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Keywords:

  • cutaneous malignant melanoma;
  • UVR exposure;
  • type of home;
  • socioeconomic status

Abstract

  1. Top of page
  2. Abstract
  3. Material and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

There is a need for more knowledge concerning the association of higher socioeconomic status (SES) with cutaneous malignant melanoma (CMM). Having a home garden is associated with a higher SES. We aimed to study the influence of having a home garden on UVR exposure behavior and risk of CMM. Register study: We collected information from Danish national registers about gender, age, type of home and CMM among persons aged 16–75 in 2002–2006. A total of 5,118 CMM cases were identified. Risk of CMM of the trunk was increased by 46% (p < 0.001, 95% confidence interval (CI): 31–63) and risk of CMM of the extremities by 34% (p < 0.001, 95% CI: 20–49) among people with home gardens. Dosimeter study: During a summer season 194 participants living in the Capital area, Denmark, equally distributed in homes with and without a garden, wore personal electronic UVR dosimeters measuring time-stamped UVR doses continuously and filled in sun exposure diaries. While no difference was found in estimated yearly UVR dose between groups, participants with a home garden had more days exposing shoulders or upper body, and upper extremities outdoors than those without a garden (p = 0.026, age adjusted). People with a home garden are at increased risk of CMM of the trunk and extremities—body sites that seems to be exposed to a higher extent among people with home gardens. People with a higher SES are more likely to have a home garden. This may partly explain the well-known association of higher SES with CMM incidence.

Exposure to ultraviolet radiation (UVR) from the sun is the primary environmental cause of cutaneous malignant melanoma (CMM).1, 2 CMM of the limbs and trunk—exposed during recreational sun exposure—accounts for the largest increase in CMM incidence.3, 4 Risk of CMM is increased among indoor workers5, 6 and people with high socioeconomic status (SES) and in areas of high population density.7, 8

The association of SES with CMM incidence may partly be explained by type of home. Danish families who live in a home with a garden have a higher disposable income.9 These families have easier access to recreational sun exposure than those without a garden. To our knowledge no prior studies have assessed sun exposure patterns among people living in different types of homes. We aimed to explore this in an open prospective observational study based on personal electronic UVR dosimetry and sun exposure diaries. Moreover, we sought to elucidate whether living in a home with a garden contributes to risk of CMM. Relations between CMM incidence and type of home were studied in a register study using data from the Danish Cancer Register, the Central Person Register and the Building and Housing Register.

Material and Methods

  1. Top of page
  2. Abstract
  3. Material and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Type of home related to CMM: Register study

Data sources
Danish civil registration system

In this study we sought to determine relations between CMM incidence and type of home. Residents in Denmark is assigned a unique identification number and registered in the Civil Registration System that was established in 1968. The system provides basic personal information including name, gender, address, date and place of birth, civil status, citizenship and identity of parents. It is computerized and updated continuously on vital status, address and spouses.10

To categorize each person on the basis of the degree of urbanization in the municipality in which the person lived, we defined three kinds of area of residence; the “capital area” denoted the city of Copenhagen and municipalities in the capital region; “town areas” denoted municipalities with the largest town inhabited by more than 10,000 inhabitants; “rural areas” denoted the rest of the municipalities in the country. Each area contained ∼1/3 of the Danish population.

Danish cancer register

The Danish Cancer Registry contains information about cancer cases in Danish residents from 1943 to the present. Hospital departments, specialist practitioners, general practitioners and pathology departments are obliged to report new cancer cases to the Danish Cancer Registry as well as any subsequent changes in the diagnosis.11 CMM has the international classification of disease code C43. The study population was all persons aged 16–75 and alive in each of the years 2002–2006. A person alive in 2002–2006 was therefore included five times. Only the first case of CMM in an individual is registered in the Danish cancer registry. We limited the analyses to people aged 16–75, because CMM among children is primarily genetically conditioned while people above 75 years of age will be more likely to live in retirement homes and exhibit different sun exposure behavior from that of the younger and middle-aged people that this study wished to focus on.

The building and housing register

This national register—founded in 1977—contains information about all buildings and types of housing in Denmark. It is continuously updated by the local authority (municipality). It provides information on the location of all buildings, their utilization and for residential housing the type of homes.12 For further analyses we focused on two types; (i) homes with a garden and (ii) homes without a garden.

By linking these three data registers using the personal identification numbers, we obtained information about type of home and type of area for all Danish citizens in relation to the incidence of CMM.

