The greatest number of casualties as the result of air pollution occurred in:

The greatest number of casualties resulting from air pollution is attributed to the Meuse River Valley in Belgium, particularly during the severe air pollution episode that occurred in December 1930. This incident is significant because it showcased the deadly impact of industrial air pollution, leading to the deaths of approximately 60 people, with thousands more suffering from respiratory issues. The conditions were exacerbated by a temperature inversion, which trapped pollutants close to the ground, creating extremely hazardous air quality.

In contrast, while other locations like London experienced significant air pollution incidents, such as the Great Smog of 1952 which also caused numerous deaths and illnesses, the immediate and acute impact seen in the Meuse River Valley during the 1930 episode is noted as having a higher casualty rate. Los Angeles is known for its smog and ongoing air quality issues but does not have a single historical incident that reached the same level of acute mortality from air pollution as that seen in the Meuse River Valley. Likewise, the Donora, PA incident in 1948 resulted in serious health effects and several deaths, but it did not match the casualty figures recorded during the Meuse River Valley episode.