Host: The 1debris you’re looking at is from one of the worst cases of 2invisible 3air pollution ever recorded, a very visible act of 4violence. This is the story of what the world did not see.

5Thomas Cahill: We watched the . . . 6destruction of the 9/11 buildings with horror. But as a scientist there was simply a building 7collapse and as the 8plumes came across our site we 9measure what they’re made of. And we found they were made up of 10extraordinary chemicals in extraordinary amounts ― more than we’d seen any place else in the world.

Host: Thomas Cahill 11heads the 12Delta Group, scientists that monitor air pollution worldwide. He was a lead 13investigator of the 14pollutants produced by 9/11. The 15conventional wisdom is that the most 16toxic air from 9/11 . But in studying the dust, Cahill made a 17startling discovery.

Thomas Cahill: To our surprise and amazement it turned out the material was mostly 18drywall, mostly 19gypsum, which in fact is 20harmless.

Host: But in the weeks that followed the air at 21Ground Zero as workers cleaned up the site.

Thomas Cahill: The 22coarse 23particles contained very 24finely 25powdered cement . . . and that cement had very 26caustic chemical 27composition, and it burned the back of your nose, your throat and your 28bronchial 29tract.

Host: As bad as these particles were, there was 30far worse material in the air, the invisible 31pollutants called 32ultra-fine 33particulates produced by the burning 34rubble 35pile.

Thomas Cahill: Ultra-fine particles are particles smaller than 0.1micrometer in 36diameter . . . which is about 1,000 to 2,000 times .

Host: These particles were the 37vaporized metals of 38tens of thousands of computers from the buildings.