Serious smog events had become a common occurrence in London since the second half of the 19th Century, with industrialisation causing an increase in air pollution that often killed hundreds of the city's inhabitants when it amassed in great quantities during winter months. Since the Second World War, severe smog episodes had occurred in November 1948 (causing an estimated 700 to 800 deaths), January 1956 (1,000 deaths), December 1957 (750 deaths) and January 1959 (over 200 deaths). The 1952 Great Smog of London had forced the government to consider new anti-pollution legislation, which eventually led to the passage of the 1956 Clean Air Act that restricted the burning of domestic fuels in urban areas with the introduction of "smokeless zones". Despite the passing of the act, residents and operators were given time to convert from the burning of domestic fuels, and so smoky fogs continued to be a problem for some time, as they were in 1956, 1957 and 1959.