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Stratospheric ozone protects us from harmful ultra-violet radiation from the Sun. However, in the troposphere, ozone is dangerous to our health and in this section we look at the damage ozone does.
A little bit of ozone is needed
Its not true that we don't want any ozone in the troposphere. We need low levels in the air to form hydroxyl radicals (OH) to clean the air of harmful chemicals. However, high concentrations of ozone are harmful to our health and damage plants. In swimming pools ozone is sometimes even used to kill bacteria. So what makes ozone so aggressive?
1. OH formation can only take place if there is a small amount of ozone in the air. scheme: Elmar Uherek.
Ozone attacks our respiratory system
We know that organic compounds made up of carbon (C) and hydrogen (H) are present in all living organisms, in plants, in animals and in our bodies. As well as C and H, these compounds may also contain oxygen (O), nitrogen (N), sulphur (S) and phosphorous (P). The main structures of organic compounds are made of carbon, and generally include C-C single bonds and C=C double bonds. C=C double bonds are found everywhere; in unsaturated fatty acids, haemoglobin, proteins, on the surface of the pulmonary alveoli in our lungs and its mucus membrane and in many other bio-molecules. Ozone attacks these C=C double bonds.
The other two main oxidants, OH and NO3, have extremely short lifetimes and react immediately they are formed. Ozone, however, can enter the lungs. Every day we breathe 20,000 liters of air into our lungs. This air carries many chemicals and particles, including ozone.
2. Not all fatty acids have double bonds. Some are (a) saturated and so just have C-C single bonds. Many of them are, however, (b) unsaturated and have C=C double bonds. Source: NTRC, Univ. of Texas Kingsville.
Ozone breaks down double bonds.
Ozone reacts with fatty acids in the lungs in the same way that it does in the air. Ozone reacts with the C=C double bonds and breaks down the molecule into very reactive radicals. These radicals then react further. The end result is inflammation of the lungs. This causes breathing problems in previously healthy people and is particularly dangerous for people with asthma. If ozone levels are high it can be dangerous to do sports or hard physical activity.
3. Our lungs have a surface area of 80 - 100 m2 and we breathe in 20,000 litres of air each day. This air contains many chemicals and particles, including ozone and this ozone attacks the alveoli and mucus membranes of our lungs. © emphysem-info.
4. The reaction of ozone with double bonds (ozonolysis) breaks down (cracks) the C=C bond. scheme: Institute for chemical education - Univ. of Duisburg, Germany.
Ozone not only breaks down the C=C double bonds in our lungs, it also attacks the double bonds of terpenes from plants, many other molecules in the air and causes damage to plant leaves. At very high concentrations ozone can even destroy the double bonds in the rubber of an air balloon and make it burst.
5. a + b) Rubber contains C=C double bonds. If you expose a balloon to a stream of ozone rich air ...
... it becomes thinner and thinner and finally the balloon bursts. Experiment: Institute for chemical education - University of Duisburg, Germany.
You find basic information about ozone and what harm it does to leaves at:
Lower atmosphere - Basics - Unit 3 - ozone