Sixty years ago thick smog descended on London, contributing to the deaths of an estimated 4,000 people.
The four-day "pea-souper", while worse than usual, was a familiar experience for Londoners.
Visibility was so poor buses and taxis ground to a halt, forcing commuters to hurry underground to use the Tube.
In the Isle of Dogs in east London, the fog was reported to be so thick people could not see their feet and animals dropped dead at the Smithfield Show.
According to the Met Office "the smoke-like pollution was so toxic it was even reported to have choked cows to death in the fields".
A Ministry of Health comparison between the excess of deaths registered in the weeks ending 13 and 20 December and those occurring in the preceding weeks produced estimates of the number of deaths caused by the smog ranging from 3,412 to 4,075.
According to the General Register Office, respiratory diseases alone accounted for 59% of the increase in deaths registered in the week ending 13 December and 76% in the following week.
The main pollutant, sulphur dioxide, was linked to coal burning which was said to have reached exceptional levels.
The legacy of the Great Smog was the Clean Air Act of 1956 which introduced a number of measures to reduce air pollution.
Furnaces could no longer emit "dark smoke" and households were offered grants towards the cost of converting their coal-burning grates to smokeless fuel.
A broad consensus believed the legislation had a major impact on improving public health.
But new pollution threats are causing concern as air pollution mortality figures remain almost the same.
Lawyer Alan Andrews, of environmental organisation ClientEarth, said an estimated 4,300 Londoners now died each year as a result of air pollution (29,000 UK-wide, according to the Department for the Environment, Food and Rural Affairs).
This is no longer because of coal fires but is put down to nitrogen dioxide caused mainly by traffic fumes, he said.
"The UK is failing to meet European Union (EU) air quality standards.
"London is thought to have the highest levels of nitrogen dioxide of any capital city in the EU, with levels on some of London's busiest roads, such as Brixton Road and Putney High Street, currently more than triple legal limits," he said.
ClientEarth says the government's own plans show 16 regions and cities will not achieve legal limits for air quality until 2020, and London will have "illegal levels of air pollution until 2025".
Dr Frank Kelly, professor of environmental health at London's King's College, said legislation worked in the 1950s to deal with pollution but new laws were needed now.
"There's been a dramatic increase in car ownership and traffic, more taxis and buses, so it's a new type of pollution. Legislation is once again required," he said.
Dr Kelly said air quality was known to be a key factor in the UK's biggest health problems - heart disease, strokes and diabetes.
"We've known this for the last decade but politicians are only just waking up to it. We need to clean the air up," he said.
Under EU air quality laws daily pollution levels must not be above the legal limit on more than 35 days in a calendar year.
But air pollution levels in London had already exceeded EU daily limits 36 times this year by April.
A Department for the Environment, Food and Rural Affairs (Defra) spokesperson said: "We want to keep improving air quality and reduce the impact it can have on human health and the environment.
"Our air quality has improved significantly in recent decades and is now generally very good, and almost all of the UK meets EU air quality limits for all pollutants.
"London, like many of our big cities, is one of the limited areas where air pollution remains an issue.
"However, air quality plans outline all the important work being done at national, regional and local level will ensure we meet EU limits as soon as possible."