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Ozone facts:

Ozone in the lower atmosphere is an air pollutant with harmful effects on the respiratory systems of animals. It will also damage the leaves of sensitive plants. However, ozone near the ground should not be mistaken with the ozone layer in the stratosphere, where it absorbs most of the sun’s ultraviolet light preventing it from reaching the Earth's surface. 
Contrary to other air pollutants ozone is not emitted directly into the atmosphere. Man made ozone is formed as a byproduct of a series of chemical reactions of precursor pollutants, such as nitrogen oxides and hydrocarbons. The presence of light is also required for the formation of ozone, hence the term photochemical smog, which describes the situation during which ozone levels can reach harmful concentration levels. These episodes also do only occur during the summer months, when temperature and solar radiation levels are high.

The formation of ozone takes some time and highest concentrations are usually found in rural areas and not in the proximity of its precursors emission sources.

Ozone indexOzone concentration rangePossible health effects
low<120 mg/m3

Effects are unlikely to be noticed even by
individuals who know they are sensitive to
air pollutants

moderate>120 to 180 mg/m3Mild effects, unlikely to require action, may
be noticed amongst sensitive individuals.
high>180 to 240 mg/m3Significant effects may be noticed by sensitive
individuals and action to avoid or reduce these
effects may be needed (e.g. reducing exposure
by spending less time in polluted areas outdoors).
Asthmatics will find that their 'reliever' inhaler is
likely to reverse the effects on the lung.
very high>240 mg/m3The effects on sensitive individuals described for
'High' levels of pollution may worsen.

More measurements results also on form of indexes can be found here.