UVR exposure patterns related to type of home: Dosimeter study

Study population

We selected the study population from a study by Thieden et al. that describes sun exposure behavior among six subgroups of the population living in the Capital area, Denmark (latitude: 56° north; longitude: 12° east). The study contains information about the participants' type of home. The study ran during the summers of 1999–2000 and was approved by the Scientific-Ethical Committees for Copenhagen and Frederiksberg (KF11-007/99). Participants gave written informed consent. All participants had Scandinavian ancestors, no history of skin cancer, atopic dermatitis, psoriasis or polymorphic light eruption. From this study population we excluded children because CMM is very rare in this age group and golfers because nearly all golfers lived in a home with a garden. Data were analyzed for 22 adolescents (age range 16–19), 81 indoor workers (age range 21–64), 43 municipal gardeners and rangers (age range 25–60) and 48 sun seekers (age range 21–63).13, 14

Ambient UVR exposure

Ambient solar UV radiation was measured continuously with an UV biometer (model 501; Solar Light Co Inc, Philadelphia, PA) on the seven-floor building at Bispebjerg Hospital in Copenhagen. The measurements are expressed in standard erythema doses (SED) where 1 SED = 100 J m−2 normalized to 298 nm using the international commission on illumination erythema action spectrum.13, 15, 16

Personal electronic UVR dosimeter

The dosimeter comprises a sensor and a datalogger in a housing together with a digital watch. The datalogger controls the sensor, which was set to measure every 8th second and store the average of the last 75 measurements every 10 min. The measurement range of the dosimeter is 0.1–23 SED per hour. It is battery-driven and can run for 145 days. The dosimeters were calibrated against the UV biometer on five cloudless days in early summer using the sun as a light source. The dosimeters were worn like a wristwatch outdoors at the least between 7 am and 7 pm. The UVR dosimeters have been previously described in detail by our study group.13, 17

Sun exposure diary

Participants filled in a diary in which they ticked “yes” or “no” in answer to the following questions: (i) Did you wear the UVR dosimeter today? (ii) Are you off work or school or on holiday today? (iii) Are you abroad today? (Country code), (iv) Did you sunbathe today? (Sitting or lying in the sun with upper body or shoulders exposed to get a tan), (v) Did you use a solarium today? (vi) Have you exposed shoulders or upper body, and upper extremities outdoors today? (vii) Have you been at the beach or by the sea today? (viii) Have you applied a sunscreen today? (SPF number) (ix) Did you get a sunburn today? (If yes: Red? Red and sore? Red and sore and blistered? On a minimal, moderate, or extensive area?).14 For further analyses participants were registered by body exposure if they sunbathed and/or exposed shoulders or upper body, and upper extremities to the sun and/or used a solarium that day.

Data analysis

Dosimeter measurements and corresponding diary information for >30 days and at least 21 days in June, July or August was required for a participant to be included for further analysis. This resulted in 194 participants with a total of 23,096 days with both UVR dosimeter and diary data. We estimated each participant's yearly UVR dose by adjusting the observation period to a year. It has previously been shown that Danish indoor workers receive a total of 3.1 SED in the winter half-year and a total of 133 SED in the summer half-year; the total UVR dose received during winter in Denmark is thus almost negligible except for holidays in sunny places and solarium use.18 In the present study, participants' estimated yearly UVR doses were calculated based on the individual measured daily doses during the study period. For missing days in the study period, e.g., days with diary data but without dosimeter measurements, the daily doses were calculated as the same part of ambient UVR found on comparable days with measurements, separating days on/off work and being inside/outside Denmark. For the remaining days in the calendar year without dosimeter and diary data, daily doses were calculated as the same part of ambient UVR found on days on/off work following the standard workweek and national holidays.13

Statistical methods

In the register study we used binary logistic regression to analyze if type of home and area of residence were predictive of CMM. The number of CMM cases and the total number of non-CMM cases and CMM cases in a given combination of age, gender, type of home and area were set as the dependent variable and trials variable respectively. The odds ratios of CMM were adjusted for age (1-year categories) and gender because these variables were considered potential confounders. Area and type of home (explanatory variables) were included in the model together. An independent samples t test for equality of means was used to analyze age differences in the study population living in the two types of homes.

