What are the sources of solar UVR exposure?

Solar UVR can reach you on the ground from three sources:

• Directly from the sun.
• Scattered from the open sky.
• Reflected from the environment.

This means that even if you are shaded from the direct sun you can still receive substantial UVR exposure from the open sky and reflective ground surfaces. Reflective surfaces can reduce the effect of protective measures. For example a person in a boat wearing a hat may still receive expose to their face from UVR reflected off the water. Also some ground and building surfaces are quite reflective to UVR including white paint, light coloured concrete and metallic surfaces. These surfaces can reflect UVR onto the skin and eyes.

What factors affect solar UVR levels?

• Sun Angle: The most important factor affecting the level of solar UVR at the earth's surface is the height of the sun in the sky. The higher the sun is in the sky, the shorter path the UVR has to travel through the atmosphere, so less is absorbed resulting in higher levels of UVR at the surface. When the sun is low in the sky the radiation has a longer path to travel so more radiation is absorbed and scattered by the atmosphere resulting in lower levels of UVR at the surface.
  
• Geographical Position: Australia generally has high levels of solar UVR in comparison with Europe and many parts of North America, due mainly to its geographical position close to the equator.
  
• Seasonal Effects: There is significantly less UVR in winter as the sun is lower in the sky. In Melbourne the levels of UVR are approximately three times higher in midsummer than in midwinter. The highest risk months are November to February and this period is generally longer at latitudes closer to the equator.
  
• Cloud Cover: Solar UVR can penetrate through light cloud cover, and on lightly overcast days the UVR level can be similar to that of a cloud-free day. Heavy cloud can reduce the intensity of UVR. Scattered cloud has a variable effect on UVR levels, which rise and fall as clouds pass in front of the sun. UVR levels can be high enough to cause sunburn on cloudy days.
  
• Ozone: This is a form of oxygen that occurs naturally in the upper atmosphere and has the ability to absorb UVC and UVB radiation. Atmospheric absorption prevents all UVC and most of the UVB from reaching ground level.
  
  Ozone levels rise and fall naturally from day to day and seasonally. Ozone over Australia is generally lowest in March. The ozone "hole" is the significant springtime (September-October) reduction in the total ozone over Antarctica. Publicity about the discovery of an ozone hole over Antarctica has increased general awareness and caused concern about possible increases in UVR levels in southern Australia.
  
  The ozone hole does not extend as far north as Australia but stratospheric winds can occasionally carry ozone-depleted air towards Australia causing a short-term decrease in ozone and a corresponding rise in UVR levels.
  
  Ozone depletion and the associated increase in solar UVR reaching the earth's surface is a major environmental issue. Other factors such as sun height and variations in cloud cover may have more local influence on the intensity of UVR reaching the ground.
  
• Scattering: Due to scattering of solar UVR by molecules and particles in the atmosphere there is often about as much UVR received from the open sky as there is directly from the sun. If you are in the shade but can see a lot of blue sky you are still exposed to UVR scattered from the sky. At times, the amount of scattered solar UVR that reaches your skin may even exceed that from the direct sun.
  
• Environment: A highly reflective environment can also increase UVR levels. Some ground and building surfaces are quite reflective to UVR. White paint, light coloured concrete, snow, water and to a lesser extent soil can reflect UVR onto the skin.
  
• Altitude: The intensity of UVR increases by about 12% for every 1000 metres increase in altitude. At higher altitudes there is less atmosphere for the UVR to pass through before it reaches the ground so less is absorbed.
  
  Consequently, people at higher altitudes can be exposed to more UVR than those at sea level. In the Australian ski fields, at an altitude of around 2000 metres, the UVR levels on clear days can be substantially higher than at sea level. The fact that snow is extremely reflective to UVR is an additional hazard.