there is a quite comprehensive paper on the combustion of polyurethane: JOURNAL OF APPLIED POLYMER SCIENCE 111, 1115-1143 (2009), DOI:10.1002/app.29131; http://onlinelibrary.wiley.com/doi/10.1002/app.29131/pdf
NIH NLM's TOXLINE database includes a publication on "Toxic Products from the Combustion and Pyrolysis of Polyurethane Foams" (I have provided a Permalink to the citation record). This is an excerpt from the abstract: " The authors conclude that polyurethane foams containing phosphorus flame retardants yield a variety of byproducts which vary according to temperature and ventilation of combustion."