With Water and Sweat, Fighting the Most Stubborn Fire
In a hot flaming fire, many toxic chemicals are incinerated, with little given off except carbon soot, carbon dioxide, water vapor and other fairly innocuous emissions.

But the relatively low temperatures of the trade center fires mean that traces of dozens of toxic chemicals and heavy metals are carried into the air, including benzene, a cancer-causing compound released when fuels are burned, and styrene, a gas emitted by burning plastic. At times the chemicals in the air at the site reach dangerous levels, particularly when fire flares up, as it did on Nov. 8.
http://www.nytimes.com/2001/11/19/nyregion/19FIRE.html?todaysheadlines&pagewanted=all