It was cold and clear the morning of December 5th, 1952 in London. People were hunkered down in their homes, huddled around their fireplaces waiting out an unusually early cold snap. The skies soon began to fill with coal smoke and soot and as the day progressed, a fog rolled in, limiting visibility throughout the city. The chimney smoke mixed with the fresh fog, turning it a sickly yellow colored pea-soup. Londoner’s went about their day as usual; heavy fog was very commonplace and their was no need for alarm. Yet over the next five days, this fog hovered over the city. The lack wind and a high pressure system combined to keep the fog cloud from moving. It continued to grow during those five fateful days, nearly covering 30 square miles, growing more dense with each passing day until people literally could not see their hands in front of their faces. Transportation came to a standstill; air travel was impossible. No ships could safely move along the waterways, and driving a car was impossible. Even the dependable British Railway system was unable to operate. Those souls who dared to step outside found themselves slipping and sliding as the walkways were covered with a greasy black film. Upon returning to their homes, those same travelers found that greasy black film also covered their exposed skin and clothing as if they had been working in a coal mine.
The local citizens called the thick haze, the Great Smog and soon found themselves having difficulties breathing. Family pets and farm animals started dying due to respiratory failure, wild birds either avoided the area or simply fell dead from the skies, and the entranceways to buildings became more blackened with soot each time the door was opened. The fog then turned it’s attention to the human population, with those already in respiratory distress succumbing to the deadly cloud first. Babies and young children and the elderly also fell victim to the cloud, which after several days began to stink like rotten eggs. Undertakers found themselves suddenly overwhelmed with corpses, so much that they ran out of available caskets. Each day more people slipped away and yet no action could be taken to alleviate the now-poisonous environment. People tried to make crude gas-masks, however they were ineffective. It seemed that many were about to give up hope when without warning, a brisk wind rolled in from the west, breaking up the thick fog and pushing the remnants far out to sea.
The casualties from the five day fog were unbelievable. Over 150,000 people were hospitalized for breathing related issues and the human death toll surpassed over 12,000. No accurate records were kept of the number of animals that died, but without the protection of a house to at least shield them somewhat, most assume that nearly all exposed animals died. The aftermath was an eye-opening message for the British Government with so many dead. Some likened it to the aftermath of German bomb attacks made during World War II. Bodies were discovered in unexpected places as many of the victims simply stopped breathing and quietly dropped in their tracks.
The British Government, after some investigation and reviewing the death toll, realized they needed to act. A few years later, the Clean Air Act of 1956 was passed by Parliament. This Act put restrictions on burning coal within areas of high population and established smoke-free zones. Those with coal fireplaces were forced to switch to alternative heating systems. This piece of legislation was the beginning of the end for the coal industry as oil and natural gas systems became the norm. Even with the new law, the change was slow and not everyone could afford to simply invest a large sum of money into their dwelling. A smaller event in 1962 killed an additional 750 people, reinforcing the validity of the law.
So What Really Happened To Make The Fog Deadly?
It was believed that the fog became toxic due to sulfates. Sulfuric acid particles formed from the sulfur dioxide that was released from the burnt coal. At the time, no one knew how this chemical change occurred and the event went as unexplained for nearly fifty years. Modern scientists, studying air pollution, ascertained that the natural fog was the catalyst for the change. Nitrogen Dioxide, another by-product of burning coal, was introduced to the naturally occurring fog. Another key aspect in the conversion of sulfur dioxide to sulfate is that it produces acidic particles, which subsequently inhibits this process. Natural fog contained larger particles of several tens of micrometers in size, and the acid formed was sufficiently diluted. Evaporation of those fog particles then left smaller acidic haze particles that covered the city.
In simpler terms, the combination of the coal smoke and a natural fog produced a rare moment for the chemical change, which in turn made the air turn deadly. The lack of wind kept the killer concoction suspended over the city and allowed the reaction to continue for several days, with terrifying results.