Fog as thick as pea soup is nothing new to Londoners. But on December 5, 1952, a dense, grimy cloud of fog descended on London, immobilizing the city and causing more than 4,000 deaths over a five-day period with an additional 8,000 deaths in the aftermath.

The intense pollution event known as The Great Smog or The Big Smoke resulted from a perfect storm of unusually cold weather along with high atmospheric pressure that led to thermal inversion, so the ground temperature was lower than that of the air above. On that nearly windless Thursday, a cap of warmer air trapped smoke from chimneys and smokestacks in a cold layer of air near the surface. At night, the layer of smog thickened and took on a sulfurous stink since the low grade domestic coal available in the years following World War II produced more sulfur dioxide than the higher quality coal that was slated for export.

Trafalgar Square, London, during The Great Smog, December 1952. (Photo credit: N. T. Stobbs, Wikimedia Commons)

To make the problem worse, a mist formed in this sheet of cool air. Water droplets collected on particles of soot and tar. Inside the droplets of water (H₂O), sulfur dioxide (SO₂) reacted to form sulfuric acid (H₂SO₄), creating “acid fog.” Particles of tar gave the smog a yellow-black color characteristic of a “peasouper.” Ordinarily, the morning the sun would have evaporated the mist, but on the morning of December 6, the sun could not penetrate the thick fog so it continued to blanket the city. The lack of wind and the warmer cap of air over the cold layer below kept the pollution in place as cold temperatures caused Londoners to burn even more coal to stay warm. Over the next two nights, the fog thickened. Soot blackened everything, and it was difficult to breathe.

When The Great Smog first settled on London, it did not raise alarm. For Londoners, smog was part of city life. Burning coal had kept homes warm and factories operating for centuries. In Bleak House (1852), Charles Dickens describes “fog down the river, where it rolls defiled among the tiers of shipping and the waterside pollutions of a great (and dirty) city.” At the turn of the century, Claude Monet visited London specifically to observe the effects of the smog on sunlight, which he captured in a well-known 1904 work, The Sun Shining through the Fog.

Claude Monet. London, Houses of Parliament. The Sun Shining through the Fog, 1904, in the Musée d’Orsay , Paris. (public domain)

It was only when undertakers started running out of coffins that the city realized the most serious effects. On December 9, a wind came and blew the fog away, but the episode caused an estimated 12,000 deaths during and after the fog (Bell, Davis, & Fletcher, 2004). Many who died were the elderly, the very young, and people already with breathing conditions; many others developed lung conditions after the fog. According the UK National Weather Service (reported in Meetham, 1981), the following pollutants were released into the air daily during The Great Smog:

• 1,000 tonnes (= metric tons) of smoke particles
• 2,000 tonnes of carbon dioxide
• 140 tonnes of hydrochloric acid
• 14 tonnes of fluorine compounds
• 370 tonnes of sulfur dioxide, which was converted into 800 tonnes of sulfuric acid

The killer fog sparked a critical reevaluation of the causes of air pollution in London. For the first time, air pollution was widely recognized as an imminent health threat in the city. The tragedy led to the Clean Air Acts of 1956 and 1968, targeting coal burning in homes and factories. Although legislation and the move to cleaner energy sources will keep London safe from another deadly fog event, air pollution continues to be a major problem on a global scale. In March 2014, The World Health Organization (WHO) released a new estimate:

[I]n 2012 around 7 million people died – one in eight of total global deaths – as a result of air pollution exposure. This finding more than doubles previous estimates and confirms that air pollution is now the world’s largest single environmental health risk. Reducing air pollution could save millions of lives.

 

FOR MORE INFORMATION

Listen to The Great London Smog podcast: Stuff You Missed in History Class, July 3, 2014.

Read about how thermal inversions trap pollution, causing a smog layer over cities in Atmospheric Structure and Pollution Sources.

For an explanation of atmospheric pressure, see our module The Composition of Earth’s Atmosphere.

REFERENCES

Bell, M. L., Davis, D. L., & Fletcher, T.  A retrospective assessment of mortality from the London Smog Episode of 1952: The role of influenza and pollution. Environmental Health Perspectives, 112(1), January 2004. <http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1241789/>

Meetham, A. R.  Atmospheric Pollution: Its History, Origins and Prevention. Pergamon, 1981.

Written by Bonnie Denmark

Bonnie Denmark holds an MA in linguistics and teacher certification in English, ESL, and Spanish. She has devoted her professional life to educational and accessibility issues as a computational linguist, multimedia curriculum developer, educator, and writer. She has also worked nationally and internationally as a language instructor, educational technology consultant, and teacher trainer. Bonnie joined the Visionlearning team as a literacy specialist in 2011, assisting the project by developing comprehension aids for science modules and creating other STEM learning materials.