Solar UV radiation
UV radiation is not part of the visible spectrum of light, nor is it thermal radiation - unlike sunlight it cannot be seen, nor can it be felt like the sun’s warmth. As our senses cannot detect UV, overexposure may be damaging our skin and eyes without our knowledge.
Australia experiences some of the highest levels of UV radiation in the world because we are close to the equator and have many clear, blue-sky days. The Earth's orbit also brings countries in the southern hemisphere (Australia included) closer to the sun in summertime than countries in the northern hemisphere during summer. Table 1 shows the average monthly UV Index across Australia.
UV radiation levels vary throughout the year and throughout the country according to:
- The height of the sun (the higher the sun is in the sky, the higher the UV radiation level)
- Distance from the equator (the closer to the equator, the higher the UV radiation level)
- The amount of cloud cover (on average, clouds reduce clear sky UV by 30%)
- Altitude (UV increases by about 4% per every 300 m)
- UV-reflective surfaces (such as snow and water).
Solar UV intensity increases with sun elevation (incident angle of sunlight), altitude and lower latitude. The ozone layer and to a lesser extent heavy, non-scattered cloud cover can reduce UV radiation. However, cloud cover does not attenuate solar UV intensity to the same degree as infrared intensity (sensation of heat), so overexposure on cloudy, relatively cooler days should also be avoided. Reflective surfaces can scatter UV radiation, with snow (85-90%) and water (particularly white foam: 30%) reflecting the most UV radiation.
Approximately 60% of the day’s total carcinogenic radiation is received between 10am and 2pm (local standard time).
Artificial UV radiation sources
Solariums are another source of UV radiation, which are now banned for commercial use in all states and territories of Australia. Solariums emitted primarily UVA since the 1980s (99% UVA and 1% UVB), with their UVB output equivalent to a high UV Index of 12, similar to the midday summer sun in the tropics. An Australian study in 2008 found that some commercial sunbeds emitted up to six times more UVA than summer sun. In 2009, the International Agency for Research on Cancer (IARC) re-classified sunbeds into the highest risk category (Group 1: carcinogenic to humans).
Inadvertent UV radiation exposure may also occur in occupational contexts, such as industrial arc welding, germicidal UVR lamps for sterilisation and disinfection and the industrial printing/photo process. Various engineering controls (such as shielding and UV radiation detectors and alarms) are used to ensure that UV radiation exposure does not reach hazardous levels.
UVA, UVB and UVC
There are three types of ultraviolet radiation as classified by the International Commission of Illumination according to the biological action spectra (i.e. the wavelength required to induce a biological effect, such as erythema, carcinogenesis): UVA (315-400 nm), UVB (280-315 nm), and UVC (100-280 nm). The stratosphere absorbs all of UVC radiation, however, UVA and UVB radiation reaches the Earth’s surface (comprising 95% and 5% of UV radiation respectively).
In 2009, the International Agency for Research on Cancer (IARC) classified the full UV radiation spectrum as carcinogenic to humans. Both UVA and UVB radiation contribute to skin damage, premature ageing and skin cancer.
The UV Index is a standardised measure used to express UV radiation intensity. It is equal to 40 times the erythemally effective power of the sun in W/m2. The UV Index at solar noon is generally in the range 0-12 and values above 11 are considered extreme. In Australia, peak daily values in summer are regularly in excess of 12-14, and can reach 16-17 at more northern latitudes. When the forecast UV Index is ≥3, sun protection is required.
Table 1: Australian capital city average daily maximum UV levels by month.
Values are rounded to the nearest whole number. Highlighted months have an average daily maximum UV less than 3 even where rounding has made it appear otherwise.
Daily sun protection times (the times when the UV Index is forecast to reach 3 or above) for locations across Australia can be accessed via the free SunSmart app (for iPhone and Android phones and tablets), the SunSmart or Bureau of Meteorology websites and in daily newspapers. Real time UV levels for Australian capital cities can also be accessed via the app and at Australian Radiation Protection and Nuclear Safety Agency.
Figure 1: SunSmart app displaying the UV level and sun protection times, for a high UV day
Images provided by SunSmart, Cancer Council Victoria.
Health effects of UV radiation
Chronic overexposure to UV radiation (particularly UVB) can also result in immunosuppression, with UVB-induced DNA damage a major trigger. Immunosuppression increases carcinogenicity of UV radiation through modulation of the immune response.
Skin damage and ageing
UV overexposure contributes to degenerative ageing of the skin, which results in wrinkling, and loss of skin tone and elasticity. UVA and UVB radiation produce high concentrations of reactive oxygen species (ROS), which result in changes in dermal connective tissues (collagen degradation etc.).
The majority of the visible signs of ageing are the result of damage to the skin caused by UV exposure. It is believed that as much as 80% of premature facial ageing - such as wrinkles and fine lines - is due to sun exposure. Other factors like cigarette smoking further contribute to premature facial wrinkling.
The positive association between self-reported lifetime sun exposure and skin damage (hyperpigmentation) and ageing (wrinkling) is particularly strong for young people.
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