A team of scientists appears to have solved the mystery behind what caused the London killer fog that claimed thousands of lives decades ago.

The Great Smog Of London

In December 1952, a thick and toxic fog covered the British capital for a period of five days. Sulphur particles mixed with fumes from burning of coal produced yellow fog that smelled like rotten egg.

The dense smog affected visibility preventing some people to see their feet. Transportation needed to be canceled. By the time the smog lifted after cold winds drove the fumes out to the North Sea, many were sick and even dead.

Experts estimate that the Great Smog of London killed more than 12,000 people and hospitalized 150,000. The toxic fog also killed thousands of animals. The event paved way to the creation of the Clean Air Act of 1956 that introduced measures to reduce air pollution such as the setting up of "smoke control areas" where only smokeless fuels were allowed.

What Caused The Killer Fog?

What exactly occurred had been a mystery for many years. The exact chemical processes that led to the worst air pollution-related catastrophe in Europe have not been fully understood over the past six decades.

By measuring the atmosphere in China, where 16 of 20 of the world's most polluted cities are found, and conducting experiments in the lab, Renyi Zhang, from the Texas A&M University in College Station, and colleagues clarified how sulfur dioxide from coal burning, which has long been blamed as the main cause of the fog, was converted into sulfuric acid.

Deadly Concoction Of Air Pollutants

Zhang said that nitrogen dioxide has something to do with the process.

"This process was facilitated by nitrogen dioxide, another co-product of coal burning, and occurred initially on natural fog," Zhang explained.

The researcher said that the process of converting sulfur dioxide to sulfate produces acidic particles. He said that natural fog has larger particles measuring several tens of micrometers and the evaporation of these fog particles left an acidic haze that blanketed the city.

"Under polluted environments, this SO2 oxidation process leads to large sulfate production rates and promotes formation of nitrate and organic matter on aqueous particles, exacerbating severe haze development," Zhang and colleagues wrote in their study published in the journal Proceedings of the National Academy of Sciences.

Air Pollution

Air pollution remains a problem worldwide with industrialized countries' reliance on power plants and vehicles that emit pollutants. According to the World Health Organization (WHO), nine out of 10 people worldwide breathe bad air.