different RCP emission scenarios and supplemented with modelling of cloud cover, aerosol concentration, and, where relevant, albedo.

To estimate the impact of changes in UV radiation exposure on morbidity and mortality due to UV radiation, in addition to an optimized registration of all skin cancer entities in Germany/Europe, an attempt should be made to describe a dose-response relationship between UV radiation/skin cancer. Furthermore, reporting procedures for other UV-related diseases, such as polymorphous light dermatoses, should be established in order to obtain an overall picture of the UV-related disease burden of the population.

There is also a need to conduct appropriate studies to clarify the possible influence of temperature on the effects of UV radiation, especially the carcinogenic effect. There is also a need for research into the possible combined health effects of UV radiation, air pollutants and meteorological factors.

Living environments within the meaning of §20 Paragraph 4 No. 2 PrävG and Social Law Book V (Law to Strengthen Health Promotion and Prevention [582]) are social systems that are significant for health and can be delimited; in particular, those of living, learning, studying, medical and nursing care, and leisure activities including sports. Accordingly, living environments are the residential environment (municipality/neighbourhood), day care centres for children, schools and other educational institutions, facilities for the elderly (old people's homes, inpatient long-term care/nursing homes), the workplace, and systems of leisure activities.

With regard to the prevention of skin cancer, behavioural patterns that determine the time spent outdoors and thus the possibility of UV exposure are of particular interest. Studies looking at this issue have almost exclusively related to the living environment "recreational systems." No studies currently exist on temperature-dependent behavioural patterns specifically in kindergartens, schools, training centres, workplaces, and facilities for the elderly, but publications do exist on the prevention of heat-related health damage (see, for example, [583]. In the latter settings, it must be taken into account that behaviour is usually influenced by and dependent on specifications and organisational boundary conditions.

The behaviour or patterns of behaviour of people in their living environments are influenced by the weather. Thermal sensation (the feeling of warmth, comfort, or cold) plays an important role here, along with cloudiness or sunshine, wind speed, and precipitation. One index that can be used to assess thermal sensation based on the human heat balance is the "perceived temperature." It often differs from the measured air temperature, since the sensation is determined not only by the air temperature but also by the meteorological variables of humidity, wind, and radiation, as well as human behaviour (especially activity and clothing) [584]. Subjective internal attitudes towards thermal conditions also seem to determine temperature-dependent behaviour, so that, for example, there are people who find heat pleasant and thus spend more time outdoors [585].

The table Leisure Behavior summarizes the information derivable from the studies on weather-dependent behaviour in the lifeworld "systems of leisure" with respect to controlling weather parameters and the behavioural patterns identified in this regard. Beyond the information in the table, it should be noted that the findings on temperature-dependent behaviour are not yet available independently of the season and thus daylight duration. As far as can be seen, the frequency and duration of outdoor activities increase with longer daylight hours and higher temperatures in the range of thermal comfort to slight heat stress. Further studies are needed to make statements on temperature dependence within individual seasons.

The explanatory details of the studies and assessments of their contribution to answering the key question can be found below. The majority of the studies were carried out in other countries, so far without testing the direct transferability of the findings to Germany.

Arana et al. [586] studied the influence of weather on public transport use in northern Spain during weekends in 2010 and 2011. The number of weekend trips for shopping and leisure decreased when it was windy and rainy and increased when the temperature rose, with a greater influence among occasional drivers than regular drivers. The study region is characterized by a temperate maritime climate with cool summers and mild winters. Average daily temperature maxima have their highest values in August at 22 °C. The statement on the increase of trips with rising temperature should therefore primarily apply to the area of thermal comfort. The range of values of the meteorological variables in the study period was not reported in the paper. The regression model uses the meteorological variables as absolute values, reflecting the annual cycle. It thus provides evidence that more trips are made by public transport (for probably recreational purposes with the possibility for outdoor stays) in temperate climate in summer, and less in rainy and windy conditions.

