The major cause of melanoma and NMSC is UV radiation exposure. Childhood and adolescent sun exposure is thought to be important in determining the lifetime potential for skin cancer. Adult exposure appears to contribute to the extent to which this potential is realised. The exact exposure needed to develop various skin cancers is not entirely clear. It is likely that both episodic and cumulative exposures are important; episodic exposures have been shown to more strongly determine the risk of melanoma. Based on a review of recalled sun exposure by period of life in studies of melanoma, the relative risk of melanoma with a history of childhood sunburn has been estimated to be 1.8, while for sunburn in adulthood it is 1.5.
In adult life, recreational (intermittent) sun exposure appears to be the strongest determinant of melanoma risk, followed by total lifetime sun exposure and occupational exposure. Fair skin, which tends to burn easily and rarely tans, is also an important risk factor for skin cancer.
In relation to NMSC, there is evidence that childhood and recreational (that is intermittent and non-occupational) sun exposure is important in determining the risk of basal cell carcinoma (BCC), while cumulative sun exposure such as occupational exposure, is the main determinant for squamous cell carcinoma (SCC).
Using actinic keratoses as a surrogate for NMSC, studies have shown the impact of childhood exposure on increasing the risk of NMSC; sunscreen use has been shown to reduce the risk in adults.
In 2009, the International Agency for Research on Cancer (IARC) added UV emitting tanning beds to its highest cancer risk category, labelling them as "carcinogenic to humans" after deeming them to be more dangerous than previously suggested. A systematic review of published studies revealed that first exposure to a solarium before the age of 35 years increases the risk of cutaneous melanoma by 75%. For all users, the risk of squamous cell carcinoma is estimated to be more than double compared with non-users.
The Australian/New Zealand standard for the solarium industry (AS/NZS 2635:2008) was revised in January 2009. The standard covers the operation of solariums and provides best practice industry guidelines, such as excluding individuals with skin type I (pale white skin that always burns, never tans) and those under 18 years of age. It also recommends that individuals with skin type II (people with white skin who burn easily, tan minimally) should not use a tanning unit.
There is no evidence supporting the view that exposure to UV radiation through solariums is ‘safe’ or that tanning in this way protects against skin cancer. The Australian/New Zealand standard for the solarium industry prohibits any claims of health benefits and ensures warning signs listing the risks must be exhibited.
See the Cancer Council position statement on Solariums for more information
Impact of climate change
The ozone layer acts as a barrier to UV radiation; its depletion over the 20th century has resulted in higher radiation levels reaching the earth’s surface. International measures to protect the ozone layer are showing signs of impact, but improvements have not yet returned ozone to pre-1970s levels.
Recently there has been an increased awareness of the interactions between ozone depletion and climate change, with global warming also thought to slow the recovery of the ozone layer. It is estimated that the increase in ambient temperature due to climate change will influence people’s behaviour and the time they spend outdoors. Therefore, skin cancer incidence is likely to continue to rise, particularly in temperate climates. Previous research has shown that people are more likely to be sunburnt in warmer weather.
Sunscreens: the evidence
There has been some debate about the role of sunscreens in skin cancer prevention and the potential association of sunscreen use with melanoma risk. Follow-up on a randomised trial published in 2010 concluded that melanoma may be preventable in adults by regular use of sunscreen. A review found mounting evidence that sunscreen can prevent SCC, but no evidence that it can prevent BCC. A summary of the evidence published in 2007 also found no conclusive evidence that broad spectrum sunscreens prevented BCC.
It has also been suggested that people may use sunscreen in order to stay longer in the sun (by reducing the risk of burning), thus increasing their risk of cutaneous melanoma. However, Gallagher et al. caution that retrospective case–control studies of melanoma and sunscreen use should be interpreted with great care, because of subject recall problems and the inevitable confounding of sunscreen use with reduced exposure.
Daily sunscreen use has been shown to be both effective and cost-efficient in preventing squamous cell carcinomas and solar keratoses. An Australian study estimated that there were 14,200 fewer cases of squamous cell carcinoma diagnosed in 2008 due to sunscreen use. The same study also found about 14% of people (or 1,729 cases) who would otherwise have developed melanoma in 2008 had their cancers prevented through regular sunscreen use. While more evidence is needed to show that modern sunscreens prevent melanoma, their use, along with other sun protection strategies, is encouraged as a means of combating the year-on-year rise in melanoma incidence.
Nanotechnology has been used in sunscreens for many years. All sunscreens in Australia are tightly regulated through the Therapeutic Goods Administration (TGA). In early 2009, the TGA reviewed the scientific literature in relation to the use of nanoparticulate zinc oxide and titanium dioxide in sunscreens, concluding that:
- The potential for titanium dioxide and zinc oxide nanoparticles in sunscreens to cause adverse effects depends primarily upon the ability of the nanoparticles to reach viable skin cells; and
- The current weight of evidence suggests that titanium dioxide and zinc oxide nanoparticles do not reach viable skin cells; rather, they remain on the surface of the skin and in the outer layer of the skin composed of non-viable cells.
Drawing on the best available evidence, our current assessment is that nanoparticulates used in sunscreens do not pose a risk. However, Cancer Council Australia continues to monitor research and welcomes any new data that sheds more light on the topic.
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