Vinyl chloride's invisible threat: Thousands of pounds are released every year in the U.S. as part of "poison plastic" manufacturing

Vinyl chloride entered the spotlight after the Feb. 3 Ohio train derailment. But the hazardous substance has been around for decades and is everywhere – from buildings and vehicle upholstery to children's toys and kitchen supplies – and factories have been emitting the EPA-designated toxic chemical into the air for years. 

The train that derailed had the manmade and volatile compound on board, prompting temporary evacuations amid concerns it could quickly impact people in the area. Then when officials decided to burn it, there were also concerns it could release phosgene, a gas that can be highly lethal and was used as a chemical weapon in WWI. 

But the derailment isn't the first time vinyl chloride has alarmed experts. They've been concerned about its potential impacts for decades.