Banwell et al. [587] conducted interviews and group discussions in Australia (Sydney) with the elderly (> 65 years), a particularly vulnerable group in heat, to shed light on their behaviour in extreme heat or heat waves. The interviews were not directly linked to an acute heat wave. There are various definitions of heatwaves in Australia, including reaching or exceeding a maximum temperature of 35°C for at least three consecutive days. There was a difference in the behaviour of the elders. In most cases, the pattern of daily activities changed towards reducing physical activities and avoiding spending time outdoors. In some cases, spending time outdoors was moved to the very early morning hours, or physical activity was moved to an air-conditioned area such as a shopping mall. In individual cases, there was no adjustment in daily routine. Consequently, this study gives the indication of predominantly avoidance strategies among elders during extreme heat.

Bélanger et al. [588] used questionnaires to investigate the influence of weather and season on the physical activity of adolescents in Canada (Montreal). Participants were 12-13 years old at the start of the study and were followed for five years. They were able to select applicable activities from 29 in weekly lists that they engaged in for at least five minutes. Because of ties to the school year, data were not collected in July and August, the warmest months on average besides June. No distinction was made between indoor and outdoor activities. Only the frequency of activity was collected, not its duration. In general, the frequency of physical activity was lower in winter and increased in the warmer months. Overall, however, activity decreased with age. Within a season, an increase of 1% in spring and winter and 2% in autumn per 10°C increase in daily mean temperature was reported, as well as a decrease in activity of 2-4% per 10 mm precipitation. The statement regarding the increase in activity with increasing temperature is likely to apply largely to the range of thermal comfort, as the summer months were predominantly not recorded. The actual range of values of the meteorological variables during the study period was not reported in the paper.

Eisinga et al. [589] evaluated the relationship between daily TV consumption and weather conditions in the Netherlands for the period 1996 to 2005. The main meteorological variables affecting TV viewing time were temperature and sunshine duration. More TV was watched when it was colder, cloudier, and wetter, with stronger winds and longer nights. A dependence on the type of TV programme offered was also found, with the conclusion that more broadcast entertainment programmes encourage greater viewing than information programmes in adverse weather conditions. This distinction is now likely to be irrelevant due to the almost unlimited availability of desirable content via streaming services. In the study, a daily mean temperature of 20°C compared to 10°C resulted in 10 to 18 minutes less TV consumption in the entertainment sector. The range of daily mean temperature values during the study period was -12.1 to 25.8°C, with a mean of 10.2°C. It was not investigated whether the interpretation that people actually spent more of their additional TV-free time outdoors when weather conditions were more favorable was correct.

Knuschke et al. [590] and Knuschke et al. [591] ound that meteorological influences, particularly air temperature, sunshine duration, and precipitation play a significant role in individual behaviour with respect to the likelihood of spending time outdoors (and thus the likelihood of individual UV exposures). The leisure time exposures of outdoor workers and indoor workers are largely identical with respect to the mean and distribution of individual UV doses, provided that the same type of leisure time behaviour is involved: the passive share was 70 to 80% while the active share was 20 to 30% of the workers. On the basis of four measurement periods in the months of February, May, September, and December, it was found that the lengths of stay on weekend days vary depending on the season and are influenced by the daily maximum temperature. Since the two variables were not considered independently, it can be assumed that the daily maximum temperature functions primarily as a seasonal indicator in this case. It was assumed that the duration of stay is a linear function of the daily maximum temperature, although it is rather unlikely that the range of values of the maximum temperature also covered summer values due to the measurement dates. Climatically, the mean daily maximum temperature in Dresden (location of the measurements) is 19°C in May and September, and 4°C in February and December. Consequently, the statement on the increase of outdoor activities with increasing temperature, which is supported by the data, is likely to refer to the range of thermal comfort here as well.

Liu et al. [592] analyzed human planning behaviour in relation to weather using data for coupon purchases in China and visits to public transportation websites worldwide. They conclude that longer-term climate forms the basis for planning responses to current weather. At higher temperatures, the propensity to plan tends to decline. The authors establish a link to social unrest, violence, and societal destabilization in times of climatic change. Within the framework of this model of thought, consequences for outdoor stays would thus also be indirectly possible.