In the dosimeter study we primarily used nonparametric statistical tests because the data were not normally distributed. We used the Mann–Whitney U test to compare possible differences in continuous measurements of sun exposure behavior between participants who lived in a home with a garden and participants without a garden. We used multiple analysis of variance when age was also included in the model. The data are presented as medians and range (min–max). Chi-square and Fischer's exact test were used for analyzing group variance regarding skin type, gender and subgroup.

The significance limit was p < 0.05. We used SPSS 19.0 for Windows (SPSS, Chicago, IL) for all data analysis.

Results

  1. Top of page
  2. Abstract
  3. Material and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

Type of home related to CMM: Register study

The study population comprised 19,882,543 individuals aged 16–75 of whom 5118 persons were diagnosed with CMM. We found that the annual increase in risk of CMM was 3% and that CMM of the upper and lower extremities was strongly associated with women, whereas CMM of the trunk and neck/scalp was more common among men (Table 1).

Table 1. Cutaneous malignant melanoma (CMM) related to type of home and area of residence, controlled for age and sex; all cases and a subdivision by anatomical site
inline image
All CMM cases

Using rural areas as a reference category, we found that the odds of CMM were increased by 29% both in the capital area and town areas. The odds of CMM among people with a garden were 30% higher than for those without a garden (Table 1). We repeated the analyses separately for each area of residence (data not shown) and still found a significantly increased risk of CMM among people with a garden in each of the three areas.

We then repeated the analyses by anatomical site, because the various locations of CMM may be differently associated with type of home and area.

CMM of the face and neck/scalp

Type of home and area did not predict risk of CMM of the face and neck/scalp (Table 1). Analyzing CMM of the face separately from CMM of the neck/scalp did not change our findings (data not shown). Neither did we find that type of home predicted risk of CMM of the face and neck/scalp when repeating the analyses separately for the capital area, town areas and rural areas (data not shown).

CMM of the trunk

The odds of CMM of the trunk were increased by 40% in the capital area and 30% in the town areas compared with rural areas. Type of home was predictive of CMM with increased odds of 46% among people having a home garden compared to those without a garden (Table 1). The risk of CMM of the trunk remained significantly increased among people with a garden when repeating the analyses separately for the capital area, town areas and rural areas (data not shown).

CMM of the extremities

The odds of CMM were increased by 26% in the capital area and by 36% in town areas compared with rural areas. The odds of CMM were 34% higher for people with a garden than for those without a garden (Table 1). Analysis of upper and lower extremities separately did not change our overall findings. Repeating the analyses separately for each of the three areas of residence, we found a significantly increased risk of CMM of the extremities among people with a garden in the capital area (data not shown).

CMM in several regions or location not specified

Again, odds of CMM were increased by 23% for people living in the capital area compared with those living in rural areas. Type of home did not predict CMM (Table 1) neither when repeating the analyses separately for the capital area, town areas and rural areas (data not shown).

UVR exposure patterns related to type of home: Dosimeter study

Table 2 shows the baseline characteristics of the participants. The four included subgroups were equally distributed between homes with a garden and homes without a garden (p = 0.756). No difference was found in skin type (p = 0.105) or gender (p = 0.773) between the two groups. Participants with a home garden were median 9.5 years older than those without a garden (p < 0.001).

Table 2. Participants' baseline characteristics
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Measurements of UVR exposure are shown in Table 3. We found no significant group difference in estimated yearly UVR exposure dose. Participants with a garden had a higher number of mean daily hours outdoor per day (hours with UVR measurements monitored by the dosimeter) than those without a garden (p = 0.046) and more days with body exposure (p = 0.034), which was mainly due to a significantly higher number of days exposing shoulders or upper body, and upper extremities outdoors (p = 0.009). While we found no difference in number of days sunbathing between the two groups, participants without a garden had more days using solarium (p = 0.021). Daily UVR dose on days exposing shoulders or upper body, and upper extremities was higher among participants without a garden (p = 0.047). Finally the two groups spent an equal number of days in Southern Europe, though participants without a garden had a higher daily UVR dose on days in Southern Europe (p = 0.030). After adjusting for age, only days with body exposure (p = 0.047, age adjusted) and days exposing shoulders or upper body and upper extremities outdoors (p = 0.026, age adjusted) remained significantly higher among participants with a home garden—body sites that accounted for the increased risk of CMM among people with a home garden. Participants were also asked about number of solarium sessions during the past 12 months before study start and no group difference was found (data not shown).

