The maximum ozone concentration that can be reached in a polluted atmosphere depends not only on the absolute concentrations of volatile organic compounds and nitrogen oxides (NOx) but also on their ratio (1). At intermediate ratios (4:1 to 10:1), conditions are favourable for the formation of appreciable concentrations of ozone. In rural areas, ozone production is usually limited by the availability of nitrogen oxides. Summer conditions favour the formation of ozone primarily because of increased ultraviolet radiation, temperature and low wind speeds with stagnation conditions, and also because of increased precursor emissions, including nitrogen oxides from soils and increased levels of volatile organic compounds. Natural sources account for about 15% of nitrogen oxide levels in the summer but may account for a larger proportion in some rural areas.