Introduction

1. Nitrogen dioxide is a gas produced by reaction of nitrogen and oxygen in combustion processes. The reaction usually takes place in two stages, the first resulting in the combination of one atom of each of the gases to form nitric oxide, this compound then being oxidised over time to produce nitrogen dioxide. Wherever nitrogen dioxide occurs, nitric oxide is also found, and these oxides of nitrogen are collectively known as NOx.
2. There are several natural sources of oxides of nitrogen in the atmosphere, including lightning and forest fires. Bacterial activity in soils and possibly plant metabolism are responsible for a proportion of the oxides of nitrogen found in the air in the United Kingdom, but by far the largest amount is formed as a consequence of combustion of the fossil fuels petrol, oil, coal and gas, especially by motor transport and non-nuclear power stations.
3. Once formed, nitrogen dioxide takes part in chemical reactions in the atmosphere that convert it to nitric acid and nitrates, both of which can be removed by rain. However, nitrates can also remain in the air as very small particles, for example as ammonium nitrate, which can be dispersed widely in the atmosphere, contributing to the airborne concentrations of small particles known as PM₁₀, (Particulate Matter less than 10µm in diameter¹).
4. Nitric oxide is a gas that is produced naturally by cells in the lung and respiratory tract, and has no harmful consequences when inhaled by man at the concentrations likely to occur in the ambient atmosphere. It will not be considered further in this report, save as a precursor of nitrogen dioxide. Nitrogen dioxide is an irritant gas which has been known for many years to have serious and sometimes fatal effects on health when inhaled in the very high concentrations associated with accidental exposures, for instance in farm silos and in mines. There is now evidence that it has more subtle effects on health at the much lower concentrations that may occur in the ambient atmosphere, both outdoors and indoors. The part played by particulate nitrate in contributing to PM₁₀ has been discussed previously by the Panel in our report on Particles.
5. In this report the Panel discuss briefly the sources of nitrogen dioxide, the methods by which it is measured in the ambient atmosphere, the concentrations found by monitoring in the United Kingdom, and the effects that it may have on health. We then consider current European Community limits and World Health Organization guidelines and make recommendations for a new Air Quality Standard for the United Kingdom. In doing so, we have taken account only of the direct effects of nitrogen dioxide on health and have not considered its well known effects on vegetation and the broader environment. We believe, however, that acceptance of, and compliance with, our recommendations will also make a contribution towards ameliorating these other undesirable effects.