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.
Throughout the 1980's 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 1980's 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.
2. 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.