Once formed, ozone can travel long distances and peak levels tend to occur in rural areas i . Committee on Medical Effects of Air Pollutants . Handbook on air pollution and health . NO 2 is probably the most important for health. It is mainly a secondary pollutant formed by spontaneous conversion of NO to NO 2 in the presence of oxygen or ozone.
Concentrations are highest in urban areas, especially on busy streets.
Peak levels occur in cold, still weather during morning and evening rush hours. High NO 2 levels are often found in conjunction with high levels of particulates and carbon monoxide i . Committee on Medical Effects of Air Pollutants . Handbook on air pollution and health . The UK distribution of outdoor CO is not accurately known.