Ozone

What is ozone?

Ozone is a form of oxygen in which three oxygen atoms join together to form a molecule of ozone (O3). This is unusual, since most oxygen atoms in the atmosphere join up in pairs to form oxygen molecules (O2).

Where is ozone found in the atmosphere?

Ozone is formed by the action of sunlight on molecules of oxygen (O2) in the stratosphere. Thus, it is mainly concentrated in a layer at altitudes between about 10and 30 km above the earth's surface. Some ozone can also be formed close to the earth's surface by the action of sunlight on, for example car exhaust fumes. This low altitude ozone (a constituent of smog) can cause a health hazard if it is breathed in.

Why is the stratospheric ozone layer important?

As well as emitted visible light, the sun also emits short-wavelength ultraviolet wavelengths. The ozone layer absorbs the shorter, more hazardous, ultraviolet wavelengths and therefore prevents them from reaching the earth's surface.

Would it matter if there was no ozone layer?

Short-wavelength ultraviolet radiation can affect human health by causing skin cancer and affecting the immune system. It can also cause various types of damage to the eyes (e.g. photokeratitis or snow blindness, and various opacities on and within the eyes, including pingueculae, pterygia and cataracts).

What is happening to the ozone layer?

The ozone layer is being damaged by various man-made chemicals (e.g. chloro-fluorocarbons, or CFC's), which may find there way to the earth's surface up to the stratosphere and destroys the ozone molecules.

A less dense ozone layer would allow more ultraviolet radiation to reach the earth's surface with consequent problems for human health, and for animals and plants in general. A single chlorine atom from a CFC can destroy as many as 100,000 ozone molecules.

Is there anything being done to stop this happening?

A large number of countries have signed the Montreal Protocol, which restricts the production of these chemicals. However, because these chemicals survive along time in the atmosphere some loss of ozone will persist for at least another 50 years. Worries remain too, that some countries may continue to produce these chemicals. Do we know whether or not there has already been a loss in atmospheric ozone?

Yes. Measurements made by satellites, rockets and from the ground all show that at some times of the year more than half the ozone over the South Pole has been lost. Fortunately, losses over Britain and other non-polar regions are very much smaller (about 3% per decade since 1979).

Does this mean that much more short wavelength ultraviolet radiation is reaching the ground?

Yes, at the South Pole. At the present time, the evidence is that ultraviolet levels have increased in Europe and other non-polar regions is much weaker, although the situation is being carefully monitored to ensure this does not occur.

When do I protect my eyes against short-wavelength ultraviolet radiation?

At present, the risks to the eyes are not substantially greater than they have been in the past. Amounts of potentially damaging solar radiation have always been greatest at around noon in mid-summer. They increase as we move towards the equator or to higher altitudes. Sand and snow tend to reflect substantial amounts of UV and therefore increase the risk of damage. Protection is almost certainly needed then, during skiing or other activities on snow at high altitudes (particularly during summer) or on the beaches, particularly when these are near the equator.

How can I protect my eyes?

Wearing a broad-brimmed hat considerably reduces the amount of light striking the eyes when walking or standing. Ordinarily clear plastic lenses, high-index glass lenses or some specialist contact lenses provide considerable protection against short-wavelength UV.

Sunglasses provide still better protection, although it is important to check that these are made to meet the British standard for sunglasses (BS 2724:1987). Sunglass shade numbers greater than 2.5 give good protection.

Do I still need protection when the sky is cloudy?

Many types of cloud have only a small effect on the amounts of UV reaching the ground. It is, then prudent to continue to protect the eyes in high-risk environments even when the sky is cloudy.

Related Eye Links:

All About Sunglasses;
Eyecare for Children;
Vision for Sport.

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