A crash earlier today between two 18-wheelers forced the evacuation for a 5-mile radius off of Interstate 24 near Nashviille, Tennessee.
According to reports, one of the large trucks was carrying chlorine tablets, among other swimming pool cleaning supplies including oxidizing agents and other corrosive material. When the two trucks collided, the truck with chlorine tablets caught fire on the side of the highway close to the tree line. Due to potentially toxic fumes, officials first called for an evacuation of all residents and businesses for a one-mile radius.
Soon afterwards, hazard materials crews formed a plan to dump water on the chemicals. Since this action had the potential to create a second, larger plume of toxic gas, authorities then extended the evacuation zone to a code red five-mile radius.
Details of what caused the crash itself are not clear at this point. The driver of the semis only suffered minor injuries. It has not been reported whether they were transported to a hospital for further treatment.
Source: http://abcnews.go.com/US/authorities-warn-toxic-air-semi-crash-spills-hazardous/story?id=44183721