Spinney et al. [593] used time diaries to investigate the influence of weather conditions on leisure behaviour in Canada (Halifax) over the course of a year. Weather data included daily maximum temperature, total precipitation (distinguished between rain and snow), daily maximum wind speed, and total snow depth. The values of daily maximum temperature ranged from -14.8°C to 31.9°C. Additionally, day length was included. While watching television dominates leisure activities with nearly 80% participation rate and a median time of 2.5 hours, only just over 4% of participants engage in outdoor sports, spending nearly an hour (median 58 minutes) outside. A quarter of study participants spend leisure time doing other outdoor activities not classified as sports, most frequently spending three quarters of an hour outside per day. Outdoor activities increase during warm days with more daylight. This confirms the seasonal effect found in the publications of Arana et al., 2014, Belanger et al., 2009, Knuschke et al., 2004 and Knuschke et al., 2007. All weather effects can explain 2.9% of the variation in participation rates in outdoor non-sports activities, and 5.8% in outdoor sports. Temperature (positive) and precipitation (negative) have the strongest influence, and for sports, so does snow depth (positive). The strongest positive influence on time spent on outdoor sports and non-sports activities is day length (season), while precipitation is negatively correlated for time spent on non-sports activities and maximum temperature, wind, and precipitation for sports activities.

Stewart et al. [585] used an online survey of 1400 college students in the southeastern United States to establish a relationship between people's individual liking or disliking of very high temperatures and their exposure to the sun. Mean temperature maxima in Athens, Georgia, ranged from 11.8 to 32.3°C, with values higher than 30°C in all summer months. Individuals who enjoy heat exposure use proportionately less sunscreen than those who avoid heat. A greater proportion of heat-averse individuals spend more time outdoors in the summer months, while this is more likely to be the case in spring and autumn for those who avoid heat. No significant difference was found between women and men in terms of heat preference. U.S. Caucasian types are more likely to be heat-averse, at 51.3%, while the other ethnic types are more likely to avoid heat, at 55.1%.

Yang et al. [594] investigated the relationship between human behaviour related to public park use and thermal comfort in the subarctic climate of northern Sweden (Umea). They combined structured interviews with measurements of meteorological parameters in July and August 2015 for objective thermal assessment of the microclimate. During the interviews from 10 a.m. to 4 p.m., maximum temperatures ranged from 12.7 to 26.6°C. Under conditions objectively classified as "light heat," most park visitors rated the ambient conditions as "thermal comfort." Forty-nine percent of locals say they prefer even more solar radiation. This example illustrates the overlay of objective measures with subjective expectations and desires, in this case, the solar affinity of people in a subarctic climate. Discrepancies between an objective thermal assessment and subjective thermal perceptions were also found by Becker et al. [595] in a hot-dry climate and were attributed in part to subjective expectations. However, the authors emphasize that it is necessary to test their hypothesis by studies.

Scientific work on temperature-dependent behaviour in other environments is currently pending.

• The question of whether a climate change-related increase in skin cancer incidence can be expected as a result of changing human behaviour patterns cannot be conclusively answered at present and requires further research.

Global warming will influence risks and health-promoting effects as a result of climate changes, e.g., ambient temperature and precipitation, through changing sun exposure behaviours [558]. The actual personal UV dose received via sun exposure depends critically on behaviour [558]. Studies have shown that the mean daily UV exposure for adults and children is in the range of 4% to 5% of daily ambient UV radiation [596]; [597]. However, there is considerable variance [527] with a range from one tenth to ten times this mean [598], depending on where one spends time, how long one spends outdoors voluntarily or occupationally, and how one protects oneself from UV radiation. This underscores the importance of individual UV exposure. Most studies on this have been conducted in Caucasian (fair-skinned) populations, so the results may not be applicable to other ethnic groups [599].

The considerations regarding the temperature-dependent behaviour patterns of citizens in different living environments (chapter 6.2.1) suggest that temperature-dependent behaviour is dependent on thermal sensation and that the frequency and duration of outdoor activities increases with higher temperatures in the range of thermal comfort to mild heat stress. These statements are valid with the caveat that the available studies do not currently allow separation of the influences of daylight duration and thermal conditions.

