The ozone layer and its effect on ultraviolet light

The Earth is surrounded by a thick atmosphere which provides us with the air we beathe, keeps the planet warm and also protects us from harmful radiation from space. One of the most important types of damaging radiation is ultra violet radiation from the Sun.
The part of the atmosphere that filters out a lot of the ultra violet radiation is called the ozone layer – a region of the atmosphere between about 15 and 40 km above the Earth's surface.

The ultra violet radiation from the Sun is divided into three regions depending on the wavelength of the radiation. These are known as UVA, UVB and UVC. Some of the properties of these three types are decribed below.

UVA

(a) UVA accounts for about 90% of the ultra violet radiation reaching the Earth's surface
(b) it has a wavelength range from 320 nm to 400 nm
(c) about 5% of the Sun's radiation is UVA
(d) it can pass through ordinary window glass
(e) the intensity of UVA radiation reaching the Earth's surface does not change with altitude, weather conditions or the time of year
(f) UVA can penetrate deep into the skin affecting underlying tissues
(g) prolonged exposure to UVA can cause long term skin damage.

UVB

(a) UVB has a wavelength range from 290 nm to 320 nm
(b) because its wavelength is shorter than UVA it cannot pass through window glass.
(c) it causes tanning and is roughly 1000 times more likely than UVA to cause sunburn.
(d) however it does helps the body with normal vitamin D production.
(e) the ozone layer is very effective at screening out UVB. For radiation with a wavelength of 290 nm, the intensity at Earth's surface is 350 million times weaker than at the top of the atmosphere
(f) unlike UVA the intensity of UVB reaching the Earth's surface varies with the season. It is more intense in the summer than in the winter.
(g) its intensity varies with weather conditions and the time of day. It is more intense at midday than in the morning or late afternoon.
(h) it is also more intense at high altitudes and near the equator.
(i) it is known to be an important factor in the development of cataracts and the growth of 90% of skin cancers
(j) you can protect yourself againt the effects of UVB by using sunscreens containg a sun protection factor

UVC

(a) UVA has a wavelength range from 200 nm to 290 nm
(b) it is absorbed in the upper atmosphere at around 35 km
(c) because of its short wavelength and high energy it would cause severe damage to living cells in both plants and animals if reached the Earth's surface

Damage to the ozone layer

A reduction in the amount of ozone in the ozone layer would allow more ultra violet radiation, especially UVC, to reach the planet's surface and this increase would have a damaging effect on both plants and animals. In the latter part of the twentieth century scientists discovered that the amount of ozone in the ozone layer was being reduced by the emission of chemicals by industrial processes on Earth. This effect was so severe that a hole appeared in the ozone layer above Antarctica.

Their studies showed that the ozone layer was being reduced in certain areas by chlorofluorocarbons (CFCs) chiefly used in refrigerators, air conditioners, propellants in some aerosol sprays and Styrofoam insulation. The damge to the ozone layer due to CFCs begins when sunlight breaks down these molecules releasing atomic chlorine. Atomic chlorine destroys the ozone which then increases the intensity of ultra violet radiation reaching the Earth's surface.Other gases that can affect the ozone layer are CH₂, nitrous oxide (N₂O), and sulphur dioxide (SO₂).

However there are some optimistic results. In 2003 scientits announced that the depletion of the ozone layer may be slowing down due to the international ban on chlorofluorocarbons. Three satellites and three ground stations confirmed that damage to the ozone layer has slowed down significantly during the past decade. It is possible that further damage may still occur due to CFCs used by nations which have not banned them, and by gases which are already in the stratosphere.

Finally remember that damage to the ozone layer and the increase in sunburn has nothing to do with global warming.

For information on global warming see Global warming.