London: The Deadly Fog of 1952

For five days in December of 1952, the fog enveloped all of London. Residents at first gave it little notice because it appeared to be no different from the familiar natural fogs that have swept over Great Britain for thousands of years.

But over the next few days, conditions deteriorated, and the sky literally turned dark. Visibility was reduced to just 1 meter in many parts of the city and tens of thousands of people had trouble breathing. By the time the fog had lifted on December 9, at least 4,000 people had died and more than 150,000 had been hospitalised.

Researchers now estimate that the total death count was likely more than 12,000 people[1]. Despite its lethal nature, the exact cause of the deadly fog has largely remained a mystery. Recently, a team of researchers has determined the likely reasons for its formation[2].

Emissions from burning coal interacted with the deadly fog, but the exact chemical processes that led to the deadly mix of pollution and fog were not fully understood. To determine what turned the fog into a killer, a team of scientists recreated the fog in a lab using results from laboratory experiments and atmospheric measurements from Beijing and Xi’an, two heavily polluted cities in China.

They found that sulfate was a big contributor to the deadly London fog. Sulfuric acid (H₂SO₄) particles, which formed from the sulfur dioxide (SO₂) that was released from the burning of coal, were also a component of the fog. But how did sulfur dioxide get turned into sulfuric acid?

The results showed that it was an oxidation of sulfur dioxde (SO₂) by nitrogen dioxide (NO₂), another by-product of coal burning, resulting in sulfuric acid (H₂SO₄). These acidic particles were suspended in the fog and thus an acidic haze was covering the city. Inhaling acids is never good for your lungs.

The air of cities in China, which is often heavily polluted, has a chemistry that’s similar to the killer fog in London. China is home to 16 of the world’s 20 most polluted cities.

The researchers said that the main difference between China’s smog and the killer London fog is that China’s haze is made up of much smaller nanoparticles.

[1] Bell, Davis: Reassessment of the lethal London fog of 1952: novel indicators of acute and chronic consequences of acute exposure to air pollution in Environmental Health Perspectives – 2001
[2] Wang et al: Persistent sulfate formation from London Fog to Chinese haze in PNAS – 2016