Richard Hull, University of Central Lancashire, UK, noted that some halogenated flame retardants may increase smoke in a fire, because they act in the gas phase, stopping combustion going to completion. However, in large fires, there is nearly always incomplete combustion, because the fire needs more air than can reach it. He also indicated that gas phase flame retardants can generate more smoke (soot particles and tar) and toxic gases (carbon monoxide and hydrogen cyanide).
Evidence from Factory Mutual1 shows that soot particles, may play a significant role in fires by radiating heat and so contributing to flame spread to other materials. Visibility in fires is a major issue for firefighters and has deteriorated considerably over past decades. Flame retardants which dilute fire gases by releasing water or nitrogen (e.g. ATH, MDH) or flame retardants which act in the solid phase by preventing polymer pyrolysis and generating protective char layers may not increase the products of incomplete combustion (such as smoke and CO).
1. A Tewarson, Combustion Efficiency and its Radiative Component, Fire Safety Journal, 2004; 39, 131.