As stated in chapter 6.1.1, initial studies indicate that an increased annual sunshine duration, as determined in Germany by the DWD, results in an increased annual sum of the erythema-effective UV irradiance. If, as a result, it can be expected for there to be a greater number of days offering conditions of thermal comfort up to mild heat stress and sunshine, then, according to Knuschke (2004) [590], it can be expected that people belonging to the active leisure behaviour type will spend more time outdoors. For this part of the population, increasing UV personal doses would be expected, which are associated with an increased risk of skin cancer. The measurements and evaluations of Knuschke (2004) [590] also show that the UV vacation dose forms a significant proportion of the annual UV exposure. Changes in holiday behaviour that may result from the effects of climate change on previously preferred holiday destinations would consequently be a not-insignificant aspect in the estimation of UV personal annual doses and the associated skin cancer risk.

In Chapter 6.1.1 it is described that with a higher number of days with stronger thermal discomfort (heat and heat waves) is to be expected. In this respect, according to the considerations in chapter 6.2.1, there are both indications of avoidance strategies and indications of the fundamental existence of a certain proportion of people with an affinity for heat who would spend more time outdoors despite the heat.

In the scientific literature to date, one indication can be found that the increase in temperature in itself seems to have an influence on the health effects of UV radiation. A study by van der Leun [543] used statistical methods to determine that for the same UV exposure, higher temperatures should increase the carcinogenicity of UV radiation. The authors use data from the US Skin Cancer Registry of the 1970s from ten regions. Because the authors' entire argument depends solely on data from a single region, the results of the analysis do not appear to be very robust. As stated in chapter 8.2 , the 2015 study [544] did not confirm the effect described above. Piacentini and colleagues [600] conducted studies on the change in non-melanoma incidence associated with temperature increases expected under different emission scenarios for the time horizon 2000 to 2200, based on and extending the study by van der Leun. Assumptions about actual human UV exposure are not directly incorporated in the study by Piacentini [600]. The quantity referred to as exposure in this study is the total available UV dose plus the temperature effect determined by van der Leun [543].

The quantity referred to as exposure in this study is the total available UV dose plus the temperature effect determined by van der Leun [543]. The changes in skin cancer incidence calculated in this study would thus be a direct function of the increase in temperature. However, the study still leaves questions unanswered, so that further investigations would be necessary to consolidate the results, also with regard to the question of whether the possible additional temperature effect is a physiological effect or an effect due to altered exposure behaviour.

Ultimately, the assumption that a warmer climate will lead to significantly increased UV radiation exposure and consequently higher skin cancer incidence cannot be conclusively proven at the present time. This also applies to a suspected additional carcinogenic temperature effect. Suitable studies need to be carried out.

Need for Research

Human behaviour plays a decisive role in real UV radiation exposure. From this derives the need for research to quantify weather-dependent behavioural habits and to clarify the extent to which climate change-induced changes in weather influence behavioural habits of different population groups in the long term in their living environments (kindergartens, schools, training centres, outdoor workplaces, facilities for senior citizens, systems of leisure activities) and on holiday, and how this increases UV radiation exposure and the likelihood of UV overexposure, also during the course of the day. Studies comparing these parameters within individual seasons are needed. On the basis of the knowledge thus gained, quantified statements are to be derived regarding climate change-related changes in the incidence of UV-related diseases, in particular skin cancer, taking into account human behaviour.

In order to offer the population protection (beyond the active or behaviour-based self-protection of the individual, e.g. textile and chemical protection) oriented to the conditions against the increasing stresses caused by heat and UV radiation in the course of climate change, the following approaches exist from an urban planning perspective: avoiding the entry of direct and indirect UV radiation, preventing heating, and creating cooling provisions [528]; [601]; [602]. In this context, it is advisable, also due to financial considerations, to implement measures that do not only serve one purpose, but have several benefits at the same time  [603]. This allows these measures to be better represented to politicians, taxpayers, investors, and developers. The approach of "Blue-Green-Infrastructures" discussed by Kabisch et al. [602] 

as a planning concept for climate impact adaptation with regard to heat avoidance and rainwater retention and its extension to include the aspect of reducing UV radiation input should be highlighted.