Table 3. Measurements of ultraviolet radiation (UVR) exposure among participants living in homes with or without a garden
inline image

Discussion

  1. Top of page
  2. Abstract
  3. Material and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

To add to our knowledge concerning the association of SES with CMM—given that Danish people of a higher socioeconomic status are more likely to have a home garden9—we aimed to study possible differences in sun exposure patterns of people living in homes with and without a garden and whether type of home predicts risk of CMM.

From the dosimeter study we found that persons with a home garden had more frequent body exposure to the sun than those without a garden. Accordingly, in our register study we found that people with a garden had a 46% higher risk of CMM of the trunk and 34% higher risk of CMM of the extremities.

The latter finding could be due to an easy access to a sunny area. For CMM of the face and neck/scalp we found no relation to type of home. It seems possible that exposing the trunk and extremities to the sun is more common among people with a home garden, whereas the face and neck/scalp are more equally exposed in the two groups. This is supported by the finding from the dosimeter study that estimated yearly UVR dose was similar in the two groups, which accords with the theory that melanoma of the face and neck/scalp is caused by continued prolonged exposure to solar radiation.4, 19

In the register study, people with a garden had a mean age of 45 and those without a garden had a mean age of 40 (p < 0.001). This low age level was due to the exclusion of persons beyond 75 years of age. One might argue that higher age explains why those with a garden are at increased risk of developing CMM of the trunk and extremities. However; we found that both age and type of home were independently prognostic of CMM of the trunk and extremities. Moreover, we added an interaction term between age and type of home to the model. The interaction term was nonsignificant in the model for all CMM cases together and for the various anatomical sites, which tells us that the age effect was the same for people in both types of home.

We found that people in the capital area and town areas had a higher risk of CMM of the trunk and extremities compared with people in rural areas, which is in line with previous findings.7, 8 SES tends to increase in more densely populated areas and is a known risk factor for CMM.8, 20 Many explanations for this relation are plausible, including that wealthy and well-educated people are more likely to practise outdoor sports activities,21 spend more money on packaged travels22 and they have easier access—in general—to the health-care system.23 Interestingly, in 2006 disposable family income among all types of Danish families living in homes with a garden, was 10–36% higher than in families without a garden.9 It seems possible that people with a higher SES—being more likely to live in homes with a garden—may be at increased risk of CMM partly because of easy access to recreational sun exposure. We must acknowledge that there are large regional differences in the number of dermatologists and traditions for seeking medical care in areas of various population density. Accordingly, a study reports on a close association of biopsy rates with increasing incidence of melanoma,24 another study on an association between greater dermatologist density and lower melanoma mortality.25 Nevertheless, this uncertainty is incorporated in the areas of residence and therefore does not affect the association of having a garden with risk of CMM.

Our study had several important strengths. There was a high subject compliance and data reliability in the dosimeter study.26 The participation period was at least a summer period and the median number of days with both dosimeter measurements and corresponding diary information was 125.5 for those with a home garden and 122.5 for those without a home garden. The register study was nationwide. A unique personal identification number assigned to all persons with residence in Denmark ensured that the registers were correctly linked. Moreover, the registration of cancer cases is obligatory in Denmark, which ensures highly valid incidence data.

A potential limitation of the dosimeter study was that the study population was not representative of the Danish population. It was selected to give an age-span of adolescents and indoor workers and subgroups with expected high UVR exposure. The population sample might have a higher UVR exposure than the Danish population due to the subgroups of gardeners and sun worshippers that are overrepresented. Still, an equal distribution of the subgroups' participants in homes with and without a garden was more important. Furthermore, the sun exposure diary did not provide information about sun exposure on lower extremities, because we had to the limit the amount of daily questions in the diary due to the long study period. Still, we consider it likely that at least a part of the lower extremities were exposed when shoulders or upper body, and upper extremities were exposed. Finally, we were not able to link behavioral aspects from the dosimeter study directly to the risk of CMM. Although our analyses from the register study show an association of having a home garden with risk of CMM, inferences about causation are uncertain. We suggest, however, that having a home garden partly explains the positive association of SES with CMM, given that people with a higher disposable income are more likely to have a garden. We rely this on our finding, that people with a home garden are at increased risk of CMM of the trunk and extremities—areas of the body that are exposed during recreational sun exposure, which seems to be practised to a greater extent among people with home gardens.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Material and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References

The authors are indebted to the volunteers for their interest and participation in the study.

References

  1. Top of page
  2. Abstract
  3. Material and Methods
  4. Results
  5. Discussion
  6. Acknowledgements
  7. References