In Europe the word "smog" and the expression "air pollution episode" are mainly associate with phases of high air pollution occurring in winter ("London smog"). This is due to the fact, that increasing emissions from the combustion of fossil fuels and stable inversion layers in the cold season lead to a higher frequency of serious "winter smog" episodes in Northern Europe. The main pollutants are sulphur dioxide (SO2) and its oxidation products "sulphates", suspended particulates (containing heavy metals, tars, organics, and other pyrolysis products), nitrogen oxides (NO and NO2), nitrates, carbon monoxide (CO), etc. With increasing industrialization the unchecked led to episodes with extreme pollutant levels that had a severe impact on public health (London smog episode, 1952): morbidity and mortality rates increased significantly. Many countries have reacted to this form of air pollution by introducing long-term abatement measures that have had substantial success in decreasing general health risks. In recent years, acute measures have been introduced to cope with the smog episodes directly, i.e. at the onset and during these phases of high pollution. These measures, termed "smog alert system", encompass reduction of vehicular traffic, reduced heating in public buildings, changing to fuel with low sulphur content, etc. These procedures have only limited effect on the overal pollution level, but may help to stop or slow down the increase of concentration in the streets in the streets in the centre of cities, thus lowering exposure of the population.
