Which is the biggest man-made disaster?

The greatest man made disaster is the Great Smog of 1952 in London. It happened over the period of 5 days (December 4 - December 8).

It was a period of severe cold weather, air pollution combined with an anticyclone and windless conditions. Sulfuric acid particles were formed from sulfur dioxide released by coal burning for residential use and power plants. This formed a thick layer of smog over the city.

The smog was highly toxic and it destroyed the lungs of many. Thousands of people died in this period and hundreds of thousands of others developed severe medical conditions.

The catastrophic events of September 11, 2001, (or 9/11) resulted in the deaths of thousands of people working in the Trade Center, hundreds of Iraqi's and Afghans as well as thousands of American and British soldiers, and it caused devastation to two Muslim countries. It divided the world into Muslim and Non-Muslim, and it sowed the seeds of suspicion and hatred between them.

It also undermined the security of nations which had to spend trillions of dollars on security measures. The total sum of infrastructure and property damage was unprecedented. New York City was unrecognizable to those who lived there. The attack caused the complete collapse of the World Trade Center.