At ground level ozone is a pollutant. Ozone scars lung tissue, which worsens respiratory diseases and can cause breathing problems and coughing, especially for young children. It also makes eyes sting and throats itch. Ozone is one of the causes of acid rain. Ozone is a summer gas, baked by strong sunlight from a mixture of unburnt fuel, oxygen and nitrogen oxides, much of which comes from car exhausts. High concentrations of ozone tend to be found not so much in the cities but in the surrounding countryside where the conversion process is completed. It has been implicated as a contributor to forest dieback, damage to agricultural crops, and human health problems. Ozone breaks down the outer cuticle of the needles of trees and opens them up for further attack from acid precipitation.
Tropospheric (lower-atmosphere) ozone is a secondary pollutant, formed from emissions of nitrogen oxides, non-methane volatile organic compounds (N-VOC), and carbon monoxide (CO). A reduction of ozone formation can be achieved only by a balanced cut in all three pollutants. Ozone formation also depends on many other, to a certain extent local, factors such as solar radiation and temperature.
Stratospheric ozone, high in the atmosphere, should not be confused with ground-level and lower-atmosphere (tropospheric) ozone. The depletion of stratospheric (high-altitude) ozone (reducing the greenhouse effect) may be compensated by increases in ozone concentration at lower altitudes, enabling the lower stratosphere to receive more solar heating and to efficiently trap outgoing heat radiation. In absolute terms, however, the increase in tropospheric ozone compensates for only a fraction of the ozone lost in the stratosphere.
Ozone is another substance which is proving increasingly harmful to trees, particularly in the Mediterranean region. It is produced by pollutants such as nitrogen dioxide and by volatile organic compounds. The source of thus dubious cocktail is road traffic, on the one hand, and various industrial activities on the other.
High tropospheric ozone levels seriously affected the USA in 1988 and western Europe in 1989. Monitoring shows that ozone in the lowest ten kilometre layer of the atmosphere over middle and higher latitudes of the northern hemisphere is increasing by more than one percent per year (mainly as a result of combustion processes, including forest and bush-fires).
Throughout the 1980s the British government environmental agencies disputed whether pollution was an important factor in the health of UK tree stocks. The UK Department of Environment and the Forestry Commission both insisted Britain's trees were healthy and suffering no ill effects from pollution. By 1994 the DOE was reporting "the available evidence suggests that ozone is likely to adversely affect tree health in parts of Britain," and calling for cuts in pollution levels. Other European environmental agencies had reported in the early 1980s that ozone, sulphur dioxide and nitrogen dioxide pollution was making trees vulnerable to pests, diseases, frost and drought.
Individuals with pre-existing respiratory diseases are especially vulnerable to the high levels of ozone found in many urban areas. One in seven children in the United Kingdom is now affected by asthma. Levels of ozone in the United Kingdom rise above the official danger level approximately 100 days each year.
The European Commission released its annual ozone reports that give an overview of ozone pollution in the European Union (EU) for 1998 and for the summer 1999. The reports conclude that ozone pollution in the EU is still a threat to human health and vegetation. Ozone concentrations measured at more than 1400 stations are still well above the thresholds for the protection of the population and vegetation set in the current Ozone Directive (1)1. Last year on average the health protection threshold of 110 micro grammes of ozone per cubic metre was exceeded on 20 to 60 days in the Mediterranean countries and 10 and 35 times in the central countries of the Union with some individual spots appearing in up to 80 days. Although data for the summer of 1999 shows a slight improvement in the central part of the EU compared to previous years, breaches of the health threshold were frequent, and even the information threshold of 180 micro grammes of ozone per cubic metre was exceeded in most countries. In Italy, Greece, France and Spain some citizens received more than 40 alerts.
Recent evidence suggests that ozone increase over the past two decades could result in a greenhouse effect comparable to that of the other greenhouse gases combined.
Belgium suffers particularly badly because the pollutants that cause ozone travel across Europe in all directions, and Belgium lies just where they meet. Its ozone levels surpassed the safety guidelines by 31 times in 1995.