Nitrogen oxides — i.e. nitrogen compounds with varying amounts of oxygen — have a very bad reputation. They are produced among other things by burning fossil fuels. In regions with heavy traffic and a lot of industry, they occur in high concentrations and are made responsible for a large number of diseases of the respiratory system. However, nitrogen oxides also occur in nature. There they play an important role in the nitrogen cycle, which ensures that nitrogen, essential for life, is available in forms that the organisms can process.