Ozone is not directly emitted, but is formed by a complex set of reactions involving nitrogen oxides and hydrocarbons in the presence of sunlight. In natural, unpolluted conditions, a cyclic balance is reached where sunlight breaks down nitrogen dioxide to form ozone, which then reacts with nitric oxide to reform nitrogen dioxide. At night, when there is no sunlight to drive the first part of the cycle, ozone is destroyed but not replaced.

In polluted conditions where the air contains hydrocarbons from fuel combustion, the balance is upset. The hydrocarbons also react to form nitrogen dioxide, which then breaks down into more ozone. As a result, both nitrogen dioxide and ozone levels increase leading, in severe cases, to summertime 'smog'.

The majority of nitrogen oxides emitted from a vehicle exhaust are in the form of nitric oxide. As it is nitric oxide that destroys ozone, ozone concentrations are actually lower next to busy roads. For the same reason, ozone annual means are higher in rural locations than in cities.

Like nitrogen dioxide, high levels of ozone can irritate and inflame the lungs. It can also cause eye irritation, migraine and coughing. It is a strong oxidising agent, so can attack materials such as rubber and pigments and damage vegetation. The international costs of ozone pollution through damage to health, crops and materials are huge.

Once formed, ozone can remain in the atmosphere for many days and is often transported over long distances. It is for this reason that a reduction in ozone levels can only be achieved through European-wide action. Studies have shown that European ozone levels have increased rapidly since 1940. Monitoring data from rural sites in the UK suggest that there was a small annual increase during the 1990s.