In relation to heat and UV radiation, urban planning and the associated spatial planning disciplines are thus required to protect people from unhealthy and unwanted exposure. This means that wherever people go about their daily lives, excessive exposure to heat and UV radiation should be avoided, i.e. specifically when they are inside buildings, when they are going about their daily business (e.g. by shading streets and squares) and when they are outdoors (e.g. schoolyards, playgrounds, sports facilities) [604]. However, even in places where people like to enjoy the sun (e.g. sunbathing areas at swimming pools, beaches, meadows, and paths in parks) [605], it is possible to increase the amount of protection available.

The following measures, which are explained in more detail in Chapter 6.3.2, appear to be sensible and possible from an urban planning point of view:

• Shading due to building development/structural-technical measures
• Reduction of albedo in built space and open space
• Shading through planting
• Cooling through planting
• Creation of cold air zones

In order for these more technical measures to take effect, measures must be taken at the organisational level. This includes, among other things, the organisation of a broad political will and the implementation of this political will with the help of appropriate working methods, laws, and regulations at the administrative or implementation level.

Shading Due to Buildings/Structural-Technical Measures

Buildings are generally capable of providing shade. Important parameters are the size or height of buildings as well as their orientation, arrangement, and design. However, it should not be ignored that building shadows are considered extremely inefficient, as a study conducted at tourist sites in Paris has shown [606]. According to this study, the least efficient building shading occurs in large squares, which is attributed to a high proportion of diffuse UV radiation, which is higher in such squares than in streets and parks. UV radiation reflected from surfaces may also contribute. Sliney [607] states that a white house paint can reflect up to 22% of UV radiation, whereas an asphalt road reflects only 4% to 9%, depending on age and colour.

Being in the shade of buildings alone cannot provide adequate protection. However, there are other structural/technical measures that provide shade [608]. These can be divided into:

• Permanent systems
• Temporary systems
• Adaptable systems
• Tension membrane structures and sun sails
• Prefabricated shade structures

Permanent systems include stable roofed structures or roof constructions that are erected for a period of at least 10 years. Examples are carport-like canopies, pavilions on playgrounds, or covered pedestrian bridges. Temporary systems such as tents, marquees, and lightweight awnings, on the other hand, are easy to erect and dismantle and are thus well suited when shade is needed only occasionally at certain locations or temporarily at different locations at the same time, or when permanent systems are unsuitable due to activities taking place at the site. Adaptable systems fill a gap here.

Tension membrane structures and awnings can be erected as both permanent and temporary solutions. They are popular because they are versatile in design, require only a few support structures, and can span large areas such as playgrounds and swimming pools as well as shopping streets/pedestrian zones in a filigree manner.

As there are many different types of space and people like to place value on special architectural design, tailor-made solutions are often used. There are also prefabricated shade constructions, where the creative scope is less, but so are the costs. This means that shading measures can be taken on a comparatively manageable budget.

It should be noted with all constructional-technical measures that not every construction or every material offers a high level of protection against UV radiation. In the case of textile solutions, it depends on how closely meshed and thick the material is, what colour it is, and what condition (old/new, wet/dry, loose/stretched) it is in. The tighter-meshed and heavier the material, the higher the demands on the load-bearing structure. Large mesh materials allow better air circulation, but also allow a higher percentage of direct UV radiation to pass through. Light-coloured materials do not heat up as much as dark ones, but scatter and reflect radiation more. Therefore, there is a trade-off to be made in the choice of materials to suit the purpose.

Reducing Albedo in Built Space and Open Space

In addition to house walls and roads, all other relevant surfaces in communities have a specific reflectivity (albedo). From the research of Sliney [607]

it can be seen that grass especially has a low albedo. A high level of protection from UV radiation can therefore be established where structural or natural shading as well as grass-covered areas are equally present. This can also be an argument for more green roofs and facades. The reflective effect of these surfaces is reduced by plant growth. At the same time, a cooling effect is achieved, preventing roof and wall surfaces from heating up and counteracting the heat island effect of cities. Due to the absorption capacity of the vegetation, the usable open space gained by green roofs is better suited for outdoor recreation than, for example, playgrounds laid out with light-coloured slabs.

