Factors that Influence UV Radiation on Human Skin

UV Radiation and the Sun

UV radiation originating from the sun is one of the leading causes of skin cancer. How much radiation reaches the skin is determined by a number of factors. This explains the variability that occurs from one day to the next and whether one gets sunburned or not. Understanding these environmental influences helps us make the appropriate decisions as to sun safety. Anytime one plans to spend time outside, protection from overexposure should take place, with all the following factors considered.

Solar Elevation Angle

Where the sun is positioned in the sky. 

The location at which the sun sits in the sky determines the level of UV radiation reaching one’s skin. One must account for the amount of atmosphere the UV has to penetrate to reach us.

  • If the sun is directly above us, the atmosphere level is lower. This results in less UV absorption and a higher level reaching us.
  • If the sun is near the horizon, there is a longer path that the UV must travel through the atmosphere. More of this UV is absorbed resulting in less that reaches us.

Ozone

The ozone layer and one’s proximity to ozone hole

Ozone is part of our atmosphere. Where there are high levels of ozone, UV will get absorbed at higher levels, thereby protecting us from excess UV and the corresponding damage to cells. The ozone layer protects life on earth by absorbing ultra violet radiation from the sun.

In the early 1980’s a hole in the ozone layer was discovered, located over the Antarctic. Antarctica is very cold. It is so cold that clouds develop in the Antarctic stratosphere. Chlorine reacts on the surface of these cloud particles.

But where did this chlorine originate? CFC’s,  found in aerosol spray cans, perfumes and paints. UV light breaks a chlorine atom off of a CFC molecule. The chlorine molecule reacts with the ozone (O3) which in turn forms a molecular oxygen (O2) and chlorine monoxide.

Over 15 years ago scientists had forecasted the possibility of man made chemicals breaking down the protective ozone layer. This resulted in the ban of CFC aerosols in the US in 1978.   In 1985 the hole in the ozone layer was discovered over Antarctica. A thinning of the ozone layer was also witnessed above the Arctic. This led to global action to curtail the use of CFC’s and related compounds in 1987. Scientists explained however that hole in the ozone would take some time to close completely due to the life span of CFC’s in the atmosphere which was 50 – 100 years.

In the year 2000, the ozone hole reached its record size of approximately 28.3 million square kilometres. Then in October 2015  news circulated of the Ozone hole over Antarctica expanding to four times the size of Australia. The ozone hole’s size currently has stabilised, with reports of it having significant healing. This 30 years after the world banned CFC’s.

Man with sunburned skin
Amazing Adriatic sea bay with sunset in Croatia

Australians have been shown to have the highest rates of skin cancer in the world with approx 1200 people dying from UV damage each year. Australian’s have exposure to radiation levels that are reportedly much higher than in other countries like Europe. This is due to Australia being located close to Antarctica’s ozone hole. The risk for sun dwellers is that there are far more severe levels of UV radiation getting through to the ground level.

Cloud Cover

The clouds and their placement in the sky do have a benefit to us in their ability to reduce the amount of UV radiation reaching the earth’s surface. It is important however to take into account the potential for the sun’s rays to reflect off the clouds, thereby increasing UV levels. This brings us to the next factor governing UV radiation potential.

Ground Surface Reflectivity

With solar ultraviolet radiation, much of the threat comes from the earth’s surface, due to its ability to reflect UV rays. Metal surfaces, concrete and beach sand all have the potential to reflect UV back onto a person, increasing the risk of cellular skin damage.

Altitude

UV radiation increases at higher altitudes. It has been found that UV increases by up to 4% for every 300m increase in altitude.

Aerosols and Pollutants

Small particles suspended in the air have the ability to scatter solar UV rays. The effect is minor, but it does impact levels of UV absorption.

Direct and Diffuse Uv

UV radiation may diffuse. Diffusion means that regardless of shade, large areas in the sky that are not filled in with cloud cover scatters the UV allowing it to reach you. Shade does offer some sun protection, but not always in adequate proportion to the sun and its positioning in the sky.