The production of smoke, carbon monoxide (CO), and carbon dioxide (CO2) were investigated with cone calorimetry testing when low-density polyethylene (LDPE), LDPE treated with an intumescent flame retardant (IFR), and LDPE treated with an IFR and ultrafine zinc borate (UZB) combusted under irradiation. The results of the testing showed that UZB could depress smoke production and reduce the amount of CO and CO2. The components of the pyrolytic gas and its contents were identified and measured with pyrolysis gas chromatography/mass spectrometry (Py-GC-MS) when LDPE, LDPE/IFR, and LDPE/IFR/UZB were pyrolyzed at 400 degrees C for 20 s. The Py-GC-MS results implied that UZB had an important influence on the components and contents of the pyrolytic gas of LDPE/IFR. UZB mechanisms of smoke suppression and toxicity reduction with respect to LDPE/IFR are proposed. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 117: 443-449, 2010