This paper briefly reviews our knowledge concerning the potential health risk of smog. It includes a review of the meteorological phenomenon, the causes of death in different age groups from London smog which occurred during the week of December 5, 6, 7, 8, 1952. During the London disaster, which caused 4,000 excess human deaths, it was deternimed the meteorological factors of almost absence of wind, low temperature, high humidity and an inversion. Hence, at ground level of air pollutants were suddenly increased and attributed to the combined action of a particulate matter with sulfur dioxide. As a result, abnormally large numbers of persons died especially, those individuals already suffering from disease of the respiratory and cardiovascular systems.