Overexposure to the sun often leads to sunburn, with the skin becoming reddened and inflamed. In extreme cases, blistering and peeling occur, and this has been linked to a higher risk of skin cancer.

The sun produces visible light and other types of electromagnetic radiation that are invisible to the human eye.

Visible light consists of wavelengths in the range of 400-700 nanometers. Ultraviolet (UV) light is a shorter wavelength, higher frequency light that literally translates to ‘beyond blue’, and is in the blue end of the colour spectrum. UV ranges from 200-400 nanometers and is further broken down into three categories: UVA (320-400nm), UVB (290-320nm) and UVC (200-290nm).

UV has different properties to visible light. For instance, if a cloud blocks a percentage of visible light on a cloudy day, it will not block the same percentage of UV. Similarly, UVA, UVB and UVC have different behavioural properties such as scattering and absorption. The earth’s atmosphere blocks most of the lower wavelength UV (almost all of UVC and most of UVB), so that on most of the UV that reaches the earth is UVA (~95%), the longest wavelength UV. Because thick clouds block UVB, the amount of UVB that makes it to the earth’s surface is heavily dependent on cloud cover.

UVB induces production of Vitamin D, which is vital for bone health, the nervous system, the immune system and controlling insulin secretion. But, overexposure to UVB can have acute and harmful effects. UVB causes direct damage to DNA, which can lead to replication mutations and eventually cancer. By contrast, UVA causes indirect damage to DNA via free radicals, and this indirect damage can lead to cancer^[1]. While UVA appears to be less carcinogenic than UVB, it is more abundant in sunlight than UVB. Hence UVA contributes significantly to the carcinogenicity of sunlight.

Melanin is the body’s defence against UV. Upon exposure to UV, the brown skin pigment increases – suntan. Melanin can absorb and disperse UV to some degree to protect the body’s precious DNA. In general, people with darker skin are more likely to withstand greater sun exposure than fair-skinned people.

Most sunburn appears to happen near the boundary of the UVA and UVB bands. This has led to the development of the ultraviolet index or UV Index, an international standard that conveys information about the strength of UV at a given location and time, and can be found for Australia on the Bureau of Meteorology website.

Due to the thinning of the ozone layer in the mid-latitude parts of the earth, Australia has a high UV risk. Without the ozone layer to stop the sun’s powerful radiation, UV can penetrate to the earth’s surface, including through clouds. So even on a cloudy day, it’s possible to get burnt. In most parts of Australia, the UV index is high-extremely high all year around, so it’s sensible to use sun protection all year. If in doubt, check the UV index forecast before heading out.

Reflected UV can be as dangerous as direct UV. Shiny surfaces such as water and snow reflect UV and can catch bushwalkers unexpectedly, particularly if the air temperature is cool and keeping warm is more to mind rather than sunbathing. Similarly, sun exposure is a problem at high elevations because UV exposure increases with altitude. Sun exposure is also a issue in countries where the malarial medication doxycycline is taken – it makes the skin more sensitive to sunburn.

Hence, sun protection may be as important in winter as it is in summer: use a hat, sunglasses, sunscreen, and a good shirt all year round.