Introduction

1. The atmosphere in which we live consists largely of oxygen and nitrogen, the first of these two gases being essential to human and animal life. The oxygen in the atmosphere has been generated by' plant life, by the process of photosynthesis which uses atmospheric carbon dioxide as a fuel, oxygen being the waste gas. Thus there is a fine balance between use of oxygen by animals (and also in combustion processes) and its production by plants. Oxygen in the atmosphere exists in two forms, the gas we need to breathe and which contains two oxygen atoms in each molecule, and a closely related substance, ozone, which consists of three oxygen atoms per molecule. Unlike oxygen, ozone exists in the lower atmosphere in very low concentrations; moreover, it is a gas which has an irritant effect on the delicate surface tissues of the body, such as eyes, nose and lungs, if present in the air in high concentrations.
2. Ozone is a naturally occurring gas, generated in the higher layers of the earth's atmosphere, the stratosphere, by the action of ultraviolet light from the sun on oxygen molecules. At this level of the atmosphere it has beneficial effects on health, in that it helps to filter out harmful ultraviolet rays that can cause skin cancers. Indeed, there is currently much concern about reduction in stratospheric ozone levels by chemicals such as chlorofluorocarbons.
3. Paradoxically, at the same time as concern has been raised about depletion of the ozone layer in the upper atmosphere, anxiety has been expressed about possible harmful effects of raised levels of ozone in the lower atmosphere, the troposphere, where we live. The major chemicals which act as ground level ozone precursors in industrialised nations are oxides of nitrogen and hydrocarbons produced predominantly by motor vehicles and industry. In some circumstances, where heavy traffic occurs in association with strong sunlight, sufficient ozone (together with other chemical pollutants) may be generated to cause people to feel irritant effects - a situation sometimes called photochemical smog.
4. In this document, the Panel discuss briefly the main sources of ozone, methods of measuring its concentration in the troposphere, and its known health effects. We then make recommendations for an Air Quality Standard for the United Kingdom. In doing so we take note of the fact that concentrations of ozone in the United Kingdom are influenced by emissions in other European countries, which may require action on a broader, international, as well as on a national front.