Playgrounds with a high proportion of sandy surfaces, where the reflectance can be as high as 18%, absolutely require good shading and, if necessary, roofing with shading elements. They are frequented by the youngest children, who require special protection, and should therefore be equipped with special protective measures.

Grasses are particularly suitable for the greening of areas that are not built over. The creation of such areas can eliminate the need for sealing. They can thus serve as retention areas for heavy precipitation and thus make a further contribution to climate adaptation.

Shading through Planting

• Planting (trees, greenery) offers a variety of positive effects and, in terms of protection from heat and UV radiation, can be used to create very efficient structures in urban and rural areas.

Trees are valuable in adapting to climate change in several ways. Not only do they provide shade depending on the density of their foliage and the size of their canopy as well as their distance from the ground [609], but they can cool their surroundings by up to 30% through evapotranspiration. For example, Streiling & Matzarakis [610] found in a study that temperatures in a city can differ significantly between sites that are heavily shaded by tree canopies and those that are less under the influence of large tree canopies. For example, 30.8°C was measured under a dense canopy, while 34.1°C was measured at a site where there were fewer trees with dense canopies. For the mean radiant temperature, 19.3°C and 21°C were recorded, respectively.

The UV protection factor of trees has been evaluated differently in different studies, ranging from two to 20 [609] depending on various factors. A denser arrangement of several trees into groups of trees usually provides more protection than single trees. Due to these variations, tree shade is not a stable parameter and should only be enjoyed in combination with individual precautions [611]. For urban and landscape planning, however, this means relying more on trees with a particularly high protection factor when creating green infrastructures. As design elements and shade providers, trees are socially accepted and easier to integrate than built structures.

Creation of Cold Air Production Zones

• By connecting cold air production zones, fresh air corridors can be created that not only have a microclimatic effect, but also serve the entire community. Connected with footpaths and cycle paths, they offer shady spaces for people to move around.

In combination with other trees and, if necessary, with bodies of water, tree locations can function as cold or fresh air production zones and thus reduce the heat island effect of urban areas. The areas on which the trees stand can, in turn, be used for decentralised rainwater management. In addition, trees store CO2, which in turn serves to reduce the greenhouse effect, and filter dust from the air, which can significantly improve the quality of the air we breathe in communities. If attractively designed, such zones invite people to stay on hot days with high radiation intensity. Buller et al. [612] have shown that cool shaded areas are readily accepted. People walking in the shade of trees are exposed to less UV radiation.

Organisational Measures

According to Knieling et al. [613], climate adaptation requires addressing related tasks along the following categories:

• Cross-sectoral task
• Cross-level tasks
• Intermediary field of action
• Transboundary requirements
• Long-term orientation
• Planning under uncertainty
• Paradigm shift in flood protection

Protection against UV radiation does not yet feature in this report but can and should be included as a task for countering the consequences of climate change. It touches on several sectors (e.g. health, water management and flood protection, urban planning, green spaces and landscape planning) and levels (local and regional) and therefore requires a level that is able to mediate between the different areas of responsibility and to represent the issue appropriately and implement measures, using a broad mix of different formal and informal instruments.

Organisational measures also include all organisational procedures to reduce high UV and heat exposure in the daily routine for each individual. As stated in the policy paper of the UV Protection Alliance [400], daily routines and the organisation of work in people's living environments should be designed in such a way that exposure to UV radiation can be avoided. This is also necessary in the area of occupational safety. Studies show that those who work outdoors have a higher risk of skin cancer than the rest of the population as a result of their activities. Since 01 January 2015, squamous cell carcinomas and their precursors, the multiple actinic keratoses, can be recognised as BK under the number BK5103 (Berufsgenossenschaft Energie). According to the German Social Accident Insurance [614], eemployers should include consideration of suitable sun protection measures in the risk assessment, especially for employees who are regularly exposed to direct sunlight for more than a quarter of an hour. The order of priority of protective measures against solar UV radiation should follow the classic prevention principle "TOP":

1. (T) technical measures (shading)
2. (O) organizational measures (regulation of time spent under the sun)
3. (P) personal measures (clothing, sunglasses, etc.)

