Around 30 Australians are diagnosed with melanoma every day and more than 1,200 die from the disease each year.
While small amounts of ultraviolet (UV) radiation are required for the production of vitamin D to keep bones and muscles strong and healthy, skin can burn from just 15 minutes of exposure to the summer sun.
In 2009, the World Health Organization (WHO) classiﬁed the whole ultraviolet spectrum and the use of solariums as carcinogenic to humans, placing them in the same category as asbestos and tobacco. The majority of skin cancers in Australia are thought to be caused by exposure to UV radiation in sunlight.
The sun emits three different types of UV radiation: UVA, UVB and UVC. While UVC rays are filtered by the ozone, 10% of UVB and 95% of UVA rays reach the earth’s surface.
The UV effects on the skin are largely dependent on the type of UV rays, the amount and intensity of UV, and the stage at which the cells on the skin are in during their normal division and renewal process.
UV can produce a number of effects within the cell including specific types of DNA damage in skin cells and, with extreme UV exposure, cell death. Some of these types of oxidative DNA and nucleotide damage, and failure of the cells to repair this damage can prompt cells to mutate, leading to the development of skin cancers.
With some cancers that develop on skin exposed to excessive sunlight, DNA damage can be identified through UV-specific DNA mutations within the tumour.
When cells are actively dividing and proliferating they are particularly vulnerable to DNA damage. So cells are equipped with mechanisms to respond to and repair DNA damage within the cell to restore the DNA structure before they continue dividing.
The cells respond by delaying progression through the cell cycle through the cooperation of cell cycle checkpoints and several biochemical pathways. This allows sufficient time for repair before the critical phases of the cell division process proceed.
In the event the DNA damage is too severe, the cells kill themselves off, preventing DNA damage being transferred to the daughter cells.
If these checkpoints are defective and do not delay the cell cycle progression to enable repair, the result may be an increase in DNA mutations and chromosomal defects. This can cause uncontrolled growth, transformation of the cell and the development of cancer.
Protecting against UV damage
A number of environmental factors influence the amount of UV radiation we are exposed to throughout the day, including the earth’s latitude, height of the sun throughout the day, cloud cover and reflection of surfaces.
In Australia, we need to balance the risk of skin cancer from too much sun exposure with maintaining adequate vitamin D levels. During summer, most people can maintain adequate vitamin D levels from a few minutes of exposure to sunlight on their face, arms and hands or the equivalent area of skin in the morning or late afternoon when the UV index is below three.