According to the DGUV, organisational protective measures are measures such as work planning, start of work, change of activity, rotating work tasks, shift planning, or arrangement of breaks, which must be communicated to the employees in the employer's instruction. If the work task permits, activities should preferably take place indoors around lunchtime. Skilful work planning should reduce daytime exposure. Of course, it is also important to seek shade during work breaks. These statements apply to all insured members of the DGUV and the municipal accident insurers, and also to children and young people in kindergartens and schools.

UV protection as an argumentation strand and guiding objective can support municipalities in consistently implementing climate protection and climate adaptation measures. The problem of skin cancer is less abstract than the concept of climate change and can thus be better communicated to the public. With guidelines for UV protection optimisation and the coupling of UV protection measures with climate protection and adaptation measures, science can support politics and administration in the implementation of suitable measures. However, this requires funding.

Laws and regulations support municipalities in the largely nationwide implementation of suitable measures because they create legal certainty. In addition, municipalities need funding programmes that make it possible to implement high-quality measures in public spaces and at public facilities.

In order to achieve the goal of largely protecting people from unwanted UV exposure, consistent greening of streets and shading of squares with trees should be aimed for, as explained in chapter 6.3.2,

which, at best, also take into account requirements of decentralised rainwater management. As a minimum, dark/low-reflective materials are to be provided as floor coverings. It should be noted here that in practice, reflective floor coverings are currently preferred in order to avoid excessive heating. This is understandable, which is why such coverings also require shading. Preferably, however, ground sealing should be avoided as far as possible and grass surfaces should be used, also as roadside greenery and design elements on squares. Here, too, these recommendations lend themselves to combination with rainwater management, because such areas, laid out as retention areas, fulfil another climate change-related purpose. The same applies to playgrounds, schoolyards, and recreational facilities.

Often, however, especially in the case of larger areas, planting cannot provide sufficient protection. In these cases, the structural measures presented (see chapter 6.3.2) can help to provide shade.

In the future, it should be ensured that all schools, kindergartens, playgrounds, and recreational facilities such as sports fields, open-air swimming pools, etc., have adequate sun protection adapted to the intended use of the facilities. What can be described as sufficient is still to be defined and continuously adjusted.

More thought could be given in future to the roofing of cycle paths and footpaths. Especially connecting routes into and out of centres, which lead through predominantly open and thus unprotected landscape, could be considered for this. Not only do the roofs protect against solar radiation, but the canopies could also keep cycling attractive in rain and snow, thus contributing to climate protection. To make them even more efficient, they could be fitted with photovoltaic systems and used to produce green electricity. Overpasses and pedestrian and cyclist bridges could also be protected and used in this way.

Need for Research

The topic of skin cancer prevention through urban planning and urban development measures still plays a relatively minor role in planning practice in Germany. It is therefore necessary to define effective measures and to create the conditions for their application in combination with existing planning approaches. It is well known that adaptation measures to reduce UV pollution bring synergy effects that are also associated with a reduction in heat pollution. Therefore, it must first be determined in which subspaces and structures UV radiation (and heat) loads to be reduced occur. Furthermore, it must be clarified which urban structures and which forms of planting are best suited to minimise UV radiation (and heat) under which conditions in different settings. Since not every form of shading or surface design is suitable or desirable for every subspace, the question of which structural-technical measures and which forms of planting can best be integrated into communities must be clarified. For this purpose, it is necessary to determine not only the UV protection factor, but also the effort, costs, design, and acceptance by different stakeholders. Above all, the question of the acceptance of different forms of proportion-related UV protection by the population, but also by politicians and decision-making bodies, is an important field of research. For the necessary planning requirements and security, it must also be investigated whether, and, if so, which, laws and regulations must be created or adapted, how the topic of urban UV (and heat) protection can be integrated into the tasks and decisions of local offices and authorities, and what the handling and routines in this topic area have been to date, possibly also in countries with more experience.